graph; that 1s, for a photograph on the scale of 1:30000, the marker is 
3 7X 3 metres. This may seem unusually large, but the cost of locating 
ground controls by measurement when markers ar not visible 1s so high 
that the cost of the material for the markers 1s negligible. 
Vertical control 
Vertical control points are usually established by a combination of 
differential levelling and barometric readings. A precision levelling line 
1s nearly always run along watercourse for later use in hydrographic 
measurements and the like. These fixes are marked, if possible, before 
the strip is flown. When the photographs are ready, they are coordinat- 
ed with the vertical control by means of the fixes. With procedure it 1s 
possible to assure satisfactory vertical control along the long axis of 
the strip. As the areas mapped are usually narrow, the x-tilt is the most 
serious in stereoscopic plotting, while the y-tilt has less effect. For this 
reason, it is sufficient to use barometric readings for vertical control 
along the edges of the model. 
Barometric readings for this purpose are taken rapidly and at low 
cost by the use of helicopters. The routes joining fixes and existing 
control points are laid out on the photographs, Station barometers are 
set up in the centre of the area to be contoured. The helicopters then 
fly the routes laid out, which are so planned that the stations can be 
visited at short intervals. The arrangement allows covering a large area. 
The control points are selected to allow landing direct at each point. 
Normally, a helicopter party can take enough readings in one day to 
provide control for 40 km of mapping (one strip). This costs about 100 
Swedish crowns per point. The accuracy is usually = 2 metres, which 
is full sufficient for the purpose at hand. 
Plotting 
For base maps, the horizontal control data obtained by ground survey 
is supplemented by means of photogrammetric triangulation. During 
recent years, two A-7 stereo-autographs have been occupied most of 
the time with triangulation for the Power Board. A-8's have also been 
used. The Ekelund triangulation method has been used almost enclusiv- 
ely; the computations are made by electronic data-processing methods. 
The triangulation not only makes possible the preparation of maps 
but also provides photo control points, that is, points oriented on the 
ground with reference to a coordinate system. These points are used to 
locate bore holes, traverses, and other terrestrial features. 
Detailing 1s done in Kelsh and Balplex multiplex instruments and in 
A-6 and A-8 stereoautographs. 
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