The visualization of the data was performed by
generating a sequence of perspective views. Figures
4 and 5 show perspective views of the "draped"
surface data, including a scene with various terrain
and building structures and another one of less
complexity. Note, that the side of the man-made
structures are only shaded but not painted.
Figure 5. Perspective view of a less complex area
Silver Spring project
The DTM extraction for the Silver Spring site
differed from the Manhattan area in a number of
ways. The scale of the photography was larger, but
more importantly, the terrain itself included
wooded, suburban areas with single family housing
units. Figure 6 shows the project area.
In dividing the project area into subsites, we had to
carefully consider the differences between the
characteristics of the suburban and downtown
sections and decide which special strategies to apply
(Helava, 1988). The critical issue in suburban type
areas was to come out with a DTM strategy that
would give "true" ground elevations in open areas
while simultaneously yielding house rooftop
elevations at the same time. Such a solution would
immediately provide deliverables, i.e., the combined
surface of natural and man-made objects. After
performing several tests on typical areas, we
selected the optimal automated DTM strategy for
our objective, with the understanding that in wooded
areas we would not obtain "true" ground elevations
but elevations of treetops and bushes instead. Only
prominent building structures in these areas would
have their rooftop elevation digitized using feature
extraction techniques.
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
F igure 6. Silver Spring project area
The downtown DTM area was generated in the same
fashion as in the Manhattan project, where "true"
ground elevations, without buildings, trees, and
other obstructions were determined using multiple
DTM editing tools. An initial 4m grid spacing was
used to generate DTMs for the two downtown sites.
Figure 7 shows a perspective view of the data
including a single family housing area. The front of
the image shows clearly the performance of the used
DTM strategy. The rooftops rise out from the rolling
surface of trees and bushes while the open areas
such as the road in the center follow the terrain.
gsm
Figure 7. Silver Spring, suburban area
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