At the time of the generation of the existing maps, issues
such as line snapping, edge match, over and under shoot
were not as important as they are in today's GIS
environment for building topological correct databases.
The original photogrammetric files might have undergone
up to three translations prior to their current use in a GIS.
Different data conversion programs and approaches
were used for these translations, most of them providing
a standard "one-to-one" translation. These translations
might have distorted the original data structure or
introduced new errors. In most instances errors and
mistakes made to the graphic elements were not
detected or corrected as long as the visual content of the
map was maintained. Users might have introduced
additional errors as a result of in-house editing of graphic
elements. The most common errors are the placement of
a graphic element on the wrong layer or the attachment
of the wrong attribute.
In order to satisfy these multiple task requirements ASI
has developed a methodology to solve the problem of
data inconsistency especially the problem of missing
attributes of existing contour in particular and the map
revision process in general. The final result of all
processing, revision and enhancement should result in
"GIS-ready" topological structured data that may form the
base for a GIS database. Especially the topographic
features that are required for the generation of a DEM
undergo the most rigorous checks.
3 Preparation Stage
A preparation stage is introduced prior to the digital
revision process to permit smooth operations.
Familiarization with the accuracy requirements, map
specifications, and required material is one of the initial
step. The preparation stage is sequenced to provide
systematic, standardized procedures that economize
operator effort and minimize error. As a primary check,
all original data is loaded onto the system and checked
for systematic errors.
Prior to the actual updating or map revision process the
original files are converted into ASI's Processing
Software (APS). During this translation each element
will, in addition to its original attributes, receive additional
attributes to allow for identification of a possible
modification. At this stage in the preparation, one of the
strategically placed quality control checks is performed-
check of compliance with the original database design.
Over the years most of the digital graphic files or maps
may have been translated from the original files to the
user's particular GIS software environment. For
example, data for an individual map sheet might be
available in three separate files, a Digital Terrain Model
(DTM) file containing breaklines and masspoints, a
contour file and a file containing all planimetric features.
In addition the user may have subsequently modified the
760
original files in a way that may have result in a loss of
attributes and/or complete loss of map data. Existing
graphic elements may have an artificial elevation value of
“0”, other erroneous elevation values or no elevation at
all and are therefore not suitable for the photogrammetric
updating process. Contour lines with missing -elevation
value cause displacement and distortion of the existing
data up to the point were stereoscopic viewing is
impossible. It is therefore necessary to introduce the
exact or approximate elevation value for each of the
graphic elements.
Part of the preparation phase is to determine the quality
and usability of the original DTM and/or contour files.
The analysis identifies and categorizes the graphic
elements into different groups, e.g., contours without
elevation tag, contours with an elevation tag that is out of
range, contours without tag but with a text string
associated, mass point file with holes, etc.
To use the client files, which contain in general the map
data, a corner file and header file must be created.
Support files that contain data effecting the appearance
of each file are set up and their environment variables
defined. These support files include files defining color,
weight, style, symbols, patterns, text feature codes,
theme numbers, source ID's, etc. When the operating
environment is established the data is loaded. The time
stamping of the new data is an operational issue. It will
facilitate the editing process as it will distinguish between
the old data and the new positional data.
The number and location of the control and check points
are based on the points generated as a result of the
analytical aerotriangulation process. The registration of
the new compilation model for integration with the “old”
map digital data can be performed in two ways. If the
ground coordinates of the lower left corner are available,
this can be accomplished by simple translation in x-and
y-direction using one corner point. Otherwise,
registration can be done by a six-parameter affine
transformation using well-identified control points.
4 Development of a DEM
Prior to the compilation of the update areas on an
analytical stereoplotter with superimposition, such as
ASI's Zeiss P-33 with superimposition, the map files are
translated to ASl's proprietary Processing Software
(APS). Within APS we are developing a DEM from all
available and in the preparation stage identified useful
data. Within APS the data is passed through numerous
routines and analysis to prepare a photogrammetric
usable file for revision. Convex hulls of the new
aerotriangulated models are generated and map sheets
are cut to match the model extension.
A DTM represents terrain relief, thus the shape of the
(topographic) surface in 3-dimensional space. To
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
