triangular irregular network (triangulation
including all measured points - breaklines and
formlines are maintained in the stored data, full
3D is possible, much greater data amount as for
regular grids)
hybrid structure (regular grid with additional
information about breaklines and formlines, also
combination with TIN areas is possible) [Kraus
K., Jansa J., Kager H., 1997]
The quality of the computed orthophotos also
depends a lot on the used DTM. Concerning
orthophotos for small scale (pixelsizes greater than
10 m) a regular grided DTM will be sufficent to
obtain good results. For large and medium scale
orthophotos the used DTM should also consist of
additional structure information. If orthophotos with
pixelsizes of 1 m or less have to be generated also
buildings must be taken into account.
2 VISUALIZATION
Digital terrain models and orthophotos are widely
used and a lot of products can be derived from them
(isolines, profiles, perspective views, intersection
with other data, volume calculations). Such results
are used either as the basic information for further
analysis or as final plots for visualization and
presentation (Figure 1).
Figure 1 DTM representation of isolines, breaklines,
formlines and outlines of buildings.
Depending on the object information and level of
complexity there are different levels of visualization
for the derived products mentioned above (wire
frame, shading, texture mapping, integration of other
data such as roads, buildings or thematics). All
these outputs can be produced with high quality and
accuracy and therefore are suitable for a lot of
applications.
A disadvantage of these outputs is the lack of
flexibility. A shaded and texture mapped perspective
view of a DTM is a very illustrative tool for giving a
more realistic appearance to landscape and larger
areas, but it is still a static view of a much more
complex model. To show the complexity, a lot of
different plots from different points of view have to
be created.
In some cases, especially when discussing with
customers and sponsors, it is of the same
importance to present results in an appropriate and
more impressive way. Imagine your customer can
move over the DTM and explore and analyze the
model in real time! As in today's world everything is
rotating and moving it seems obvious to look for a
possibility to perform interactive, dynamic, real time
DTM visualization and animation.
The problems that arise are not issues of basic
algorithms (computer graphics has reached a high
level of sophistication), they are rather problems of
handling very large data sets under time-
constrained conditions. Some of the requirements
for efficient visualization are distributed worlds,
levels of detail and image compression.
"Distributed worlds" means that the whole DTM is
divided into different parts, each one representing
an own smaller DTM. So, only the visible parts
(determined by using a viewing pyramid) need to be
loaded. Using this concept, the amount of data to be
hold in memory at the same time can be reduced by
approximately 75 percent on average.
A further increase of performance can be achieved
by utilisation of levels of detail (LOD). "LOD is a
mechanism used in computer graphics to improve
the drawing speed of complex scenes [Clark J.H.,
1976]. Each object is stored several times in
different levels of quality (levels of detail). During
visualization each object is drawn in the optimal
level of detail. The chosen level depends on the size
of the object in the current view. Objects that appear
small can be drawn in little detail (and therefore very
fast) without loosing quality; in contrast, objects near
the point of view that cover a lot of space on the
screen need to be rendered in full quality." [Kofler
M., Rehatschek H., Gruber M., 1996]. Although the
total amount of data is increased by storing different
LOD in a pyramid structure, the current memory
demand during interactive visualization can again be
reduced dramatically.
As a last way of data reduction, image compression
should be mentioned here. When visualizing a DTM
