Abstract: Geovisualization involves the depiction of spatial data in an attempt to facilitate the interpretation of observational and simulated datasets through which Earth's surface and solid Earth processes may be understood. Numerous techniques can be applied to imagery, digital elevation models, and other geographic information system data layers to explore for patterns and depict landscape characteristics. Given the rapid proliferation of remotely sensed data and high-resolution digital elevation models, the focus is on the visualization of satellite imagery and terrain morphology, where manual human interpretation plays a fundamental role in the study of geomorphic processes and the mapping of landforms. A treatment of some techniques is provided that can be used to enhance satellite imagery and the visualization of the topography to improve landform identification as part of geomorphological mapping. Visual interaction with spatial data is an important part of exploring and understanding geomorphological datasets, and a variety of methods exist ranging across simple overlay, panning and zooming, 2.5D, 3D, and temporal analyses. Specific visualization outputs are also covered that focus on static and interactive methods of dissemination. Geomorphological mapping legends and the cartographic principles for map design are discussed, followed by details of dynamic web-based mapping systems that allow for greater immersive use by end users and the effective dissemination of data.
Summary (7 min read)
- Geoscientists aim to develop an understanding of the processes that create landforms, and in order to study landforms must be able to visually perceive them.
- Geovisualisation relates to topics covered in other chapters in this volume particularly remote sensing (XR: 3.1), digital terrain modelling (XR: 3.9), geomorphometry (XR: 3.10) and spatial analysis/geocomputation (XR: 3.14).
- Section 3 explicitly deals with Visual Processing and the optimisation of imagery for landform visualisation.
- Section 4 introduces the mechanics of various methods by which a user can interact with data once they have been prepared for viewing.
- Finally, the scope of the chapter is stated.
2.1 Historical Context
- The visual interpretation of data and conceptual ideas has been a key aspect of human understanding (e.g. Kraak and Ormeling, 2010).
- In particular the emergence of the GIS (Geographic Information System) through the wider field of geomatics has provided a digital tool through which disparate datasets can be stored, manipulated, analysed and communicated.
- Such large datasets are most often generated by remote sensing of various kinds; Smith and Pain (2009) review currently available datasets.
- In terms of impact upon geomorphology, the generation of digital elevation models (DEMs) (see XR: 3.9) has arguably had the largest effect.
2.2 Geomorphology and Geovisualisation
- Geovisualisation is much utilized in geomorphology for the exploration and analysis of spatiotemproal data.
- Whilst a spatial framework is not a requirement for geomorphological study (e.g. Smith and McClung, 1997), it is natural in many studies to use “space” as the organising paradigm (e.g. Benetti et al, 2010).
- The definition proposes that the term “visualisation” include four primary functions : i) ‘exploration’ of datasets e.g. in order to find landforms ii) ‘analysis’ e.g. of patterns and relationships between the landforms iii) ‘synthesis’ e.g. generating an overview and understanding of the origin of the landforms and iv) ‘presentation’ of the findings.
- Indeed, it could be argued that in the past cartography was geovisualisation; that is, cartography embodied the sum of geovisual techniques that were possible before practical, pervasive desktop computing.
- The meaning of the term geovisualisation is therefore debated, but in its widest, intuitive, sense it is ‘the visual depiction of spatial data’.
2.3 Applications and Emergent Technologies
- Some applications of geovisualisation are depicted in Figure 4, with the axes of the cube illustrating the task being performed, the user doing the task, and the degree of interaction with the data being visualised.
- Within this, MacEachren et al’s (2004) four functions (Section 2.2) move sequentially from highly interactive exploration of the data by specialists in order to generate basic observational knowledge, to presenting synthesised information to the public involving little interaction with the data.
- Two trends not well represented by the four functions proposed within the geovisualisation cube are the growth in both public interaction with data and data distribution to the widest possible audience.
- The UK government has released significant quantities of data (http://data.gov.uk), and UK research councils also require scientists to place their data in repositories.
- GoogleEarth performs similar tasks in an accessible way, encouraging map-making by the public at large.
3 Visual Processing
- Geomorphologists are interested in the geovisualisation of spatial data in order to understand the Earth’s surface.
- Automated and semi-automated mapping techniques use a variety of algorithmic approaches to identify landforms (e.g. Hillier & Watts, 2004; Van Asselen & Seijmonsbergen, 2006; Seijmonsbergen et al, 2011).
- On land, passive airborne systems, such as aeromagnetics (detecting subsurface magnetic features), and active systems, such as airborne electromagnetics (3D conductivity), provide subsurface data (Smith and Pain, 2009).
- The use of automated and semi-automated techniques that integrate a variety of datasets is therefore expected to become increasingly prominent.
3.1 Detection of Landforms
- Manual mapping requires the visual detection of individual landforms, then recording their morphology on to some kind of basemap.
- This is dependent upon the data source, image pre-processing and the skill of the interpreter.
- Angle along the length of elongated landforms (e.g. drumlins), also known as 2. Azimuth Biasing.
- The minimum resolvable planform size of detectable landforms can only be reduced by increasing the spatial resolution of data (imagery or DEM) being used (Smith et al, 2006).
- Bias is minimised through the acquisition of imagery with a high illumination elevation (i.e. closer to overhead), however this minimises the landform signal strength so that features are not clearly observable .
3.2 Enhancement of Satellite Imagery
- With the acquisition of appropriate satellite imagery, it is necessary to process or enhance the imagery to provide the best possible visualisation of landforms.
- Enhancement is largely based upon a sub-set of the standard image processing techniques, namely those applied in remote sensing (e.g. Lillesand et al, 2008; Mather, 2004) that have proven useful for geomorphological visualisation.
- Standardised contrast enhancements such as a linear stretch, histogram stretch or standard deviation stretch should be utilised in order to maximise the visible contrast within the image.
- Inter-drumlin regions in his study contained greater moisture availability which impacted upon land cover, resulting in lower reflectance in VNIR bands, and thereby better differentiating drumlins.
- Whilst DEMs record elevation rather than reflectance, using the terminology of imagery they may be thought of as a single ‘band’ and therefore any of the techniques described above can be applied to them.
3.3 Enhancement of DEMs
- DEMs directly record elevation and therefore landscape shape can be inferred from them.
- DEMs are treated differently from satellite imagery, although some aspects of processing are common.
- By default most image-processing software display DEMs as a greyscale image with a palette of shades representing height that linearly varies between the maximum and minimum heights in the dataset; a simple ‘panchromatic’ display .
- In such images small, low amplitude, features are not easily visible against larger-scale higher amplitude features typical in landscapes.
- It is therefore necessary to focus on the component of the landscape containing features of interest, enhancing the ‘landform signal strength’.
3.3.1 Regional-Residual Separation
- ‘Regional-Residual Separation’ (RRS) is the act of isolating landforms, ideally completely and uniquely, in to one component so that they may be studied independently.
- The skill in performing a successful regional-residual separation is always determining a property that makes the features you wish to isolate distinctively different.
- Widely ranging shapes and sizes within a class of feature (e.g. seamounts, drumlins).
3.3.2 Land Surface Parameters
- In order to facilitate mapping, parameters derived from elevation as represented in a DEM are commonly calculated.
- Figure 6 illustrates the problem with a DEM illuminated parallel and orthogonal to the principle landform orientation, and with an animation where the illumination azimuth is rotated through 360º at 5º intervals; for the latter, landforms both appear/disappear and change shape.
- It is therefore common to utilise relief shaded images generated from multiple azimuths.
- Roughness, the variability of elevation of a topographic surface at a given scale, has also been used extensively as an LSP for landform characterisation .
- The parameter is sensitive to changes in local relief, where multi-scalar expressions of openness (i.e. distance) will have an impact.
3.4 Recommendations for Terrain Visualisation
- A range of techniques designed to best enhance satellite imagery and DEMs for landform detection have been outlined in the previous sections.
- Simple methods can be effective in highlighting individual landforms of interest, relegating all other aspects of terrain to “noise” (e.g. Hillier & Smith, 2008).
- The technique of relief shading highlights subtle topographic features well, is widely implemented in software, fast to compute, and very useful when appropriate care is taken to allow for azimuthal bias.
- This does not correct the problem, however, only making the interpreter aware of it.
- Openness and roughness also offer illumination-free visualisation methodologies that may be appropriate for certain applications.
4 Visual Interaction
- Visual Processing, as introduced in the previous section, presented a selection of techniques for the static visualisation of terrain.
- That is, the outputs are fixed 2D entities, however it is often necessary to combine and interact dynamically with data in order to explore it more in more detail.
- This section introduces common techniques to dynamically interact with spatial data and then explores increasingly sophisticated methods for interacting with geomorphological data as 2D planes and 2.5D surfaces, before outlining how surface data can complement true 3D volumetric data.
- Raster outputs of terrain are usually displayed and inspected upon a video display unit (VDU) using the red, green and blue (RGB) additive model of mixing colours .
- Given the sensitivity of the human eye limits colour perception to RGB, the colour cube allows the mixing of the three primary colours at different intensities to provide the full gamut of possible viewable colours.
- The technique is widely used in remote sensing and, within geovisualisation, allows the interactive inspection of multiple terrain datasets.
- Figure 13 presents an example of how a false colour composite can be used to interpret geomorphology; this combines topographic openness (green; 501x501 m kernel) and slope angle (red; 3x3 m kernel).
- A watershed (top left/light blue) is clearly depicted with drainage channels running away from it (red); below this are moraine ridges (centre left/light purple), gypsum sink holes (centre/red) beneath till and mass movement (centre/light red/yellow).
4.2 Digitisation and Overlay
- The main output of visual processing is a raster dataset, or datasets, created using the techniques outlined above.
- The process is generically known as “digitisation” and can form a body of work for the production of geomorphological maps (e.g. Hughes et al, 2010) or inputs for further quantitative processing (e.g. Smith and Rose, 2009).
- Layers can be stacked (e.g. overlain) and reordered, allowing the interpreter to interact with different data layers and visually inspect their interaction.
- The ability to manage large datasets and interact with them, allowing detailed inspection is an important, although simple, feature of a digital workflow.
- Over larger areas lines and points can also be used to represent some area features.
4.3 2D to 21/2D In Space
- A simple way to interact with data is through activities such as zooming, panning, rotating, and performing simple contrast enhancements.
- Google Earth, for example, provides an elegant interface for the navigation of a single set of imagery for anywhere on Earth; this is a remarkable achievement.
- Whilst somewhat restricted, these “global” measures can be used to explore features of individual datasets.
- Animations provide a powerful methodology for data visualisation by depicting attribute change to vector or raster data.
- For DEMs, timestages in a modelled landscape evolution could be similarly displayed.
4.4 3D in Space
- It is natural to move from 2.5D to full 3D visual interaction with data, either in terms of estimating volumes, or being displayed in conjunction with true 3D volumetric data (e.g. seismic reflection data).
- As noted above, DEMs do not provide volumetric data, however where it is possible to define a lower boundary through an understanding of a geomorphological basal surface (e.g. Sclater et al, 1975; White, 1993; Wessel, 1998; Hillier & Watts, 2004; Smith et al, 2009), volume can be estimated.
- Airborne radiometric, magnetic and electromagnetic measurements of the sub-surface, for instance, could be integrated.
- 3D visualisation packages integrating data such as outcrop sedimentary logs, strikeand-dip measurements, and horizon interpretations with photography draped over a LiDAR DEM have been developed for sub-aerial work (e.g. Fabuel Perez et al, 2010).
- Through the use of polarised glasses, the human visual system assembles the display in to a 3D image.
4.5 Virtual Globes
- Virtual globes, or Earth browsers, and online maps have become a central part of the internet since the first release of NASA’s World Wind (http://worldwind.arc.nasa.gov).
- The geosciences community has embraced the use of virtual globes with emerging applications in many fields: from simple terrain inspection and feature mapping (Sato & Harp, 2009; Welsh & Davies, in press) to data visualisation and facilitated geo-data exchange (http://www.usgs.gov).
- The National Snow and Ice Data Centre offers information about ICEsat data, NASA’s Ice, Cloud and Land Elevation satellite (http://nsidc.org/data/virtual_globes/glas/anchorage.kml).
- This may involve graphical outputs for peer-reviewed research papers or direct public consumption.
- This section summarises static and interactive visualisation products used in geomorphology and introduces some examples of web mapping and WebGIS.
5.1.1 Legend Systems
- Traditional geomorphological maps differ from other thematic maps in that qualitative information prevails over quantitative or classified data.
- Most geomorphological maps are compiled using descriptive symbols to represent landforms and processes.
- Many different symbol sets and mapping systems have evolved during the 20th century in different countries with different thematic emphases and varying usage of symbols and colour .
- Even though attempts were made to create a general legend for geomorphological maps by the International Geographical Union (IGU) in the 1960s (Demek et al., 1972), no universally applicable legend system has been established.
- While the former style results in multi-coloured maps with several stacked layers of data that tend to be overloaded with information and may be hard to interpret (e.g. the German system; Barsch & Liedtke, 1980), the latter usually come in black and white and represent a reduction of information for practical purposes (e.g. the British system; Evans, 1990).
5.1.2 Map Design
- Geomorphological maps are complex thematic maps that place demands on cartographic visualisation techniques in order to provide a comprehensible and readable map.
- Rock glaciers for example could be represented by a single point symbol on small scale maps, or by the assemblage of line and area symbols that differentiate the step height of the rock glacier front, furrows and ridges and the accumulation of boulders and blocks on top of the rock glacier, if the map scale increases.
- If colour is used, variation of colour characteristics, hue (colour variation), value and chroma , are the most powerful tools to emphasise certain aspects of the map.
- In many geomorphological legend systems colours are applied to represent variations in landform genesis (Barsch & Liedtke, 1980), process domains (Gustavsson et al., 2006), or lithology (Pasuto et al., 1999).
- In order to engage map-users and enable them to develop an understanding of the meaning of the map, a visual sense of the symbols and their attributes that correspond to the intention of the cartographer is required (Robinson et al., 1995).
5.2 Digital Mapping
- Digital map creation is performed either using vector graphics software (e.g. Adobe Illustrator) or GIS software.
- The results of GIS analyses are often compiled into maps and consequently GIS software includes mapping facilities and some graphic design capabilities.
- Special symbol editors are provided to compose and define the symbol set for the map (e.g. in ArcGIS).
- Generating geomorphological maps using a GIS enables numerous possibilities for the dissemination of research outputs extending beyond simple paper products.
- These techniques are outlined in the following sections.
5.2.1 Open Standards
- Data distribution and access in distributed web-based geospatial infrastructures need to be specified to achieve interoperability in a way that different applications (e.g. geodatabases, mapservers or clients) on various platforms (e.g. Linux, Microsoft Windows) can interact and communicate with each other.
- The specific needs for interoperable geospatial technologies are implemented in specifications or standards describing the basic data models to represent different geographical features.
- The standards or specifications are the main outcomes of the OGC and appear as technical documents that detail interfaces or encodings.
- There are currently more than 30 standards defined, the most prominent of which are web services, also known as OpenGIS Web Services (OWS), specifically: i) Web Map Service (WMS) - providing map images, ii) Web Feature Service (WFS) - to retrieve feature descriptions and, iii) Web Coverage Service (WCS) - preparing coverage objects from a requested region.
- XML is an acronym for Extensible Markup Language which is a set of rules for document encoding, comparable to HTML (Hypertext Markup language).
- A GeoPDF, an OGC standard, includes one or multiple map frames within a Portable Document Format (PDF) page associated with a coordinate reference system (Graves & Carl, 2009).
- It enables the sharing of geospatially referenced maps and data in PDF documents.
- Multiple, independent map frames with individual spatial reference systems are possible within a GeoPDF for example for map overlays or insets.
- A GeoPDF enables fundamental GIS functionality outside specialised GIS documents, turning the formerly static PDF maps into interactive, portable, geo-referenced maps.
- Some geospatial data providers, such as the United States Geological Survey (USGS) or the Australian Hydrographic Service (AHS), have already started publishing interactive maps using the GeoPDF format (http://store.usgs.gov).
5.2.3 Principles of Web mapping and WebGIS
- Web mapping is a common way of presenting dynamic maps online.
- The application employs MapServer generating the maps as WMS, the spatial database management system PostgreSQL (http://www.postgresql.org) maintaining the geometries and the web mapping client Mapbender (http://www.mapbender.org).
- Due to MapServer’s powerful cartographic engine, complex geomorphological symbols can be implemented and displayed.
- The WebGIS map thus uses the same symbols as the printed map of the same area (Otto & Dikau, 2004).
- Thus, WebGIS applications are powerful tools to disseminate geospatial information to users from different organisations (e.g. local authorities, environmental agencies).
- Geovisualisation has garnered considerable interest as a term covering a wide swathe of activities ranging from exploration, through to analysis, synthesis and presentation.
- An intuitive, useful definition is 'the visual depiction of spatial data', and this chapter has focused upon the use of geovisualisation in geomorphology under this definition.
- Automated and semi-automated techniques also deliver landform parameters, but are beyond the scope of this chapter.
- This can then be used for the generation of output that is appropriate for the intended end-user, be that a printed map or web-mapping system.
- Dynamic visualisation techniques, and particularly areas such as temporal and 3D analysis will gradually improve, along with products designed to engage end users such as augmented reality.
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Cites background from "3.11 Geovisualization"
...Geovisualization and mapping of bathymetry is an evolving field (Smith et al. 2013; Alves et al. 2014; Resch et al. 2014)....
...…in the literature include Alves et al. (2014) who found that bathymetric features have a profound effect on oil spillmovement in theAegean Sea; Smith et al. (2013) who highlight the use of visual processing, visual interaction, and visual outputs, all of which form part of the…...
...Recent examples of the use of geovisualization and/or bathymetry in the literature include Alves et al. (2014) who found that bathymetric features have a profound effect on oil spillmovement in theAegean Sea; Smith et al. (2013) who highlight the use of visual processing, visual interaction, and visual outputs, all of which form part of the geovisualization of terrain; and Resch et al. (2014) who examined the display of bathymetry as a three-dimensional (3D) time series for geovisualization purposes....
Cites methods from "3.11 Geovisualization"
...…to support this approach through the use of aspects of the process of visualization that include the ‘exploration’ of data (MacEachren et al. 2004, Smith et al. 2013); the aims of this being to (i) support students throughout their course, (ii) introduce the key statistical concept of…...
...These aspects of visualization have been described in a framework consisting of four functions; from ‘exploration’ and ‘analysis’ of data to the ‘synthesis’ and ‘presentation’ of information (MacEachren et al. 2004, Smith et al. 2013)....
...However, the authors of this paper chose to integrate visualization through using mapping technology (MacEachren et al. 2004, Smith et al. 2013), because mapping technology is seen as being increasingly important for employers and social science research in a range of areas, such as poverty,…...
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...When representing landscape more generally, then the subject reduces to one of communication or more specifically geovisualisation (Smith et al., 2013)....
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