Fig. 5 The completed 3-d model of a redevelopment site 
of certain buildings. Enquiries as to the availability of 
suitable vertical aerial photography were made but 
unfortunately the only suitable photography was 3 years 
old. However because it was at a scale of 1:5000 it was 
felt that it would be useable. In many other parts of the UK 
the age of the photography would have been 
inconsequential, but as London is still undergoing major 
redevelopment it was important that recent photography 
be used to get the most from the photogrammetric work 
and reduce the amount of field completion required. It was 
decided to supplement the vertical photography with 
oblique photography (Fig. 3) taken from the roof of a 
conveniently situated building on the site. This together 
with the terrestrial photography that was required for the 
detail facade work was taken with a Zeiss Jena UMK 
10/1318. 
All sets of photography were studied and suitable control 
points selected which were coordinated by field survey to 
give 3-dimensional coordinates based upon the Ordnance 
Survey National Grid so that any subsequent site survey 
or additions could be more easily linked with the model. 
The accuracy requirement for this project varied 
according to the subject, detailed facades +/-30mm, 
building lines +/-150mm. This meant that the control 
survey was potentially a two phase operation, but it was 
decided to coordinate all control points to the higher 
precision and allow the photographic scale to determine 
the final accuracy. 
Experience has shown that it is necessary to work closely 
with clients to obtain a good understanding of what further 
use will be made of the finished product. In this way the 
most suitable methods of 3-d digitising can be employed 
on the IMA in order that further workstation editing time 
can be reduced. 
Initially features are digitised as line string elements and 
where possible higher order elements such as shapes. 
These data act as a background template from which 
shapes representing roofs and walls are interpolated. This 
process inevitably leads to the degradation of the accuracy 
    
of the original line strings but this compromise situation 
is presently unchangeable due to the geometric 
restrictions imposed by most CAD systems. 
This particular project did not require surface modelling 
to be carried out on all features. Detailed facades (Fig. 4) 
were required to be represented by 3-dimensional line 
strings, which retain the original photogrammetric 
accuracy. Surfacing of these facades would have 
compromised the accuracy and would not have been cost 
effective. The main blocks or masses of buildings were 
surfaced by the placement of shapes and projected 
elements but ensuring a homogeneous fit between the 
idealised blocks and the facades caused some problems 
mainly due to the overhanging eaves of the buildings. This 
problem was overcome by the client who simply moved 
the facade detail forward so that it fitted just over the 
surface! This obviously compromised the absolute 
accuracy required by the specification, but the relative 
accuracy of each facade was maintained. 
The representation of the ground surface was derived from 
a triangulated digital terrain model with the kerb edges 
being projected down to the road surface for a more 
realistic effect. The triangles were colour coded according 
to whether they represented roads,pavements or grass 
areas. The advantage of having a ground surface for the 
buildings to sit on not only gives a better visual effect, but 
also makes the creation of the building surfaces simpler 
as the building walls can be projected through the ground 
surface. This can produce a dramatic time saving in the 
editing process, especially on sloping sites. 
Crane 
EPU were commissioned to carry out surveys on three 
gantry cranes used in separate hydroelectric power 
stations in northern Italy. The power stations were 
constructed during the 1930s' and over the ensuing period 
the original engineering drawings have been lost. The 
generators within each plant needed to be replaced by 
more efficient modern ones. To achieve this the gantry