8. Data Visualisation
The results of the DTM and GIS modelling operations discussed in the previous chapter need to be communicated to the users. This is normally done in a graphical format. Visualisation plays an important role within Digital Terrain Modelling that is "closely linked to interpretation: results of interpretation steps need to be displayed, and interpretation operations may in turn lead to improvements in visualisation. Moreover, graphics themselves may directly support decision making (through visual interpretation) without involving any quantitative analysis" (Weibel & Heller 1991). The value of the visualisation depends on their ability to effectively communicate the modelling results and support interpretation.
8.1 Orthographic Display Techniques
Contour lines are the most commonly used method for displaying and analysing relief. Unfortunately they do not present the user with an immediate impression of the landscape topographic form, which can be represented by analytical hillshading. This allows the user to readily distinguish the landform, however no information is provided about the absolute terrain elevations. The most common method used to compute this display technique is based on a model of illumination. Individual light intensities are computed for each DTM facet and shaded accordingly. By portraying the results using grey scales and continuous tone techniques the results resemble aerial photographs. To aid human perception a light source of 45 degrees above the horizon in the north-west is normally used (Burrough 1986).
8.2 Perspective Display Techniques
"We are now at a point in history of cartographic design and production where it can supersede all that has gone before, so why is it not doing so? When at last it has been released from the constraints of the past, it seems afraid to let go the apron strings of previous generations, slavishly reproducing computer imitations of styles developed some 50 years ago. Now is the time for maps to come alive, no need for symbology, the elements of the map can now be what they mean" (Collinson 1997). Many map users have a very limited knowledge of interpretation and the new tool available to those wishing to visualise geographic data are developing rapidly. Traditional cartographic images, such as contours make little attempt to give the landscape any perspective. Collinson (1997) argues that "as a means of portraying relief in a meaningful way, contours fail because they do not match out human perception of landscape. They are symbols of the thing, not the thing itself. People should not have to be taught to read maps. Maps should be self-explanatory." He continues that we are not making full use of the new technologies on offer and that the maps on offer do not represent the 'whole' picture.
To aid the end viewer most proprietary GIS packages now offer the ability to produce three-dimensional images of the landscape. Whilst orthographic displays offer the advantage that the entire surface is visible and relatively undistorted, whilst perspective displays provide more convincing visualisations, and again the display may be extended to include cartographic features. These views can be generated from any viewpoint, either within or outside the landscape. Additional realism can be added by the inclusion of the sky, fog or haze. With the growing usage of GIS this display method is expanding as users increasingly exploit the technology, although their creation is still expensive in computational terms. The growth of the Internet has also created new tools that GIS operators can make use of.
8.3 Advanced Visualisation Techniques
"Putting the real world inside a computer is one of the main aims of GIS" (Smith 1997) and the use of advanced visualisation techniques can help make a GIS more accessible to new users. VRML (Virtual Reality Modelling Language) is an object based language that enables the construction of objects in three dimensions with minimal user effort. McCarthy & Descartes (1998) suggest that "VRML could be used as a portable format between GIS systems". The Centre for Advanced for Spatial Analysis (1998) is undertaking an ongoing project to develop VRML worlds using ArcView and its' Avenue Macro Language. Although still experimental their results are impressive and they see this an important area in GIS development.
8.4 The Presentation & Visualisation Methods Used
Whilst the analysis results can be displayed in either two or three dimensions the most important aspect of their presentation is to ensure an ease of understanding. While the production of three dimensional images may sound impressive there is little point in producing such displays if the results are less clear than those produced with simple two dimensional maps. Arc/Info allows the creation of perspective view and from previous experience they can significantly aid presentation and enhance the quality of the results. However, after experimentation this approach was discarded. The views created simply gave no impression of elevation when the viewshed or accessibility results were draped over the surface lattice. Using the Gridcomposite command it is possible to added the results of a Hillshade analysis but unfortunately due to the resolution of the grid the results were very coarse. The Gridcomposite command can also be used to add the effect to orthographic displays but the same problems occurred. Undeterred a method to get around this problem was sort in order to fulfil one of the aims of this research, namely to produce results in an effective but impressive manner.
Whilst the use of VRML is a possibility my knowledge of it is limited. With the growth of free software available over the Internet a search was undertaken for visualisation software. LandSerf is available free to download from the University of Leicester. It is an experimental visualisation tool designed to operate on various raster data formats. Whilst the images presented on the associated instruction pages look very impressive after experiment the display of my results suffered the same coarse image problem as their display in Arc/Info. Therefore a decision was made to display the results in a simple orthographic format. An advantage of using ArcView is its simplicity in producing maps.
However, just because a map can easily be it does not automatically mean that it will serve the purpose it was designed to do. It is very easy to add cartographic features simply because you can. The maps produced to display the analysis results are designed to be informative without displaying purposeless information. Hence on the visibility result maps' elevation is not supplied, simply implied using colour scales. Likewise only the main roads and rivers are illustrated to give a simple guide to geographic location.Last Updated 23-03-1999