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Optical Methods for Assessing Fade Margins 10-3
Elevation (deg)
90
75
60
45
30
15
0
0 45 90 135 180 225 270 315 360
Azimuth (deg)
Figure 10-2: Skyline derived from image shown in Figure 10-1.
10.3 Skyline Statistics of Austin and San Antonio, Texas
Using the methodology described in Section 10-2, databases for four locations were
derived by Akturan and Vogel [1995a]. The locations consisted of a rural and suburban
area in Austin, Texas and central business districts in Austin and San Antonio, Texas.
For the rural case, 39 images were acquired along a sparsely tree-lined road at a spacing
of approximately one image per mile. In the suburb, 86 images were taken in a
neighborhood with single-family houses and relatively small trees. In the central
business district (CBD) of Austin and San Antonio, 106 and 96 images were obtained,
respectively. The mean, standard deviation, and the 90 th percentile (percentage of angles
below) elevation angles were averaged in each environment and are summarized in Table
10-1. A small difference exists between the results of the rural and suburban Austin
cases. Neither the suburban nor the rural environments had significant tree shadowing.
The suburb area was comprised of one- and two-story houses. The 90 th percentile of the
skyline elevation angle follows a normal distribution, except in the suburb. San Antonio
and Austin have similar skyline statistics for the central business district.
Table 10-1: Average skyline statistics for four locations derived by optical methods.
Location Mean Elevation (°) Stand. Dev. (°) 90% (Angles Below)
Rural Austin 5.3 3.7 10.9
Suburban Austin 7.4 5.8 16.6
Austin CBD 25.8 20.2 55.2
San Antonio CBD 27.7 18.1 53.5
As an example, the skyline histogram for the central business district of San Antonio is
given in Figure 10-3. This histogram follows with good approximation the log normal
fit,
( ln( θ / c)
μ )
⎡ 2
1 − − ⎤
f ( θ ) =
exp⎢
2 ⎥ , (10-1)
( θ / c)
σ 2π
⎢⎣
2σ
⎥⎦