Analysis of grids and surfaces - Spatial Data Analysis: An Introduction for GIS Users - page 164

Figure 10.9 Viewing direction.

Figure 10.10 Top: invisible cell; bottom: visible cell (both shaded grey).

In this case, if the obstacle is higher than 5.986 units then the target will not be visible
to the viewer.
When the height of the obstacle ( h o ) is known, the length of the possible line of sight
that is blocked by the obstacle can be computed using (Li et al. , 2004):

h

d =

o

¥ d (10.6)

to vo

hh +

v o

h is approach could be applied to all cells in a raster image, but more ei cient algo-
rithms exist. Figure 10.11 shows a DEM with a viewpoint superimposed (A) and the
viewshed from this location (B).
Note that the viewer height must be specii ed and not simply the elevation or build-
ing height at the viewing location. Of course, visibility can only be meaningfully