Thanks to Bernd who sent me the horizo10.dat file in which a much easier
method is used for defining the horizon line.
There is a "hor" command which isn't documented in the horizon.dat file.

Just insert these lines into the horizon.dat file:

hor 32 0 0
0  25
10 21
20 20
30 16
40 22
50 18
60 16
70 22
80 19
90 16
100 22
110 20
120 15
130  6
140 10
150  6
160  6
170  6
180  7
190  8
200  8
210  7
220  7
230  9
240  9
250  8
260 14
270 16
280  8
290 12
300 17
310 21
320 24
330 26
340 28
350 29
360 25
hend

I also like the example from Ulrich in which he creates the "NE", SE",
SW" and "NW" symbols.

By the way, if you have several horizon files and you want to switch
between them, you can create one batch file for each horizon, with this
content:

copy d:\guide9\my_horizon_1.dat d:\guide9\horizon.dat

Put the batch files on the desktop and execute one of them by
double-clicking. It will automatically overwrite the horizon.dat file
with one of your own horizon files. You can change the horizon file
while Guide is running. After overwriting the horizon file, just click
in the Guide window to refresh the graphics and then you see the new
horizon.

Thanks to all who responded
Michael