PHOTOGRAMMETRIC TRIANGULATION WITH SEPARATELY 
ORIENTA TED STEREO MODELS 
by LENNART EKELUND, Stockholm, Sweden. 
Since 1946 the author has been studying the problem of space triangulation at 
the Division of Photogrammetry at the Royal Institute of Technology, Stockholm. 
The investigations were among other things concentrated on the systematic errors 
and the possibility of eliminating them in plotting instruments. Bertil Hallert, 
professor of photogrammetry at the Royal Institute of Technology, has given me 
the most valuable support in this work. As a result of the investigations a method 
was developed, which has found considerable employment during the last two 
years at the Photogrammetric Order Division of the Geographical Survey Office 
of Sweden when producing an accurate control survey for special maps, scale 
1:2.000—1:10.000. In this paper a short description of the principle of the method 
is given and an example of the application of the method is reported. A report 
on the method was delivered to the Royal Institute of Technology in May 1949 
and was published in *Fotogrammetriska Meddelanden" from the division of 
photogrammetry in March 1950. [2] 
I. Introduction. 
The geodetic control net is generally sparse in Sweden and in great parts of 
the country there is no geodetic net at all. This is why it is often necessary to base 
special maps on photogrammetric control survey, which aims at the determination 
of coordinates of pass points for the fitting in of the stereoscopic models into the 
map, as well as of suitable points marked out on the ground, which replace geo- 
detic control points at the use of the map. In areas lacking geodetic control net 
the photogrammetric triangulations must be based on local geodetic measurements 
without coordinate connection (bases or local transit traverses). Then it is gene- 
rally not possible to eliminate by computation the bends of the strips which nor- 
mally arise at ordinary stereo triangulation. This is specially the case at the mapp- 
ing of such band-shaped districts as are covered by only one or a few strips (e.g. 
rivers, roads, rail-roads, power-lines etc.). 
Evidently it is necessary to find out a method for triangulation where the 
bend of the strip is eliminated already at the work in the instrument. 
Also the elevation point net is sparse in Sweden. At special mappings on large 
scales this net must therefore be rendered denser. The demands of accuracy in the 
contour lines are, however, for special mappings on large and average scales so 
heavy that this densening must be executed with geodetic methods. Only in those 
cases when one need not make full use of the maximum elevation accuracy of the 
separate model plotting, the densening of the elevation point net can be made by a 
general stereo triangulation. This form of triangulation makes special demands on 
the localization of the geodetic control points. Thus the control points must not 
occur as separate points but as groups of points with 4—5 points in each group. 
The groups must if possible be situated with equal intervals in the strips etc. Even 
if these claimes are fulfilled the accuracy in the photogrammetric determination 
of the Z-coordinates is relatively low in comparison with the result obtained if 
each stereo model is orientated with the guidance of elevation points or other