The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B3b. Beijing 2008 
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and installation. The physical structure of stairways can range 
from typical straight-run examples, to quite complex, curved, 
multi-level structures that can even double or triple back on 
themselves. All stairways present a geometrical challenge for 
photogrammetry, due to the narrow fields of view and obvious 
physical constraints that are placed on possible imaging 
locations. 
Traditional automated photogrammetric techniques using white 
retro targets and highly controlled image exposure are generally 
not suitable for stairway measurement. Firstly, it is near 
impossible to place targets exactly on the points and features of 
interest, such as straight line edges of stair intersections; risers, 
which are the vertical portions of each step; and winders, which 
are steps narrower on one side to facilitate a change of direction 
in a stairway. Hybrid measurement helps by allowing automatic 
network orientation via the use of coded targets, and then the 
subsequent manual and semi-automatic measurement of 
important stair features in the normally exposed imagery. The 
stairway will still be clearly visible in each photograph, 
allowing the user to see and therefore measure the necessary 
elements. This is illustrated in Figure 3. In the left image, the 
photogrammetric network is shown, with approximately 30 
images and 40 coded targets, measured fully automatically. The 
right image shows examples of the manual measurement that 
has taken place, including points at the intersection of each step 
and the walls either side, and also some semi-automatic 
measurement, including scale bars, lines to represent each step, 
and ‘stair-plates’ which are linked to a user-defined database for 
ease of implementation. 
In a hybrid measurement context, verifiable accuracies of about 
1:50,000 have been achieved for the automatic component of 
stairway measurement, using a 12 megapixel Nikon D200 with 
fixed-focus 18mm lens. Achievable accuracy levels for 
manually measured untargeted features are roughly an order of 
magnitude lower, yet certainly fall within the required tolerance 
for stairlift design and installation. 
Figure 3. Hybrid measurement of a stairway for lift installation. 
5.2 Vehicle Crush Measurement 
Another application particularly suited to hybrid measurement 
is the accurate reconstruction of traffic accidents, and especially 
the measurement of damaged vehicles. Normally exposed 
images are required so that the operator can manually digitize 
unsignalized features, which typically constitute the primary 
features of interest, such as skid marks; debris on the road; 
surrounding buildings; traffic signs; road markings, and so on. 
Operator-assisted surface extraction may also assist in vehicle 
‘crush’ analysis. The number of image stations illustrated in 
Figures 4a highlights the need for fully automatic network 
orientation, while the abundance of natural features to be 
measured, including the curves shown in Figure 4b, illustrates 
the need for well exposed imagery. This is not generally 
conducive to automatic image scanning and multi-image 
orientation as it places an extra burden on these processes in 
terms of complexity and required computation time. 
->u ■ ■ mb a > b 
(a) 
Figure 4. Vehicle crush measurement, illustrating: (a) automatic 
orientation and (b) manual feature dimensioning. 
6. CONCLUDING REMARKS 
There are a growing number of photogrammetric measurement 
tasks which can substantially benefit from fully automatic 
network orientation and 3D target point determination, coupled 
with the ability for subsequent semi-automatic and manual 
feature extraction from normally exposed imagery. The authors 
have dubbed this all-encompassing approach ‘hybrid 
measurement’. Potential users include law enforcement 
agencies performing traffic accident reconstruction and forensic 
or crime scene measurement, architects interested in modelling 
buildings for which no CAD models exist, and ship building 
companies. 
The practical developments of hybrid measurement reported in 
this paper, namely the adoption of red retro-reflective targeting; 
the new system of coded-targets, fully automatic network 
orientation and image point correspondence determination, as