Cl PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey 
Figure 3 Rectified photography montage of a tiled floor from 
Windsor Castle, UK 
As with analogue rectification control can range from scaled 
distances to full 3-D control. Some packages also feature 
perspective correction which, assuming the subject contains 
horizontal and vertical lines, means only one distance 
measurement is required for scaling. If a number of images 
are to be montaged together, it will be necessary to have full 
control in the form of co-ordinated targets or detail points. 
Many rectification packages have a montaging routine but 
often the results are not particularly satisfactory. An 
alternative is to montage the images using photographic 
manipulation software such as Photoshop. As well as 
allowing adjustment of contrast, colour etc, the images can be 
cut along lines of detail and feathered. In this approach it is 
essential to have full control so that targets can be matched 
together pixel by pixel. It is also helpful to import an image 
of the control that can be overlaid as a check on the accuracy 
of the montaging. 
The English Heritage Metric Survey Team uses Rolliemetric 
MSR and Photoshop to produce digitally rectified montages. 
The final image is then attached to an AutoCAD R.2000 
drawing containing the control and a border for plotting out 
at the required scale using an HP DesignJet 1050C. 
5.2 Orthophotography 
Many historic floors tend not to be flat; especially mosaics 
that have been buried and then excavated (see Fig. 4). The 
availability of digital photogrammetric workstations (DPWs) 
means it is now possible to record these subjects accurately 
by producing an orthophotograph (Clowes 1997). In an 
orthophotograph variations in scale due to relief as well as 
tilts can be corrected. 
In order to produce an orthophotograph, stereo-photography 
is required. This can be acquired in exactly the same way as 
described for rectified photography but at least twice as many 
photographs are required to cover the same area. The 
photography is usually scanned with a dedicated 
photogrammetric scanner. Interior orientation is often 
performed at this stage. 
The imagery is then loaded onto the DPW and the relative 
and exterior orientations undertaken. Aerial triangulation 
methods can be used to set up whole strips or blocks of 
photography. Here an economy in the amount of control 
required can be achieved by using a bundle adjustment. 
Fewer control points are required because tie points are used 
to join the models together. 
Once the models are set up a Digital Elevation Model (DEM) 
is produced using automatic stereo-matching. It is often 
necessary to undertake some manual editing, however. The 
finished DEM can then be used to produce an 
orthophotograph or can be used in its own right as a model of 
the surface of the floor. A DEM grid spacing of 10 cm is 
usually sufficient to map any undulations accurately enough 
to result in an accurate orthophotograph at architectural 
scales i.e. 1:50 or 1:20. To create the orthophotograph it is 
necessary to identify the images required and then indicate 
the area of each that is to be used. Seam polygons are used to 
determine the lines along which each image is to be cut. 
These polygons butt-join although the images can be 
feathered together thus producing one orthophotograph 
without visible joints. The orthphotograph routine can also 
be set to automatically balance the colour and exposure 
across the whole final image. This can have mixed results 
because in the case of an archaeological excavation, for 
example, areas of earth may appear in the images and skew 
the colour of the mosaic detail. As with rectified photography 
the orthophotograph can be attached to an AutoCAD drawing 
for printing. The English Heritage Metric Survey Team use a 
Leica Geosystems DPW running SocetSet V.4.4.1. 
Figure 4 Orthophotograph of a temporarily revealed Roman 
mosaic at Lopen, Somerset, UK