17 
positioning of the photographs within each flight line so that each sheet is 
covered by one conventionally rectified photo (in the case of flat terrain) 
or by two successive stereo models (in the case of non-flat terrain), either 
a 90% forward overlap should be observed or pin-pointed photography 
should be taken. In this connection the automated exposure command 
system being developed at the ITC (the ITC Blockphot system) is to be 
recommended. 
1.5.8 Chronological Summary of the Procedure Proposed is then as follows. 
I Choice of photo scale and camera type (section 1.5.6) 
II Choice of photo map sheet size (section 1.5.7) 
III Establishment of a grid of permanent and 
air-visible, readily identifiable, reference points (section 1.5.4) 
IV Photo flight (section 1.5.7) 
V  Aero-triangulation and block-adjustment (section 1.5.4) 
Vla Stereo plot of contours and spot heights, if 
required (section 1.5.3) 
VIb Production of photo maps (section 1.5.5) 
VII Field definition of cadastral boundaries (section 1.5.1) 
VIII Production of conventional line maps of the 
cadastral situation by tracing from the photo 
maps, if desired (section 1.5.5) 
IX High precision stereo-restitution for recon- 
struction of cadastral boundaries, if and when 
really necessary (section 1.5.2) 
2 Urban Survey 
n3 
Introduction 
In urban areas the application of field survey encounters particular 
difficulties: 
a the traffic in the streets, which makes a field survey time-consuming and 
even dangerous; 
b the backyards, which are not easily accessible and require complicated 
geometrical constructions of the measuring lines;