COMPUTER - ASSISTED NUMERICAL ORIENTATION FOR THE 
223 
REVISION OF THE SWISS NATIONAL MAP 1:25000 
by Ch.Eidenbenz and D.Schneider, dipl.Ing ETH 
Federal Office of Topography, CH-3084 Wabern, Switzerland 
1 Introduction 
The Federal Office of Topography of Switzerland is, according to 
official order, author and administrator of the Swiss National Maps. 
These maps were completed in February 1979 and include 249 sheets at the 
scale 1:25'000, 78 sheets at 1:50'000, 23 sheets at 1:100'000, 4 sheets 
at 1:200'000 and one sheet at 1:500'000. All of these maps are purely 
topographical maps, extremely accurate and with a high information 
density. 
Since 1968 the maps were being systematically revised, using strictly 
new aerial photography and stereo-compilation to collect the new infor 
mation. 
Today, after the completion of the map series, revision is our main 
task. 
2 The revision procedure 
In 1968 the Office decided to start a regular revision program for all 
maps, based on a cycle of 6 years. This program runs quite successfully. 
Actually we are now in the middle of the third revision cycle. The steps 
of the program are the following: 
Every year we acquire aerial photographs of one sixth of the country at 
an approximate image scale of 1:25'000. 
In the first step, the new photographs and the six-year old maps are 
compared in the office using stereo-interpretation and any differences 
are registered in a sketch-map. In the second step, this sketch-map is 
then completed with a field check, a task which we consider to be very 
important. In the third step, all new elements indicated in the sketch- 
map are photogrammetrically compiled. We use three WILD A8 and one WILD 
A7 stereo-plotters for this task. The plotting base consists of a 
coated glass plate containing a copy of the old map. For the smaller 
scales (1:50'000 and smaller) we reduce this information and select the 
scale-relevant features. 
Because of meteorological and administrative reasons, signalization of 
control-points in our country would be too expensive and time-consuming. 
Therefore, we use exclusively map elements (crossroads, houses, spot 
heights etc.) for ground control. Experience has shown that we can 
maintain our accuracy standards of ±0.1-0.3 mm in planimetry and ±1-2 m 
in height.