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Selective sampling is supported with functions for
measurement of randomly located spot heights and
characteristic terrain lines.
Systematic sampling can be done using profile or
grid measurements. Once profile/grid parameters are
set, upon each redraw of actual data window, theoretical
locations for profile/grid points are marked on the
screen. Of course, systematic sampling can be
performed also by proper movements of stereoplotter
handwheels to accomplish nearly regular distance
between points during grid measurement.
Progressive sampling is based on analysis of TIN
DTM generated from the points measured. This concept
has already been presented in several papers (Bill,
1986; Mann, 1988; Reinhardt, 1988). After the TIN is
created and the surface calculated, curvature of each
side if triangle is tested, and if it exceeds the specified
limit (based on the required height accuracy) additional
measurement is suggested. Locations of these
measurements are marked on the screen until
densification step is finished.
Contour measurement is supported with several
criteria for automatic on-line selection of contour points
during dynamic contour digitizaton. These are: distance,
time, and curvature (tube) criteria. Combination of these
criteria is also possible.
Standard functions for interactive data manipulations
and editing are developed: browse, delete, undelete and
attribute changes. All operations are supported by on-
line help and designed in a same manner as the rest of
the MapSoft functions in order to make training and
work as easy as possible. Several workstations
connected in a computer network can simultaneously
use and update data stored within the common data
base.
4.3. DTM creation
Irregularly distributed height points and other height
information are dominant in case of data acquisition on
analogue stereoplotter which cannot drive measuring
mark on the specified position. DTM modelling based on
TIN (Triangulated Irregular Network) has therefore been
chosen, since it provides very efficient and simple
processing of such data.
Triangulation: Several algorithms for the TIN
generation have been implemented and tested, each of
which respects terrain lines. These are: continuous
development of the TIN (McCullagh, 1980), radial sweep
algorithm (Mirante, 1982) and point insertion algorithm
(Reinhardt, 1988). Each of these algorithms result in
Delaunay triangulation with well known properties,
constrained by terrain lines. The best results are
achieved with point insertion algorithm and it will be
briefly described.
Initial triangulation consisting of four imaginary vertices
located outside active DTM area is created. After that
terrain lines are processed sequentally. If the line length
exceeds specified value, then the line is divided. The
heights for the new points are interpolated by linear
interpolation from the end line points. Each line segment
is subsequently processed as a separate line. For each
line end points are first inserted into the TIN. Insertion of
a point is done by finding the existing TIN triangle that
the point falls in. The triangle is after that divided into
three new triangles. This is followed by local TIN
optimization which is performed until Delaunay criterion
is satisfied (Sibson, 1978). When the both line points are
inserted, triangulation is again modified to ensure that
the line forms edge in triangulation. Whenever possible,
swapping of the alternative diagonals for the
quadrilateral consisted of the two TIN triangles with the
common edge is performed. Otherwise, new point is
inserted into the TIN and its height is interpolated using
linear interpolation from the end points of the line.
After processing of terrain lines, single height points are
inserted into triangulation using the same algorithm.
Using this algorithm it is also possible to obtain the TIN
with the minimum sum of all distances. To achieve this,
only minor modification of the swapping algorithm for
local triangle optimization is necessary. Spatial triangle
sorting for faster manipulations is perfomed after
triangulation is finally completed.
Surface creation is done by finite elements method.
Three methods for interpolation and terrain surface
presentation are supported: linear, third order and fifth
order polynomial interpolation. The simplest and fastest
solution is linear interpolation. In that case terrain
representation is done with triangular facets. The
calculated surface is continuous but not smooth. This is
primarily used for initial checks of the data.
For high quality terrain modelling triangular surface
patches are represented by high order polynomials.
Third or fifth order polynomials can be used optionally.
In each TIN node, partial derivatives are estimated using
information on neighbouring nodes (Akima, 1974, 1978).
If fifth order polynomial is chosen, then first and second
derivatives are calculated. This yields five derivatives for
each TIN node. In case of third order polynomial, only
three values for first derivatives are calculated. Where
breakline exists, two or more sets of derivatives are
calculated for each node.
Terms of the polynomial are calculated from the heights
and derivatives of the three triangle points (Barnhil,
1981; Sekulovié, 1984). The result after calculation and
connection of all triangular polynomial patches is a
continuous and smooth terrain surface. The only
exception is across breaklines where smoothing is
intentionally avoided.
209
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996