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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