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1. High altitude flight, about 5 300 m above the ground. 
?. Low altitude flight, about 2 300 m above the ground. 
Normally, a wide angle camera is used, negative size 
23 x 25 cm (9 X 9 inches) f = 153 mm. 
Low altitude flight is normaly arranged East—West with 
courses plotted across the middle of the intended map sheets. 
The format of the map sheets is 48 X 64 cms. 
For preference, high altitude flight should be arranged 
so that each high altitude strip covers two low altitude strips. 
Photography is carried out in the period from 15. April 
to 15. September, preferably from 2 hours before noon until 
2 hours after noon. 
Marking. 
3efore photography can commence, the following must be 
marked: 
l. All existing trigonometrical points, i.e. N.G.O. points. 
2. All property boundary marks. 
Marking of Trig Points. 
In accordance with the conditions laid down for the map- 
ping work, N.G.O. shall provide one point per Sq.km, evenly 
distributed over the area to be mapped. 
Marking of these points is usually carried out by the 
province. The marks used are at least 1.0 x 1.0 m. 
Marking of Property Boundary Points. 
Marking of property boundary points is carried out by 
the Re-Allotment Authority, which has appointed an Area 
Manager in each province which commenced the production 
of economic maps. The area managers organize marking work, 
which is actually based on the private initiative of the indi- 
vidual landowner. The intention is to mark on the maps all 
agricultural properties of over 0.5 hectares, with boundaries 
and registry numbers. 
To interest the individual landowner sufficiently in this 
marking work to ensure that he actually carries it out in a 
satisfactory way has proved to be one of the most difficult 
tasks in connection with the production of economic maps. 
For this reason, an early start is made in canvassing the land-