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