(to be done by the user himself rather than by the map maker) into metric 
photogrammetry is an absolutely necessary factor. The agrarian re-develop- 
ment, resulting in a transformation of the rural areas, requires a regular up- 
dating and a frequent remaking of the maps”. The above necessities are 
easily obtainable with photo maps. 
Van Twembeke proposes to distinguish between two sorts of annota- 
tions to be made on the photo maps, or on special overlays. 
a) “permanent” elements: 
- height information, in the form of contours and spot heights 
- the network of reference points 
- the names 
- the land classification 
- the map grid 
b) “changeable” elements 
- the structure of cadastral parcels 
- structure of land use 
the rural areas classified in agricultural zones with an economic 
function and others with a social function 
- forest areas, forest inventory data 
- road classification. 
He goes on “A periodical revision will be highly simplified if the 
permanent part of the documentation is carefully established at the outset, 
and if the successive photography of the area is executed by the so-called 
‘imposed nadir’ technique”. 
3.1.3 United States 
Large scale orthophoto maps are widely produced: vide the papers 
presented to Orthophoto Workshop II (California, January 1973) and Ortho- 
photo Workshop III (Texas, June 1975). I quote one illustrative example 
(Ryser: A Multi-Purpose Mapping Program) from Workshop II: “The entire 
land area of San Mateo County has been remapped at scale 1” = 400” 
(1:4,800). The many conflicting user demands were met with multiple 
overlay techniques by extracting different types of information (boundaries, 
drainage, etc) onto separate but matching overlay sheets. In lieu of customary