manipulated, and analyzed interactively. 
During digitizing of new objects the topo- 
logy is built up in realtime. The program 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Objects 
INPUT 
with attributes 
and topology HEIGHT 
MODELS 
4 Y 
TIN GRID CONTOUR 
point 
objects 
Model <+5— Model Model 
points points with object: 
with xyz- >| attributes: polygon 
coordinates height attributes: 
slope heights 
: OUTPUT curvature Zax: Zein) 
aspect area 
Objects: P 
Gon i perimeter 
po 
topology: topology: 
attributes: pology ope ogy 
no yes 
slope, aspect, 
height, area, 
perimeter 
topology: 
yes 
Fig. 4 The object-oriented height model 
uses objects in input and output. 
Various height models (TIN, GRID, 
CONTOUR) which contain objects 
and attributes can be derived. 
"MODELER" can be used in combination with 
the programs for image processing ("IMA- 
GER"), for mapping ("MAPPER"), and for ana- 
lyzing ("ANALYSTT), All of these programs 
are of the same size and relatively big 
(ca. 13 Mb). The hierarchy of objects is 
designed graphically by means of the pro- 
gram "ADMINISTRATOR", 
The advantages of the object-oriented and 
topologically structured height model are 
based on its higher "intelligence". The in- 
put data as well as the derived results can 
be automatically validated.For example, the 
height values of drains shall decrease con 
tinuously, or the generated height model 
Should not have unrealistic peaks, or the 
digitized contours should not cross. The 
height models can be generated with less 
blunders and are more accurate. The possibi- 
lity of working either with GRID models or 
with TIN models allows an adaption to the 
task to be fulfilled. For example, a TIN 
model allows a more accurate modelling of 
the terrain, but the generation of a Slope 
map is best done by means of a GRID model. 
The conversion from GRID models to TIN mo- 
dels is solved via the generation of 
point-objects; redundant height data are 
then eliminated without altering the height 
model significantly. The editing of topolo- 
gically structured data, e.g. contour 
lines, is much simpler. And the most impor- 
tant is, that the objects (with their 
attributes) can be analyzed and new ap- 
plications can be created. For example, 
one can easily derive slope maps from GRID 
models or shade contours. From TIN models 
one can generate a thematic output such as 
height or slope polygons. The area for a 
slope class will be topologically struc- 
tured and attributed with Slope values. 
Furthermore, by means of general functions 
in "MODELER" one can convert 2 D-objects 
into 3 D-objects, or height values can be 
manipulated. In this way, a flooding ean be 
simulated, and the extent of the water body 
can be mapped, or a dam ean be designed, 
and the extension of the reservoir can be 
displayed. 
It can be seen from the description above, 
that the term "object-oriented" relates to 
the management of data or the way the user 
classifies and manipulates the data. There- 
fore, it is sometimes called object-orien- 
ted management (Fritsch, 1991). There are 
also object-oriented data bases which are 
based on "object-oriented" programming. 
Such data base models are first under de- 
velopment. These two things‘ should not be 
mixed up. 
3. SOME PRACTICAL EXAMPLES 
These practical examples are carried out on 
the GIS workstation of the University of 
Aalborg. It consists of an INTERPRO 6280 
computer with 48 Mb memory and 600 Mb hard- 
disc, it allows processing with 14 MIPS. 
The 2 Mb pixel screen has a high perfor- 
mance graphic processor (EDGE II) which 
allows fast displays and millions of co- 
lours at a large screen (68.6 cm-diagonal). 
3.1 Generation of a height model from pho- 
togrammetric data 
A height model 
tos have to be 
is required, when orthopho- 
produced. Today photogramme- 
tric companies (as well as other institu- 
tions involved in the production of photo- 
grammetric data) use analytical plotters 
and collect height data by profiling. The 
distance between profiles corresponds to 
the slit length used in the analytical or- 
thoprojectors, e.g. 16 mm in the scale of 
the analog orthophoto. Within the profile 
data are recorded in shorter intervals. The 
accuracy of the collected heights can be 
relatively high ( 6h = 0.1 o/oo of the fly- 
ing height when static measurements are 
used), but the heights between the collec- 
ted points will be found by interpolation 
only. It now depends on the Shape of the 
terrain and the method of interpolation, 
how big the errors in the generated 
height model will be. These errors will 
Show up in the orthophoto, especially when 
several orthophotos have to be assembled in- 
to a mosaic. In order to improve the aceu- 
racy of the height model additional break- 
lines can be collected in the stereoplotter 
870 
and then be used for the generation of the 
height model. 
In the first part of the test about 8000 
regularly distributed pointsfrom a photo- 
grammetric model were collected for a 
2 km x 3 km area. The photoscale was 
1 : 18 000, the distances between the 
points were 24 m (within the profile) and 
32 m (between the profiles). The accuracy 
of the measured points was about 6} = 0.3m 
All these points were used to produce a 
height model. The matrix of points is then 
oriented to north, and the distances bet-