Geography Reference
In-Depth Information
Gradient (deg.)
V
alue
High : 15.39
Low : 0.00
Inland water
0
20
40 km
Figure 10.8
Gradient in degrees.
(routing), irradiance, watershed, cost surfaces, and visibility maps. Algorithms also
exist for processing DEMs—in particular, removal of 'anomalous' pits or peaks is ot en
a concern. h is section considers two classes of widely used DEM-based approaches:
visibility analysis and derivation of cost surfaces.
Visibility analysis is concerned with the identii cation of areas in the landscape that
are visible from a given location (or a set of locations). Many GIS of er tools for gener-
ating viewsheds—areas that are visible from a given point. Figure 10.9 indicates a
viewing cell in black and the pixel that is to be tested for visibility is grey. Figure 10.10
indicates the two possible cases: that the cell is invisible or visible. By testing the line of
sight to each cell, a map of visible cells (the viewshed) can be generated.
h e line of sight from one location to another can be assessed using:
dh
+
dh
h
=
vo
t
to
v
(10.5)
crit
dd
+
to
vo
where
h
crit
is the critical value for the height of an obstacle,
d
to
is the distance between
the target and the obstacle,
d
vo
is the distance between the viewer and the obstacle,
h
t
is
the height of the target, and
h
v
is the height of the viewer. If the height of the obstacle
is less than
h
crit
then the target is visible (Li
et al.
, 2004). As an example, for
d
to
= 24.5,
d
vo
= 12,
h
t
= 8, and
h
v
= 5:
12
¥+
8
24.5
¥
5
218.5
h
=
=
=
5.986
crit
24.5
+
12
36.5
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