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considered a value z in a 3D coordinate system (Burrough & McDonnell,
1998). Such surfaces are called statistical, they are relatively continuous and can
be processed, analysed and represented in the same way as the relief of the earth's
surface. In case of the earth's surface the altitude corresponds to the value z .
14.2 Accuracy in Relief Expression
The aim of DEM is to represent the variability of relief as best as possible. However,
the quality of such representation is limited by many factors arising from the
character of the creation of the model. The load of inaccuracies in the model then
influences all analyses performed for this model.
Except for the newest DEMs derived from stereo pairs of aerial or radar photos
(using the IFSAR technology, for more see GEODIS, 2008) the majority of DEMs
in the Czech Republic are created from digitalised analogue topographical maps of
large scales. These maps may not only be outdated but they are also loaded with
errors. Their digitalisation augments the errors. We have to keep in mind that it is
not possible to perform accurate analyses using inaccurate data and it is essential to
maintain a critical approach to the assessment of the results (Voženílek et al., 2001).
There are several sources of errors and inaccuracies in the expression of spatial
(3D) properties of relief.
Type of source data : The type of source data influences the accuracy of relief
expression. The most common source data are contour lines or height fields (grids of
points with their elevation). Both of these ways represent a discrete expression of a
continuous relief. Therefore, it is true that the higher the number of accurate discrete
objects (contour lines, height points) and the better their distribution (according to
the configuration of the relief ), the less inaccuracies in relief expression, and thus,
in the computer models.
Scale : Thanks to cartographic generalisation the smaller the map scale, the sim-
pler and more generalised the map content, including the expression of altimetry
(contour lines, elevation points). When the scale changes, there are changes in the
basic contour line interval, contour line course and curvature, as well as the density
of elevation points. On a map with a smaller scale a relief expressed by contour
lines of 10 m is more smoothed down than a relief with contour lines of 5 m. Except
for different expression of detailed surface shapes compared to the real relief a
modelled surface has different parameters (area, slope, slope length, etc.). Generally
it is true that the smaller the scale, the bigger the inaccuracies in relief expression
and the higher the influence on the results of computer modelling. A more exact
assessment (quantification) of this dependency will be the subject of further research
by the author.
Number of dimensions in the model : The basic influence of the accuracy of
relief expression is the change from real 3D to 2D relief expression of maps
that most often serve for derivation of values representing the relief in computer
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