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
