Big Earth Data: a comprehensive analysis of visualization analytics issues

ABSTRACT Big Earth Data analysis is a complex task requiring the integration of many skills and technologies. This paper provides a comprehensive review of the technology and terminology within the Big Earth Data problem space and presents examples of state-of-the-art projects in each major branch of Big Earth Data research. Current issues within Big Earth Data research are highlighted and potential future solutions identified.


Introduction
Over the last 30 years the rate of production and collection of Big Earth Data has been increasing rapidly (Gantz & Reinsel, 2012;Mahdavi-Amiri, Alderson, & Samavati, 2015), with Earth Observation data alone doubling in volume every 2 years (Riensel, Gantz, & Rydning, 2017). This rise in data volume is accompanied by an equivalent increase in velocity and variety (Soubra, 2012) and these bring along with them further issues in terms of verifying the data and determining what value the data holds . While this problem is not unique to Big Earth Data (Chen, Mao, & Liu, 2014;Guo, Wang, Chen, & Liang, 2014;Wu, Zhu, Wu, & Ding, 2014), it is an area in which many different types of organizations interact: Governmental, research, commercial, and nonprofits, which gives rise to unique collaboration, interaction (Liu, Li, Li, & Wu, 2016;Philip Chen & Zhang, 2014), and communication (Chi et al., 2016;Nativi et al., 2015) problems.
This review paper draws together the background, terminology, and technology of Big Earth Data systems to provide a comprehensive overview for researchers in the field. The paper will introduce the terminology of the field (including the five Vs above) and then cover: the Big Data problem space; existing visualization platforms and tools-the state of the art in terms of storage, management, visualization, mining, and processing; and finally presenting current issues and future solutions to Big Earth Data problems.

The five Vs
The characteristics of Big Earth Data systems are typically classified as the five Vs, velocity, variety, volume, value, and veracity. These effect all big data systems; however, the scale and effects of Big Earth Data mean that we must consider these with particular attention. We define these terms further.

Velocity
Velocity refers to either the rate at which new data are being generated or to the rate at which data must be processed to be useful.

Variety
Variety refers to the number of unique types of data present in a Big Data project. Typically, this will have an impact on the ability to utilize structured data storage systems such as databases efficiently and effectively (Soubra, 2012).

Volume
Volume refers to the amount of data associated with a Big Data project, most big data projects will involve a minimum of 10s of terabytes of data with many projects being in the multipetabyte range. With this scale of data traditional database single-device database systems will struggle to manage and utilize the data effectively. If these data are unstructured then this storage becomes more complex as many entries are effectively unique.

Value
Value refers to the concept that data must provide some usable output or knowledge in order to have made collecting and storing it worthwhile. Value can be seen in many Big Earth Data studies such as climate investigations ; however, the data value density is often very low with only a small subset of the complete data set being valuable to each project. This low data value density, however, high data value clustering requires highly effective data storage, retrieval, and processing techniques to efficiently acquire information from the dataset.

Veracity
Veracity refers to the overall quality of the data available. The quality of data can be impacted by noise or abnormalities in the original data gathering process, or through improper processing (Lakshen, Vraneš, & Janev, 2016). Big Data presents some unique issues due to the methods of data collection-slight variations in both the state of the recording equipment and the environmental conditions of the subject can dramatically alter the quality of incoming data. Postprocessing is often required to reorientate and reorganize the data set, however care must be taken while doing this to protect the veracity of the data (Batini et al., 2017).
Veracity can also be impacted during the presentation of data, poor usage of visualization methods can completely create a distorted view of the data leading to invalid conclusions being drawn and communicated (Bresciani & Eppler, 2018).

Geo/Earth Science
Geo and Earth Science are the terms which have taken hold as the primary terms for the study of the planet Earth at all levels-though primarily broken down into four branches of study: the lithosphere (surface topology, geology, and effects), the hydrosphere (the effects of surface and subsurface water), the atmosphere, and the biosphere (ecological effects including human factors). Until fairly recently Earth Science has been a very hands on branch of science involving direct observation and mapping of effects and results of experimentation "in the field." As we gain more and more capability to observe remotely this has given rise to a special area: Earth Observation.
Earth Observation is then a specialization of Earth/Geo Science specifically focused on our ability to gather data remotely. Earth Observation focuses on the gathering of information about the Earth under categories such as physical geography, chemical and biological composition, natural resources, and human factors primarily through remote-sensing technologies (EO-MINERS, 2018) and backed up by direct Earth surface and subsurface surveying techniques (van der Meer, 2018). These data are then utilized to allow for the extrapolation and mapping of trends including: weather forecasting, natural disaster tracking and monitoring, disease and health monitoring, or climate change analysis.
Groups such as ESA (European Space Agency), NASA (National Aeronautics and Space Administration), and Google, gather terabytes (TB) of data about the planet each day from various satellite platforms, such as Landsat (Land Remote-Sensing Satellite), MODIS (Moderate Resolution Imaging Spectroradiometer), EOS (Earth Observing System), and GPS (Global Positioning System). That feeds large volumes of free data into the Big Earth Data system allowing for an increasing complexity of analysis (NASA, 2018c).

Structured data
Structured data are organized data which have a known structure, size, and format, making it suitable for indexing and automated metadata creation allowing for the creation of searchable, indexable, and retrievable data management systems. Structured data will typically include data such as temperature measurements, rainfall, or other well-structured data.

Unstructured data
Unstructured data refers to all other forms of data (Taylor, 2018) and comprises data of variable types, sizes, multidata set collections, and other arbitrary data formats. Typically, common data such as audio, video, and images will fall under the category of unstructured due to variable size, format, and resolutions.

Data cubes
A data cube is at its simplest a three-dimensional (3D) array of data with meaning applied to layers within the cube as well as to the rows and columns (illustrated in Figure 1). Data cubes are a way of applying structure to unstructured data or to provide an aggregate data set that pulls together multiple individual data sources. An example of this would be the changes in rainfall over a geographic area-each layer within the cube would represent a time period with the value in each cell being the rainfall value at a geographic location indicated by the row and column.

The Big Earth Data problem space
Data acquisition in Big Earth Data is a massive challenge, data are collected from an extremely varied number of sources, from satellites beaming back gigabytes of data to earth each day to remote sensors in internet of things devices providing a constant stream of poll data. Storing these data in a way such as to make it useable and compatible with other datasets requires significant time investing into management strategies to ensure usability of the data. Once acquired initial processing is performed to increase the usefulness of the data, extracting the useful features relevant for the current task, and adding metadata describing the information extracted for future reference, from here the data can be combined with other datasets to produce meaningful and useful results.
Big Earth Data as a problem space is too large to sensibly discuss with any form of structure. We have instead broken the problem space down into: storage, processing, visualization, management, and mining as the five primary problem areas. Figure 1. Illustration of the elements of a data cube, from left to right a cell which contains a single data element; a layer which contains a related set of data elements; and a cube which contains multiple related sets of data elements, e.g. geographic, hydrological, and human effects over time.

Data storage
The storage issues of Big Earth Data are generally similar to those of any Big Data project, commonly referred to as the three Vs of Big Data (Gewirtz, 2018): volume, velocity, and variety (with the addition of two supplemental Vs Variety and Value to make 5). Data being gathered by Big Earth projects are growing in line with Moore's law (Moore, 2006) to fill the available space. Riensel et al. (2017) show this trend is likely to continue, with an expected 163 ZB (Zetabytes) of data being generated in 2025. This dramatic increase has largely been made possible due to the shift/evolution in computing from centralized datacenters of the 1980s, which were often underutilized and extremely expensive, to the more dynamic and responsive cloud computing systems utilize in modern data centers.

Basic concepts and technology
The increased use of distributed computing technologies and the growth of available cloud computing facilities (Assunção, Calheiros, Bianchi, Netto, & Buyya, 2015;Yang, Huang, Li, Liu, & Hu, 2017) have begun to address the core issues of distributing/sharing and processing the extremely large data sets involved in Big Earth Data projects (Munroe, 2017). At the same time large scale simplified processing technologies have been developed to enable realistic use of these distributed computing platforms. With HADOOP style open source projects (Fernández et al., 2014;Patel, Birla, & Nair, 2012) it is now feasible for independent researchers to manage Big Earth Data projects.

Data cubes
Data cubes are a way of representing large data sets over a common series, however, as Baumann points out (Baumann, 2017), there are issues with standardization of data cubescurrently no such standardization exists. Each layer of the cube is made up of a multidimensional array of cells with each cell containing the data (e.g. a data point, complex structure, data value, or even another data cube). These layers each represent the same measurement points for each cell, but modified by an external series (Technopedia, 2018).
While the lack of standardization can make it difficult to deal with even data sets where commonalities exist due to packaging order, representation, and axes. Metadata (Devillers et al., 2007) can be attached to data cubes to give information on how to understand, manage, and process the data within the cube. These data can be provided as purely Human readable formats or more ideally in automatically processable formats allowing cubes to be refactored by web services and data processing languages such as XSLT (W3C, 2017a). Ironically the lack of standardization applies to the metadata attached to data cubes as well and can lead to unnecessary confusion and difficulty in processing (Harinarayan, Rajaraman, & Ullman, 1996).
As part of the EarthServer2 (Earth Server, 2015), an attempt is being made to combine two common standards, i.e. XPATH (W3C, 2017b) and WCPS (Open Geospatial Consortium, 2009) into the new xWCPS (EarthServer, 2016) standard which hopefully addresses the issues associated with data cube, and data cube metadata standardization (Baumann, 2017).

Structured and unstructured data
Big Earth Data comes in structured and unstructured forms: • Instruments making regular-timed measurements, e.g. a temperature sensor may be structured due to regular and consistent timing of measurements combined with a known range of values (minimum and maximum temperatures). • Instruments that produce data at inconsistent times, locations, or contain other similarly unpredictable components are classed as unstructured data, e.g. an opportunistic sensor network tracking wildlife (inconsistent temporal and geolocation characteristics). • Instruments which produce data so varied that classification or organization into traditional models management system results in trivial gains to storage size (Buckler, 2015) would also be classed as unstructured. Examples of this would include satellite observation data (images) or traffic camera footage (video). Table 1 describes the characteristics of structured and unstructured data in terms of the core characteristics of the data, where the data can reasonably be stored, how processing and management can be implemented, and how solutions need to scale to manage that data.
To aid in the understanding of these classifications we provide the following examples of structured data ( Figure 2) and unstructured data ( Figure 3).
Both datatypes are generated in GIS systems, depending upon the methods used to gather the data: sources such as images from satellites, aeroplanes, and UAV create large repositories of unstructured data; while other sources such as GPS readings and Internet of things compatible sensors (e.g. metrological sensors) gather large quantities of structured data.   With both structured and unstructured data there is the potential for additional metadata that acts to introduce additional information about/associated with the original data allowing us to contextualize or understand the data better. While metadata can provide some structure or information about unstructured data we cannot realistically rely on this as metadata as it is often incomplete, missing, incoherent, wrong, or a combination of all of these.

Database management systems
As discussed above, the distinct characteristics of structured and unstructured data require different storage techniques, these can be broadly broken down into: (a) traditional relational database models, (b) NOSQL style alternative solutions including: keyvalue storage; graph databases; and document databases. As highlighted by Yang, Huang, Li, Liu, & Hu (2016), the combination of SQL and NoSQL databases into a big data context (i.e. high scalability, fault tolerance, and availability) allow for the creation of Big Data specific cloud style platforms, e.g. Knowledge as a Service (KaaS).
To obtain optimal performance the type of database selected must be suitable for the project. The AGDC (The Australian Geoscience Data Cube) project (Lewis et al., 2017) shows significant change to the database model from a fully relational database in version 1 to a combination minimalist relational database with NoSQL in version 2. This change facilitated the addition of multiple different sensor data types and integration of the associated metadata types into the project. The integration of the metadata presented a unique benefit/difficulty as the data were often bespoke to the dataset making storing it in a traditional relational database in a meaningful way challenging. The inherently bespoke nature of nonrelational databases means interaction with external datasets can be complicated by the unique data models and interactions involved in multiple data sets. This can be addressed by the effective use of metadata to identify the contents and structure of the data stored within; however, this is again hampered by the lack of standardization of metadata discussed earlier.
While optimal storage size is a key consideration of Big Earth Data, a more pressing issue comes in the form of access speed. Querying a relational database requires the assembling of data from multiple tables each requiring an additional processing to be performed by the server. Figure 4 illustrates the issues encountered when accessing large volumes of data in quick succession, the overhead of reassembling these records quickly adds up incurring a significant overhead which only increases as the data set gets larger. As nonrelational databases do not have these joins access times for individual records are faster and more suited to the mass processing performed in Big Earth Data analysis (Taylor, 2018). The nature of Big Earth Data sets is that they are nonrepetitive and largely unique and so cannot fully leverage the data dereplication inherent in an SQL database.
Relational databases typically scale vertically-increasing the performance of the host machine to increase processing speed. Vertical scaling models are not ideal for handling very large datasets as processing large volumes of data quickly and efficiently require a large increase in the performance of the host machine, which then has to rely on significantly more expensive high performance parts, and may approach and reach a performance cap (Krishnaprasad, 2001). Nonrelational databases in contrast typically scale horizontally-they increase the number of resources working on the problem at once and distribute the load across multiple host machines. By distributing the load over an increased number of resources cheaper and more replaceable hardware can be utilized which makes horizontal scaling generally cheaper at the expense of increased infrastructure and management systems (VCloudNews, 2016). Utilizing this horizontal scaling model also allows the system to fit naturally into the dynamic, and adaptive cloud computing systems that exist to leverage large scale distributed computing projects.
Querying these databases and retrieving the data in a useable format is a huge task and requires matching the processing language to the appropriate platform. While there are many languages that exist to perform this task there are only two major standards that dictate how these languages operate: WCPS (Web Coverage Processing Service) (Open Geospatial Consortium, 2009) is used to control the retrieval of raster based data; XPATH (XML Path Language) (W3C, 2017b) is designed to control the retrieval of data stored in an XML format.
Currently with mixed data (such as images with an attached xml metadata file), neither solution is able to fully handle the problem, one solution maybe in the form of xWCPS (XPATH Enabled WCPS) (EarthServer, 2016), a new language which aims to combine the strengths of xPATH and WCPS into a single language eliminating the need for multiple languages.

Cloud storage systems
The increasing availability and reduced cost of cloud-based storage and processing has greatly aided the ability of researchers to analyze and process data (Assunção et al., 2015). While techniques and tools for large scale distributed computing have been around for decades the ability to dynamically and responsively acquire, utilize, and importantly release processing and storage power on demand has brought these systems to the forefront of large scale data storage and management. Cloud services are typically available as public, private and hybrid varieties based upon the security and budget of the organization. The key differences between localized storage solutions and cloud storage are summarized in Table 3.
Most projects turn to distributed cloud computing systems to handle issues such as temporary data storage and processing requirements. Software solutions such as Amazon S3 (https://aws.amazon.com/s3/) and Google file services (Ghemawat, Gobioff, & Leung, 2003). Distributed across many hardware elements, these packages offer a large, and expandable storage capacity, data redundancy and basic analytics functionality for the data. At current there are no specific cloud computing services aimed at Big Earth Data and the specific challenges of the Earth Science domain; however, general purpose cloud services are being built upon to create processing platforms that mirror and manage the data types, flow, and output that we expect.
Cloud computing platforms do introduce one major issue into our pipeline: vendor lock in. Each cloud platform requires its own management and processing functions be observed unless we build from raw hardware up, this can introduce significant issues with data migration (Ramachandran et al., 2018).

HADOOP FS and other distributed storage systems
There are many in use distributed file systems designed for big data problems. Typically, these file systems are specialized to provide storage for small files (or block data) while transitioning from a single machine paradigm to either network attached storage (StoneFly, 2018) or a storage area network (Sabeesh, 2017) paradigm (Park & Kim, 2018) while adding load balancing, copy consistency, and deduplication systems (Yang et al., 2017).
In 2003, Google published Google File System (GFS) which is a large-scale extensible distributed file system (Ghemawat et al., 2003) after Google put forward the GFS, Apache realized an open source implementation of GFS and launched its own project-Hadoop whose key storage technology is the Hadoop Distributed File System (HDFS) (Shvachko, 2010). Hadoop created a new way for massive Earth data storage and management, attributed to its high throughput of massive data, high performance distributed database and effective use of distributed and heterogeneous computing capacity.

State of the art
Pan & Zhang (2012) put forward a Hadoop-based Earth Observation image processing system (HBRSIPS). The system redefines the storage structure to suit for the characteristics of remote sensing images and deploys the data processing algorithms in cloud computing environment. Cheng et al. presented an Earth data management system for massive Earth data based on HBase and GeoSOT (Wang, Cheng, Wu, Wu, & Teng, 2015). The basic idea of the system is splitting Earth Observation images into patches and storing them into HBase based on GeoSOT global discrete grid system and further processing the data in a distributed manner. George Vafiadis et al. proposed a Hadoop-based remote sensing image cloud storage model based on cloud computing framework (Vafiadis, Moulos, Sykas, Baglatzi, & Vohora, 2013). Its storage is a multilayer structure, supporting image processing and image data searching of various algorithms.
Examples of these kinds of specialized distributed file systems include: • Google Earth and Google Maps take advantage of GFS and BigTable (https://cloud. google.com/bigtable/) to manage the global Earth data. • Bing Maps store the Earth data based on the distributed cluster storage system and cloud computing platform of Microsoft's data centers, where all the Earth data are structured using quad-tree tile layers (Beureuter, 2015). • ArcGIS Image Server is Esri's solution for storing, analyzing, and serving massive collections of diverse Earth Observation imagery and raster geospatial data. It is designed to take advantage of the storage and computational power provided by the cloud with a new approach to distributed storage and distributed computation. • NASA's utilizes Earth Observation System Data and Information System (EOSDIS) which is used to store and manage all the Earth data it gathers. EOSDIS follows a distributed and open storage architecture (NASA, 2018b), where data are stored in an extensible multilayer data format (Ramachandran et al., 2018).
While it is easy to highlight that the five Vs of big data will continue to give use trouble in the coming years (Hashem et al., 2015). It is also clear that the horizontal scaling models of NoSQL, HADOOP, and similar technologies address the scaling issue well in terms of storage capacity and capability (Pourabbas, 2014). The increasing capability to deploy the processing capability to the data in a structured layered model allows us to address scaling issues appropriately and well, and with modern containerized data centers (Diamanti, 2018). We can even address dynamic deployment of resources to areas of need.
4.6. Data processing 4.6.1. Distributed computing systems Distributed computing is the concept of taking a computationally intensive task and breaking it into multiple tasks and distributing these tasks to multiple systems essentially turning a single large linear task into multiple smaller ones which can be processed in parallel speeding up execution times (Dean & Ghemawat, 2010;Lämmel, 2008). This concept is far from new and has been the core of multiuser computing systems since the 1960s, however, as tasks grow larger we need to move from manual parallelization of tasks to the automated application of processing to data, and them the subsequent aggregation and integration of that data.
To aid in this process many frameworks and paradigms have been created that work to reduce the complexity of parallelization while aiding in the automated management and control of task flows. The primary frameworks in use currently are HADOOP and its derivatives and the Apache suite (STORM, SPARK, SAMZA, FLINK).

Frameworks and paradigms
To execute these queries in a reasonable amount of time, distributed computing is needed to effectively share the workload across multiple computational resources. There are two different types of distributed computing frameworks, batch only frameworks like map reduce (Dean & Ghemawat, 2008), where events are processed as blocks for more efficiency with volume of tasks at the cost of immediate speed or stream-only frameworks such as Apache Storm (http://storm.apache.org) where events are processed on receipt which sacrifice efficiency to deliver more immediate individual results. Hybrid systems, such as Spark (https://spark.apache.org) and Flink (https://flink.apache.org), capable of operating in both modes as a hybrid system but require more in depth management to ensure the correct method is used for the correct situation.
The MapReduce (Dean & Ghemawat, 2010) framework which is designed for processing parallelizable problems across large datasets is a possible solution for this task. The MapReduce programming model is composed of a map function and a reduce function. The map function splits the input data into independent chunks which are distributed to available computational resources and processed in a completely parallel manner. The reduce function is responsible for collecting all of these individual results into the final query result which is then provided as an output to the user (IBM, 2018). This divide and conquer solution effectively reduces the waiting time between submitting a query and receiving a response. Benefitting from the high efficiency of distributed computing solutions, analysts can effectively access to systems, which can process queries at a reasonable pace, without incurring a prohibitively expensive cost.
Indexing and searching of data is now handled by frameworks such as Hadoop (Patel et al., 2012), Spark, and Flink. These packages handle data storage, and distributed processing, as well as effectively managing the indexing of the data. Indexing significantly speeds up the retrieval of data by creating a list of where things are within the dataset as queries are run. As the index builds up, it gains Knowledge about what data are stored where, effectively skipping/reducing the processing requirement for the search phase, retrieving the relevant data from the indexed location. This system is not without its problems though, large datasets themselves can generate indexes of vast sizes requiring their own storage management solutions, reassembly of the indexed data added an overhead in terms of processing, with transition speeds between nodes becoming a significant limiting factor in processing speeds, finally keeping the index up to date requires either sharing changes to the dataset across the whole system or risk inaccurate links within the database. Each of these frameworks has benefits and flaws under specific datasets and so the choice of frameworks needs to be calibrated against the dataset in use for optimal performance (Inoubli, Aridhi, Mezni, Maddouri, & Nguifo, 2018).

Cloud processing systems
EarthServer is one of the major systems used for Big Earth Data analysis. In 2015, the second iteration of the EarthServer project was launched (Earth Server, 2015) to expand the functionality of the original EarthServer project. Baumann et al. (2016) outline the improvements EarthServer 2 aims to bring over the original project. A lot of this is just expanding the project to increase capacity, but most interestingly is the push for the creation and implementation of standards for Big Earth Data storage, processing, and retrieval. Given the success of the first project, the results of EarthServer 2 will likely have a significant impact on how Big Earth Data is processed.
The ArcGIS platform (http://desktop.arcgis.com/en/) is another major visualization tool facilitating the construction of visualizations in addition to the analysis, management, and sharing of GIS data. ARCGIS represents a complete system for GIS analysis using visual models and scripting to combine outputs from multiple processing tools automating workflows, increasing the effectiveness of the GIS researcher. Unlike most other platforms ARCGIS attempts to address the researcher familiarity, query complexity, and feedback issues discussed below through the provision of a Unix style program integration visual programming language.
For researchers on a smaller budget, open source solutions exist to the Big Data puzzle. QGIS (https://qgis.org/en/site/) while not as visually impressive as the commercial projects identified above, has all the features a GIS researcher needs-data input, management, analysis, and visualization as well as the capability to add modules in Python. Due to its open source nature, QGIS can be added to by any researcher in an open manner through add-on modules as well as editing core functionality to fit their needs and (in the spirit of open source software) feed these improvements back into the software.

Complexity in language
Most of the existing tools for data processing introduce a level of complexity to the process-they often utilize SQL (Structured Query Language) like languages to produce queries over data. When it comes to using these queries, the process is a little more complex-all require the understanding of an SQL like language which, due to the lack of standards, varies widely in their implementation from standardized SQL. The actual processing of these queries also leaves a lot to be desired with most implementations lacking many basic text editing functions and providing bad support and feedback for query design and execution (Kong, Zhang, & Stonebraker, 2015).
Most packages simply implement a text window into which a query is entered, and no feedback is given when queries fail. While there are many parsing tools for SQL and similar languages it is often more useful to see where something has failed rather than simply that it has.
Plymouth Marine Laboratory (http://earthserver.pml.ac.uk/www) provides an example implementation of EarthServer technologies, which provides visualizations, Figure 5, relating to various sets of ocean data. The example query highlights two significant problems with the current system. Firstly, there is a significant amount of complexity within the query making it difficult to follow exactly what it is doing. Individuals unfamiliar with WCPS will have significant trouble getting the desired output from the system. Second, as with most packages of this type, when a query is entered incorrectly, no feedback is provided to the user, in this case infinitely hanging at the processing query stage, never progressing, and never informing the user there is a problem.
Modern IDE's (Integrated Development Environment), such as Visual Studio 2018 (https://visualstudio.microsoft.com) and NetBeans (https://netbeans.org) or text editors such as notepad++ (https://notepad-plus-plus.org) have implemented systems familiar to users of word processors to highlight problems to users, but even without this most software packages will at least return an error code when a query fails, while implementing the systems of mainstream software development environments may be a bit of a stretch, providing the user an error code would at least mean it could be looked up in the manual. Another significant challenge in complex programming comes from fixing problems in algorithms and queries, traditional DBMS such as phpMyAdmin and PostgreSQL provide basic error messages (see Figure 6) but these are often cryptic and to a novice user can be of no help at all in solving the problem. As visual programming systems are designed to lower the skill requirement for implementing GIS queries, reducing the probability for mistakes to be made, by not allowing incorrect data types to be entered or preventing modules from being connected to incompatible ones, however no system is fool proof and therefore care must be paid to ensure these error messages are able to be processed by novice users (Munson & Schilling, 2016). Plain English instructions effectively communicate the problem to novice user better than the error codes traditionally used in software development (usually acceptable due to these not reaching the end user), though both allow the user to utilize the internet to find resources which may be helpful to their problem. 4.6.5. Approximate query processing One of the largest challenges for data visualization is the processing of the data, which is a computationally intensive process taking a significant amount of time to complete due to size of the data. This processing time is undesirable as it reduces productivity and even small interruptions have been shown to have a significant impact on an analyst's performance (Adamczyk & Bailey, 2004).
Approximate query processing is the idea that we can speed up the processing of a query by approximating its output from a subsample of data (Fisher, Popov, Drucker, & Schraefel, 2012;Moritz, Fisher, Ding, & Wang, 2017). When applied correctly, AQP performed via subsampling of data can produce results accurate enough to make reasonable decisions (displaying the confidence of certainty the final result will be similar to this output Figure 7) without the need for full processing of the query.
In any AQP process, it is still important to run the full query, while for most data providing the subsampling are appropriate the full query will resemble a more accurate version of the AQP query this is always an approximation not a guarantee, as such there exist cases where the full query will not be reflected in the subsampled query. Consider Simpsons paradox as an example of this, the subsampled data show one thing while the fully sampled data show the inverse (Wagner, 1982). AQP certainly shows potential to allow predictive visualization of Big Earth Data; however, a practical implementation of this has yet to surface. 4.6.5.1. Workflow concerns. Implementing new systems and workflows is not as easy as it first appears. Bad updates often face public backlash, recent Snapchat (Constine, 2018) and Twitter (Titcomb, 2017) updates caused massive backlash from the user base, resulting in users switching to competing services. While these are high profile examples, they do illustrate how easy it is to alienate a user base when decisions about a piece of software's usability are taken without due consideration. On the flip side software updates can have a positive response, The ribbon interaction system (Microsoft, 2007) has standardized the controls between many core office applications and beyond on the windows operating system, increasing usability by removing the confusing cluster of icons and menus previously used. This is not to say that the change was easy, a review by Toms Hardware (https://www. tomshardware.co.uk) (Yam, 2011) showed initial productivity was hampered due to unfamiliarity with the system despite evidence to the contrary (Neagu, 2017) that the systems were mechanically more efficient. This suggests that there is more to consider than the mechanical effectiveness of a UI (user interface) when implementing it, usability is key, and while changes may be technically more efficient, disrupting the established mental models of the user will certainly cause difficulties at least in the short term (Brown, 1998).

Basic concepts and technology
Data visualizers commonly perform two distinct roles. The first is the processing of data into a useable form, detecting desired features, and removing unwanted ones. This is achieved by utilizing a suite of feature selection (e.g. removing attributes and range checks) and pattern recognition algorithms (e.g. data cleaning techniques, normalization, and subsampling) within the environment. Secondly, visualizers produce graphs, images, and plots used to convey the data to the analysist. This can be as simple as producing a common graph or chart from data values or creating more complex outputs, such as raster images or vector plots of the data. Tools are also provided to assist with the extraction of information such as area/shape detection algorithms, and filters such as variance filter for edge detection and Fourier transform. Each different package however will provide unique features depending upon its purpose and the constraints under which it operates. 4.7.1.1. Visual queries. An alternative does exist however, visual programming systems such as Scratch (https://scratch.mit.edu), Kodu (https://www.kodugamelab.com), and the Blueprints system (https://docs.unrealengine.com/en-us/Engine/Blueprints) from the unreal game development engine, do address some of the issues raised here. By using simple drag and drop systems, these applications remove the need to learn a complex programming language such as C++, C#, or Java (illustrated in Figure 8). They also reduce the chance of making simple mistakes such as typos by removing the need to enter anything into the system other than the parameters which drive the query.  proved this concept with the grid workflow system, which shows a drag and drop system is possible however its effectiveness had not been fully tested. Through the implementation of a visual query language, it may be possible to utilize these systems to remove the need to learn a language or provide a wrapper to make these systems language neutral removing the need to learn multiple languages (Costagliola, Tortora, Orefice, & de Lucia, 1995).
Whether ultimately implementing a system such as this would actually prove to be more work that upgrading the text input to the standard of a modern IDE, it certainly has the potential to greatly improve the usability of these tools and reduce the skill level required for their usage.

State of the art
Most common tools for big data analysis such as Zoomdata (https://www.zoomdata. com) Tableu (https://www.tableau.com), and Plotly (https://plot.ly), are of some use to Big Earth Data analysis, however, to get the most out of the data, particularly for Earth visualizations, specialist tools are required for visualizing raster and vector data such as ArcGIS (), GRASS GIS (https://grass.osgeo.org), and Mapbox (https://www.mapbox.com). While these tools can perform the task, they certainly fall behind the more mainstream big data analytics tools when it comes to usability (Haklay & Jones, 2008;Komarkova, Sedlak, Habrman, & Cermakova, 2017) likely due to the smaller target audience as a specialist application. There is still room to improve the usability of these tools and bring them up to a higher quality standard.
NASA provides serval public visualization tools for viewing satellite data Worldview (https://worldview.earthdata.nasa.gov) and Giovanni (https://giovanni.gsfc.nasa.gov/gio vanni). Worldview provides a major source of geographic visualization to individuals. This data has been obtained from a wide variety of different satellite scans produced by NASA and its partners over the last 18 years. Its primary purpose is to show "The entire Earth as it looks 'right now'." This said it is not an analytical tool, providing few ways to manipulate the data and only simple visualizations. Giovanni offers more complexity in terms of allowing data filtering and plotting, animation and many different plot types. Neither tool allows the importation of other data sets.

Future research
Looking to the future, the rise of AR (Augmented Reality) and VR (Virtual Reality) technologies have seen widespread adoption in the fields of architecture and engineering (Joyce, 2017;Quirk, 2017), data visualization is likely to follow soon (Donalek et al., 2014;Tarolli, 2017). These technologies show the potential to have a significant impact on data discovery, allowing new and novel interactions to aid with the task (Olshannikova, Ometov, Koucheryavy, & Olsson, 2015). What is critical however is the need to overcome the traditional issues related to the visualization of volumes angles and curves (Evergreen, 2016) which VR and AR aim to exploit. 4.7.3.1. Virtual and augmented reality. The future of virtual reality is always uncertain, every new iteration improves the technology slightly and yet, so far none have managed to achieve true mainstream penetration. Films like Minority Report (Spielberg, 2002) have been promising the widespread distribution AR and VR systems into modern life and yet this future has yet to materialize. Mainly this is due to these systems over promising (Drummond et al., 2014) and largely failing to deliver on this promise.
The desire for VR systems is largely to do with removing the limitations of current interaction methodologies (Garbaya et al., 2014), while traditional desktop displays can produce pseudo-3D images, but with the addition of stereo-3D effects object recognition and perception can be improved (Bennet, 2006). Also while traditional interaction technologies (such as keyboard and mouse, trackballs, etc.) perform adequately, they are not what would be considered natural interaction methods, and the potential exists for gesture-based systems being able to improve on these methods (Nielsen, Störring, Moeslund, & Granum, 2003).
However, the current generation of hardware may be due to buck the trend both the Oculus rift (https://www.oculus.com) and the HTC Vive (https://www.vive.com/uk) seem to being going strong, with both entering a new hardware iteration soon, and attracting significant investment and support from blue-chip companies, particularly from engineering and architecture firms (Joyce, 2017;Rotella, 2018) who are already taking advantage that the novel visualizations the hardware provides. While this does not represent a significant investment by these firms, who spend tens of thousands of ponds every year on infrastructure, it does show that the technology is beginning to permeate industry and a desire to utilize the technology, beyond a gaming setting is present. 4.7.3.2. Adoption. Both AR/VR and gesture control systems are currently seeing a round of strong commercial development. Many competing products exist in the VR field (HTC Corporation, 2018;Oculus, 2018) and commercial AR technologies (Microsoft, 2018) are on the horizon. However, previous technologies have shown that distribution of a technology and getting mainstream penetration is a challenging task. Generally, only products which provide a major quality of life boost (such as smart phones) (Hall & Khan, 2003) survive long enough to become more than a flash in the pan novelty. VR and AR represent the potential for real opportunities and challenges in visual analytics, being able to render and manipulate 3D visualizations in real-world space in front of a user. They may completely change the way we think about visualizations maybe reversing some of the currently known facts about visualization relating to the perception of volume curves and angles (Evergreen, 2016), ushering in a new wave of visualization technologies enhancing data usefulness and communication in ways we have never experienced before. 4.7.3.3. Gesture control. As well as new visualization methods, potentially gesture control systems such as the leap motion (https://www.leapmotion.com) could provide an innovative leap similar to that produced by the mouse in the late 1990s, due to the ability to produce a near infinite number of gestures assigned to a unique amount of functions, not to mention the ability to assign natural motion to actual tasks such as picking up and manipulating objects or graphs. Research into gesture control and its effect is still ongoing (Malizia & Bellucci, 2012). Primarily researching into the design of gesture control systems and the mechanics of usage of gestures, however a shortage of research exists to demonstrate its effect on data visualization and its effect on visualization confidence.

Data processing
Due to the large volumes of data being generated in Earth Science/Earth Observation fields it is an obvious step to take to attempt to apply machine learning techniques to the data that we have in order to produce new insights and new metadata JIN (Jin, Wah, Cheng, & Wang, 2015). Multiple attempts have been made to produce a unified framework for Big Data analysis such as the work by Wu et al. (2014), and to define the standards and future of the field (Fan and Bifet, 2013). The nature of the data, however, often adds an additional challenge to perform this meta-analysis due to the highly variable and unstructured nature of much of the data. Additional problems are typical of Big Data-scalability and parallelism. There are benefits to applying these techniques though as an example drawing from the Healthcare industry as an example, Herland, Khoshgoftaar, and Wald (2014) and Yuan et al. (2013) have shown that massive savings can be made by relying on machine learning rather than human analysis of data. There are many projects underway to approach these issues and make data mining of Big Data more feasible in addition to the frameworks we have looked at before, these include: NIMBLE (Ghoting, Kambadur, Pednault, & Kannan, 2011), BC-PDM (Yu et al., 2012), PEGASUS (Kang, Tsourakakis, & Faloutsos, 2009), and Graphlab (Low et al., 2012). Gandomi and Haider (Gandomi & Haider, 2015) provide an excellent overview of simple processing techniques.
Data mining techniques vary greatly in complexity, ranging from simple-rule-based systems up to complex neural networks, for a general overview it is likely best to consider a book such as Witten, Frank, Hall, & Pal, 2016, van der Aalst, 2018, or Han, 2017. We can consider most data mining techniques under the following categories: The simplest level of data mining is performing pattern analysis-that is looking at the data for general trends, e.g. spikes in traffic in coastal areas when the temperature rises (Giannotti, Nanni, Pinelli, & Pedreschi, 2007;Luigi, Oana, Mihai, & Simona, 2012).
• Classification (DataminingTools Inc, 2018) This involves the grouping of attributes and variables together in order to provide a simpler and aggregated view of a topic, i.e. we draw together factors of "town location" in order to determine its appeal as a holiday destination.
• Association (Borne, 2014;Wesley, 2012) Association is similar to pattern tracking, however, involves the tracking of highly correlated events. Natural disasters as an example often have many social and human factors that are associated with their destruction such as spikes in twitter traffic.
• Anomaly/outlier Detection (Agrawal & Agrawal, 2015;Chandola & Kumar, 2016) Anomalies and outliers are very important in data mining as they often define unusual events or tell us more about a pattern than we have seen normally. Changes in seasonal migrations or spikes in volcano characteristics might fall under this category. These also have an effect on technologies such as AQP where these outliers might make the subsampling error prone.
• Clustering Analysis (OriginLab, 2018;SAS Institute, 2018a) Clustering is a technique similar to classification but involves the grouping of specific sets of data based on characteristics. From this we can then derive large amounts of metadata about groups based on their shared characteristics that may be missed in a simple classification.
• Regression Analysis (Luigi et al., 2012;Michael, 2013) Regression is typically used for planning and modelling in that they attempt to derive the value or likelihood of a variable or characteristic given the presence of other known attributes. This can be useful in some situations where there is a consistency in performance, however, can also be utilized to disprove these kinds of simple relationships such as the geyser spouting at Yellow Stone National Park.
• Prediction (Chou, Li, Chen, & Wu, 2010;Grzymala-Busse, 2005) Prediction is often the most valuable of data mining techniques-the ability to predict the future with some certainty allows us to act and make choices with knowledge. Weather forecasting is a good example of this field.

Future developments
It is clear from current trends that there are going to be a large number of competitors in this field. The primary motivators seem to be toward integration and analysis (one size fits all) solutions such as SPSS and a secondary drive toward analytical platforms that can be tailored toward specific ends (the R programming language).

CASEarth
CASEarth is a Big Earth Data project launched recently by the CAS (Chinese Academy Science), Guo (2017) outlines the primary objective for CASEarth to establish an international center for Big Earth Data science, which will have three main aims, to develop the world's leading Big Earth Data infrastructure, a world class Big Earth Data platform and construct a decision support system. This seems to suggest the project will not only address the physical issues affecting Big Earth Data, particularly the bottlenecking issues with distribution and processing of data but aims to establish support for the analysis of data and collaboration between disciplines. Overall the project aims to produce an international center for Big Earth Data collaborating a wide range of institutions and projects to produce a world-leading platform and toolset for the analysis of Big Earth Data.
One of the key aspects of the CASEarth project is its integration with government systems which will provide live feedback from a wide range of usage scenarios to inform tool design. Hopefully, this should have a significant impact on the usability of these tools as the project progresses reacting to feedback and hopefully see some of the mainstream usability features integrated into the program, significantly improving their usability and hopefully creating a blueprint for tool development for Big Earth Data visualization.
On 15 January 2019, Chinese Academy of Sciences (CAS) published the Big Earth Data sharing service platform. This platform provides systematic, multivariate, dynamic, continuous, and standardized Big Earth Data in a shared fashion, and promotes the formation of the new sharing mode of Earth Science data by building an integrated data sharing system where data, computing and service are involved. At present, the platform shares about 5 PB data, including 1.8 PB Earth Observation data, 2.6 PB biological and ecological data, 0.4 PB atmospheric and ocean data, and 0.2 PB fundamental geographical data. With the improvement of the hardware infrastructure, the platform will share more data successively, and update the data in a speed of 3 PB per year.
The most important components of the Big Earth Data sharing service platform include two core systems, i.e. data sharing and storage system and CASEarth Databank system, and one regional system, i.e. digital silk road Big Earth Data system. The data sharing and storage system is the window dedicated for resource publishing and sharing service of Big Earth Data. CASEarth Databank system offers instantly accessed long-temporal multiresource Earth Observation data to users all over the world. The digital silk road Big Earth Data system mainly involves a number of thematic data sets for regional resource, environment, climate, disaster, and heritage of the Belt and Road initiative.

Conclusions
Having analyzed the present and future technologies of Big Earth Data visualization analysis, it is clear that the volume, variety, and veracity of Earth data being collected about the world we live in is growing rapidly and we are only just beginning to scratch the surface of what we can do with this data. Significant challenges remain in storing and processing this data however head roads are beginning to be made into these issues. However, the visualization analytics tools are still technical and complex systems which have yet to take advantage of the advance in human computer interaction (HCI) over the past 20 years and with interaction technologies on the horizon. We need to urgently review these technologies to bring them up to date with other advances in the field, growth in this field shows no sign of slowing down and we must grow our visualization tools if we hope to keep up.

Disclosure statement
No potential conflict of interest was reported by the authors.

Data availability statement
Data sharing is not applicable to this article as no new data were created or analysed in this study.