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° Map accuracy standards are less stringent
than in primary mapping.
Secondary mapping implies that a
primary digital base map already exists
which can provide a dense network of
reference features for the restitution of
images.
* Resolution of digital images used in most
secondary mapping applications need not
be as high as for primary mapping.
The requirements which a secondary soft
copy mapping facility must fulfill can be
summarized as follows:
* All operational procedures must be easily
understood and able to be followed by
users with limited photogrammetric or
cartographic skills .
* Image restitution must be based on
sufficiently rigorous mathematical
models to assure that the map accuracy
standards and specification are met.
* Means must be provided for the easy
integration of the mapping output with
existing data bases.
The pseudo-stereo mapping technique is
designed to satisfy these requirements.
2. PSEUDO-STEREO IMAGE
RESTITUTION
2.1 The Technique
Monoscopic measurement and digitization in
images displayed on the screen of a
cartographic workstation form the basis of
this mapping technique. A significant
element of this scheme is its implementation
in a geographic information system (GIS).
Thus, information stored in the GIS can serve
as ancillary data to support the mapping
operation while the newly acquired
information can readily be merged with
existing files.
Image monoplotting is the simplest mapping
scheme which can be offered to non
photogrammetrists. In the past, single image
photogrammetry has been used routinely for
transferring details from photographs onto
maps in instruments built on the camera
lucida principle. The residual effects of tilt,
relief displacement and other geometric
anomalies inherent in photographs are,
139
however, limiting factors in the application of
these devices. Possible means to alleviate
these problems in soft copy photogrammetry
are to perform the digitization in a digital
orthoimage or to correct the relief
displacement at the digitized points with the
help of a digital elevation model (DEM).
Pseudo-stereo mapping is design for the case
when orthoimages or DEMs are not
available, and, although the digital images
provide stereoscopic coverage, the
workstation is not equipped for stereo
viewing.
2.2 The Procedure
The pseudo-stereo mapping technique has
been implemented in a GIS [Chen, 1993]
which is capable of handling digital images
in raster form as well as digital maps in
vector form [Derenyi, 1991]. The main steps
are as follows (Figure 1):
l. The exterior orientation parameters of
both photographs of a stereo pair are
determined by space resection. Image
coordinates of the ground control points
(GCPs), needed for this purpose, can be
measured monoscopically on the display
screen. When a digital map of the area is
available in the GIS, the GCPs can be
picked in a double window display of the
image and the map. An alternate step is
to acquire the orientation parameters from
some'' other "sources such“ as
aerotriangulation.
2. The planimetric features to be mapped
are digitized in Photo 1 on the screen.
This is achieved by free hand cursor
control using a mouse. Zooming,
planning and real time image
enhancement facilities assist in this
operations.
3. Corresponding image points are located
in Photo 2 by image matching. This
operation is performed in batch mode
after a group of points has been digitized.
4. The object space coordinates of the
digitized points are computed by space
intersection and stored in a new map file
or added to an existing file. This step is
also a batch operation.
2.3 Image Matching
The key issue in pseudo-stereo mapping is to
locate the corresponding point in the second
