The visibility of points in a landscape from one or more locations has many applications. These include studies of scenic quality, sound reduction, urban design, civil and military observation needs and telecommunications planning, amongst others. Many GIS packages offer visibility analysis, with varying degrees of functionality. Two principal functions are provided: line of sight, which is essentially a point-to-point operation; and viewshed, which is typically a point or point set to surface operation. Packages such as ArcGIS with Spatial or 3DAnalyst and MapInfo with Vertical Profiler provide a broad set of facilities, whilst programs such as TNTMips, Idrisi and Manifold provide a subset of these functions. Typically these operations are performed on grid files, but a similar, vector-based procedure known as Isovist analysis has been developed by Rana (2004b). This includes an add-on “Isovist Analyst” for ArcView (see further, Section 6.3.3, Isovist analysis and space syntax).
For large distances the curvature of the Earth and optical refraction may be important, and packages often provide options to automatically adjust for these factors. For radio-wave modeling, Earth curvature tends to have an enlarged effect, which is generally modeled as 4/3 times the basic Earth curvature effect (possibly adjusted for different wavelengths). Other radio-wave modeling factors, such as Fresnel and non-linear signal decay effects (attenuation, interference, reflection, refraction) are not provided for, but may be incorporated as application-specific models using the programming facilities of the package.
An additional and in many ways more important issue is the question of offsets from the surface. Lines of sight are rarely if ever made from the surface itself — the observer’s height and/or a structure (such as a radio mast) provide the vertical position from which computations should be made (the same question may arise for target points, such as communication or observation towers). A further and very important factor requiring consideration is the impact of surface uncertainty and obstructions (such as vegetation, road embankments, signs and buildings) on such computations. Lines of sight and viewsheds are highly susceptible to such factors, principally because of the low angles involved. For example, with zero offset from a surface, visibility is equivalent to shining a powerful light beam from the observation point and scanning around in a full circle. Any small rise or obstruction, such as a 1 meter high mound of earth or a bush, especially if close to the observation point, will cast a very large shadow behind it. Scaling this problem up to include larger objects over a range of distances and variable curvature of the landscape highlights the need to view results with caution. Raising and lowering the offset height of the observation point(s), moving the observation points and inspecting the results is one way to evaluate the sensitivity of results to such factors. Another is to increment and/or decrement the surface where known obstructions are understood to exist (e.g. by adding an estimated vegetation height and/or buildings height raster to a DEM).