Supplementary processing is needed to upgrade and/or condition raw
DTM data. Common operations are interpolation, compression, resamp-
ling, aggregation and/or data segmenting. If raw data are arranged
in epipolar profiles (e.g., in cylindric coordinates), they need be
converted into a regular grid in a uniform terrestrial coordinate
system, which implies resampling. Before being entered into a data
base, the DTM data can be compressed (7).
4. Graphic display
Displays permit data verification and editing, and sometimes manual
support during operation. Images, parallax data, DTM data and qua-
lity indicators can be displayed. Image display can be mono or
stereo. Different techniques can be used for stereo, e.g., anagly-
phic, by polarised light, and others. For visual data verification
and interactive editing, parallax data can be superimposed on the
corresponding image pair.
Quality indicators, such as similarity estimates, error distribu-
tions, etc., can be displayed separately, in combinations, or super-
imposed on corresponding images.
5. Quality control
Quality control is an essential part of the process. It requires,
however, definition of suitable assessment criteria (or quality
indicators) and adequate reference (or check) data. The criteria
should be defined with utmost care; standard errors, error distribu-
tions (spatial and/or statistical), and corresponding thresholds are
commonly used. Another important criterion is the fidelity of a
surface (or profile) reconstructed from DTM data (8), and the cor-
responding terrain resolution (i.e., limiting spatial frequency).
Suitable references for quality assessment are geometric conditions
(for geometrically regular terrain objects), check points, check
profiles, a-priori known morphometric features, and some neighbour-
hood characteristics (e.g., local terrain roughness).
Trends in post-processing
The trends can be summarised as follows: Accuracy and reliability of
DTM data increase by integrating external information and by proces-
sing the data collectively. Interactive editing by means of a gra-
phic display terminal tends to be a requirement. Supplementary ope-
rations (for DTM) are being extended and refined, i.e, to attain
higher fidelity of DTM, and quality control is being improved.
VI. CONCLUSION
Recent advances in micro-electronics have a strong impact on automa-
tion in photogrammetry. In digital off-line techniques for automatic
DTM production, emphasis is being displaced from smaller towards
larger photo-scales. The process stages and corresponding operations
are beoming increasingly sophisticated and thus capable of handling
more difficult situations than before. Effective multi-stage image
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