B. Large-scale mapping (1:1000 to 1:4000) 
Large-scale maps are prepared for detailed studies of sites for dams, 
power stations, and the like; they are usually made to a scale of 1:2000 
—1 m, Photogrammetric methods are also the chief means of preparing 
these maps for the Power Board. 
Taking photographs 
The photographs are taken at altitudes of from 600 to 1500 metres. 
The tendency is to work more and more at the latter altitude. Strips for 
triangulation are taken at about 2500 metres. 
Control 
Horizontal control points are established by using the existing chains 
as starting points. The geodimeter is of great help in this work. Various 
procedures are used. A chain of polygons with sides two to three km in 
length may be run between existing stations through the area of inter- 
est, or pique-points may be run in from the existing stations. The latter 
method is the more rapid and the less costly, although it does not allow 
superflous observations it is often used nonetheless, because the geodi- 
meter is foolproof” and also because ordinary chains of polygons are 
usually laid out between derivative control points, thus affording a 
measure of control. 
Vertical control is established by differential levelling. 
Marking stations 
Ordinarily the horizontal control points are marked. Sometimes the 
vertical control points are marked as well, if there is little detail which 
could be used for reference. The markers are made large, even when 
the photographs are to be taken at low altitude, so as to make certain 
that they will be visible on the prints. Great care is taken to obtain high 
contrast in the photographic copy. 
Plotting 
The model triangulation is done chiefly on the A-7 and the A-8 in 
order to obtain horizontal control and photo control points. However, 
since the introduction of the NASM-4 geodimeter, we have found that 
in some cases the extablishment of control points by terrestrial mea- 
surement is cheaper than photogrammetric triangulation. This is not a 
general rule; each case must be judged on its own, assessibility on the 
ground being the decisive consideration. It is most important to be 
awake to be possibilities offered by combinations of geodesy and pho- 
togrammetry, This has become a live question since the development of 
the geodimeter and the tellurometer.