previous point can be used as first approximation. Then increment dZ is
applied in "down" and/or "up" sequence.
Geometric criticality occurs when =4-90°, i.e., the ray is nearly tan-
gential to the DTM surface. The accuracy and time efficiency depend on
the height range and roughness of the terrain, and the increment size dZ.
Fig. 7: ITC iterative (feedback) process
- The "Digital Mono Plotting System" (ITC, [6]) uses a feedback iterative
process (figure 7). For an initial approximate Zl-level the correspond-
ing Xl, Yl are caluclated (by means of the collinearity model).
Subsequently the height Z2 is determined by interpolation from the DTM
data. If A4Z1-121-Z2!1 is small enough («ThZ), the process can be
terminated. Otherwise X2, Y2 are calucalated for the Z2-level, and sub-
sequently Z3 is determined by interpolation. If 422 - 122-231! « ThZ, the
process can be terminated, Otherwise X3, Y3 are calculated for the Z3-
level, and subsequently
4 D2?- (X2 - x3)^« (Y2 - Y3)?. If 4D2 is small enough («ThD), the
process can be terminated. Otherwise Z4 is determined by interpolation,
etc. The process is continued until AZi or ADi is small enough. Hence,
the successive Zi values are self-adapted.
Geometric critical situations occur when «= 90- (non or slowly
converging) and especially when «> 90 -/4 (diverging process). In such
situations each (next) iteration cycle should use the mean value
Zi=(Zi+Z(i-1):2 instead of Zi as input, which permits fast convergence.
7. Extensions and refinements
The AMP system capabilities can be extend and/or improved by providing
the local control data from existing data bases (for differential
mapping), by a virtual overlay of the DTM grid on image, and by inclusion
of differential heights in the "image-to-DTM surface" transformation.
- For differential mapping, the contol, tie and check points, and local
DTM data can usually be extracted from the existing data base (e.g.,
topographic maps). Additional software is needed to condition such data
for the control of AMP.
- Virtual overlay of the DTM grid on the image is attained by digital
transformation (collinearity model). Hence, for any image point, the
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