4.4 DTM analysis and data verification
The calculated model of terrain surface and the TIN
topology are used for all subsequent analysis. These
analyses currently include: interpolation of heights,
contour construction, fast interpolation of heights in
regular grid and profiles, evaluation of DTM quality and
calculations of volumes. Only the functions that are
essential for primary data verification will be briefly
discussed in the paper.
Interpolation of height at specified point is done
simply by determining which triangle the point falls in. In
case of linear interpolation height is calculated from the
condition the point belongs to the triangular facete. In
case of smooth interpolation using high order
Verification of DTM: Visual check of the acquired data
can be done by inspecting data distribution, generated
TIN or constructed contours. All these DTM elements
and results can be plotted at any time on screen or as
output on standard plotting devices (Figure 2). Once
display of some of these elements is turned on, it
remains active until it is cancelled or a new DTM or its
product is generated. All standard display functions for
zooming and moving are available. Display of
MapSoft's entities for horizontal terrain presentation,
combined with display of height information is also
enabled.
Rigorous analytical check of generated DTM is
supported by calculations of differences between
measured or known heights at chosen locations and
Figure 2 : Visual verification of the acquired DTM data within MapSopft
polynomials, the height is the value of the polynomial for
that triangle. This operation is supported in on-line mode
when MapSoft's cursor is moved across the screen.
Contour construction: In case of linear interpolation
contour segments are simply intersections of horizontal
plane containing the given contour and triangular facets.
In order to get smooth contours polynomial interpolation
is used. Each TIN triangle is divided into the specified
number of subtriangles. Heights for subtriangle vertices
are calculated as values of the polynomial for the main
triangle. After that, contour segments are calculated
using linear interpolation within subtriangles. Two
options for contour construction are available. One of
them are non-sorted contour segments. This option is
faster, but it only satisfies requirements of on-screen
plotting. For additional smoothing of contours using
spline functions and for plotting on pen plotters, the
option with sorting and connecting of contour segments
is required.
heights interpolated from DTM at the same locations. It
is possible to move measuring mark of the
photogrammetric instrument and to track in on-line
mode differences between interpolated and measured
heights. The results can also be written in the report file
together with some statistics concerning these
differences. The same applies if file with control points is
available.
Analysis of the curvature of triangle sides is used to
establish if additional measurements are necessary. If a
selected treshold depending on the desired height
accuracy is exceeded for triangle side, additional
measurement is suggested. This is done by marking
locations for all such measuremens on the screen. After
new measurements being made, a new analysis is
performed and so on. Procedure is to be repeated until
there is no need for further measurement. Using this
procedure, it is possible to obtain more objective results.
210
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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