441
The accuracy s time-efficiency and reliability depend primarily on the
precision of the measuring equipment and on the skill and care of the
personnel involved. Thorough preparation is essential. Potential sources
of errors and failures are the identification of control and tie points,
especially on generalised maps, and separate measurements of differential
heights and planimetry. In the latter case, mistakes in indexing or in
the sequence of measurement are likely to occur (Radwan, 1980). Errors
in DTM and in differential heights cause radial displacements which may
be significant near photo-edges when terrain surface is rough (Beotra,
1981; Besenicar, 1976).
The updated information should be compatible with the existing graphical
and the anticipated digital data base system(s). Compatibility concerns
the data structures, formats and codes and the (added) relational data,
i.e., to provide flexibility in manipulating and representing the basic
data.
CONCLUSION
The recent achievements in the field of data base technology give reason
to reconsider the existing techniques for updating maps. An up-to-date
approach should serve two purposes, i.e., to update the existing map
series and to contribute to a gradual transition from graphical to digital
data bases. By accumulating the changes, adding new data items and rela
tions and converting the existing data into digital format, a digital data
base can be gradually built-up.
It seems rational to anticipate that there will be an increasing transi
tion from the original mapping to updating the existing data bases. The
traditional economic and performance considerations about the alternatives
of "updating” or "re-mapping" are no longer applicable to digital data
bases. Correct "old" data should be preserved and supplemented by the
"changed" and "added" new data, even if more than 50% of the total in
formation has changed. Re-mapping is justified, however, if a higher
accuracy is required.
The operations typical for updating by DMP are involved in the preparation
mensuration, and the geometric transformations. The properties of DMP in
updating are listed below, separated according to technical and manage
ment views.
The technical properties are:
- Control points and local DTM data can be extracted from existing topo
graphic maps. Aerial triangulation and full DTM coverage are therefore
not required;
- Operation can be restricted to local zones of changes (except change
identification and external orientation);
- The transformation and correction algorithms are exchangeable, which
provides versatility and flexibility;
- The DMP output is suitable for updating graphical and digital data bases
- The DMP software can be made compatible with other existing data base
software systems;
- The height of any point can be determined as a by-product.
Some properties important for work management include:
- The process is phased and thus operations can be parallel and decentra
lised ;
- Existing equipment can be used;
- Each operation employs specific skills;
- The control data, inclusive DTM, need not be available in the phases
preceding the transformation stages.