‘or 8 ween points are 50 m. Figure 5 shows a 3-D 
ruc- presentation of the generated height model 
les. 
1ections 
ects 
an be 
' can be 
er body 
ned, 
an be T uu so 
2 E 
UU 
above, 
tes to 
e user 
There- 
orien- 
e are 
h are 
ng. 
r de- 
ot be Fig. 5 Isometric representation of a 
height model. 
Profile lines are colour-coded 
according to their height. The 
height model is used to produce a 
out on digital orthophoto. 
of 
280 In this form the height model ean be used 
b hard- for the generation of digital orthophotos. 
IPS. More details about this application can be 
For- found in (Höhle, 1991). In the second part 
ich of the test breaklines were also collected 
co- in the analytical plotter, again by an ex- 
ronal). perienced operator. The breakpoints and 
the regularly distributed points as well 
1_pho- as some check points were treatedas "ob- 
jects". The input of all these points re- 
quires a conversion from ASCII to binary 
10pho- format. These so-called xyz-files which 
'ramme- are established for each object ean then 
itu- be read in "MODELER". The ASCII input 
hoto- files have to be of a certain format as 
‚ers well, where the units of measurements, the 
The sequence of coordinates, the sequence of 
| to point numbers for a line have to be added 
l or- and arranged in a certain way. The con- 
e of version from the photogrammetric data file 
file to the ASCII input file has been solved by 
S... The a newly established program. A TIN model 
be was first generated from the regularly 
e fly- distributed points and from the breaklines 
re and later converted to the final GRID mo- 
llec- del. A display of the differences between 
tion the two generated models revealed that the 
the improvements (when using additional break- 
on, lines for modelling) reached up to 3 m. 
This confirms the assumption, that additi- 
11 onally collected breaklines considerably 
when improved the quality of the height model. 
ed in- 
accu- 3.2 Generation of & height model from digi- 
reak- tal map data 
lotter 
the Digital maps which are produced by photo- 
grammetry have also z-coordinates for the 
individual objects and their elements 
000 (points). Objects which are on the surface 
oto- of the terrain can be used for the genera- 
tion of a height model. In Denmark, digi- 
tal maps are available for the whole coun- 
try (T0, T1, T2, T3 data). 
and : 
racy TO-data covering the rural areas are pro- 
= 0.3m duced from photography 1 : 20 000 to 
a 1: 30 000. This means, that objects of 
then about 1 m x 1m on the ground ean be iden- 
oet - tified. The produced data base has 14 dif- 
ferent objects, their heights have an accu- 
racy of 6, = 1 m for well defined points 
or: 83 = 1.5 n for all other points and 
lines. These data can be bought for a very 
modest price. The main application of this 
data base is the recording and updating of 
utilities (gas,telefon, etc.). The ex- 
change of all these data occurs by means 
of the Danish Exchange Format (DSFL, 1986), 
(Alexandersen, 1991). The format is object- 
oriented as well. In the data files header 
are also information about the used photo- 
graphy and control points. In built-up 
areas more detailed map data are produced 
(T1, T2, T3 data). The seale of photogra- 
phy is between 1 : 4 000 $0 1i + 20 000 
which means that objects down to 15 cm x 
15 cm on the ground can be identified and 
the accuracy of the heights of well de- 
fined points is @p = 0.15 nm at the best. 
The number of the recorded objects differs 
from i6.(TO) to 60 (T3). In order to take 
advantage of these easily available T-data 
for the generation of & height model in 
TIGRIS "MODELER", à conversion program was 
created (DSFL2ADF). By means of this pro- 
gram the objects in the National Exchange 
Format (DSFL-format) are translated into 
the objects of the height model. A cross 
reference table like in table 2 had to be 
established. The output of the conversion 
program are files which contain all the 
data for one object type (e.g. breakline). 
In this way some height models have been 
created. Their accuracy, however, is not 
homogenious. It depends on the density of 
the planimetric data and the purpose of the 
application. For the generation of ortho- 
photos this may sometimes be sufficient, 
because a higher accuracy is required for 
areas where objects, especially man made 
ones, are. In this context it may be men- 
tioned, that in Denmark scanned and rec- 
tified aerial photographs are available 
on CD-ROMs (Jydsk Telefon, 1992). The 
application of the digital photographs 
(pixel size 2.5 m x 2.5 m on the ground, 
64 grey values) is within Geographic In- 
formation Systems. Their rectification is 
done by means of digital map data (TO data). 
3.3 Display of height model by means of 
grey values 
The attribute of a GRID model can be 
heights, but also many other parameters 
(e.g. slope, aspect, temperature, ete.). 
The heights can be converted into grey va- 
lues. Such a display can be seen in fis.6. 
It is used for visual inspection in the 
orthophoto production, in order to make 
sure, that there are no positional dis- 
placements or other blunders. 
3.4 Thematic maps for slope and soil ero- 
sivity 
The inclination of the terrain surface at 
a particular point is called slope. Slope 
values can be computed for the entire 
height model. From TIN models, for example, 
areas (polygons) of slope classes can be 
derived. These polygons are the objects, 
their attributes are slope values. At the 
beginning of the process one Specifies the 
interval, e.g. 159, and the range, e.g. 
00 - 909 , All polygons for the defined 
slope class are calculated and displayed. 
871