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indicate in a GIS the elevation of a graphic element an
elevation attribute may be attached. Other features of
the terrain such as hydrography, land use, cultural, etc.,
are traditionally represented in 2-D. f necessary the
third dimension can be added to the two-dimensional
digital map files by the use of a DEM.
The photogrammetric procedure for generating DEM's is
to measure stereoscopically the elevation of terrain
points in an analytical stereoplotter. The point
distribution can be regular, quasi regular, or random, with
a certain average density. From these observations a
(regular) grid is interpolated which represents the DEM.
Normally, the average density of the derived grid is 2-3
times higher than the number of the originally observed
terrain points.
To solve the problem of incorrect or missing elevation
information ASI constructs a preliminary DEM using all
photogrammetric established points within a model in
addition of any recoverable elevation values from the
existing DTM or contour file. As a first step a dense
mesh is generated, overlaid and intersected with the two
dimensional index contours and checked against the first
DEM iteration. At that point preliminary elevations are
given to the index contours to the nearest 10 feet.
Depending on the quality of the original data a search for
intersecting index contours is expanded to 20 feet or until
a satisfactory match is achieved. Intermediate contours
are included in the next processing step where
intersections with the mesh are calculated and the
elevation value determined.
The elevation attribute is attached to the contours to
make them three dimensional. The newly generated
DEM serves as the base for the draping process of the
planimetric and topographic features necessary for the
photogrammetric revision and acquisition process. In
order to generate topological correct contours automatic
processes are used to close gaps and delete overlapping
line segments within contours lines. Any planimetric
displacement of contour lines that may have been
originated from graphic editing or cartographic
displacement will cause an additional error in the DEM
and cannot be corrected.
Existing spot elevations may be used as an indicator of
the correctness of the contours and the DEM in general.
Elevation data can be analyzed to determine whether or
not these data are accurate enough for a particular
application (Zhing Lee,1988). To further enhance the
accuracy of the DEM breaklines must be introduced.
Coordinate values of selected planimetric features, e.g.,
hydrographic features, road edges, etc., are compared
with the DTM file. If a match can be achieved, the
planimetric features will be reclassified as breaklines and
the correct elevation value from the DTM as an attribute
attached. It is understandable that not all planimetric
features will match or can be classified as breaklines.
761
5 Mapping and Map Revision
In order to detect the changes and make revisions it is
necessary to view the new aerial photography and the
existing data stereoscopically at the same time. Only
when reviewed stereoscopically changes can be
identified and the map revision performed.
Map revision operations of existing vector or point data
from aerial photographs require several steps:
interpretation, change detection, collection of new data,
and integration of old and new data in the database.
One of the effective methods for extraction of new data
is by superimposition of the existing digital data over
recent acquired aerial photography and update the
database by collecting the new information from the
stereo model (Regan & Amenakis, 1994). The
superimposition and the data collection can be performed
in several ways (e.g., Welch, 1989, Bouloucos et al.,
1992). Map revision or modifications can pertain to a
change of the thematic description, to a change of the z-
coordinate (elevation) or to a change of position (x, y).
After the topographic and draped planimetric features are
downloaded to the Zeiss P-33 equipped with
superimposition, the photogrammetric technician visually
scans the model for any feature changes or additions by
comparing the superimposed old map to the features in
the stereo model. Terrain changes are identified and
“fenced” to ensure seamlessness between the old and
new contour and DTM data set. Inside the fenced area
all existing vectors and points are deleted and replaced
by points collected through photogrammetric compilation
methods. Furthermore, in tagging all updated features
we are able to identify the areas of changes. Contours
within the updated area are generated from newly
collected masspoints and breaklines using the Triangular
Irregular Network (TIN) concept. Extensive data
processing is used to edit the entire map. This is the
stage where automatic editing and enhancement of the
data use APS software to develop topology and the
continuity of the data. Plots in different colors may be
generated and are used in the following quality control
check.
6 Final QC Process
Translation to the target system based on specifications
and database design completes the updating process.
Virtually all of our processing procedures were developed
for quality control, topological structuring and aesthetic
enhancement of cartographic data. To ensure that all
data was correctly translated, the files are interactively
examined on a graphics workstation by a GIS specialist.
Special attention is given to the continuity of the existing
and new contours through edge match techniques. Edge
matching is the term used to describe the process of
making two adjacent digital files graphically and
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
