|Rectified image of the west front of Holy Trinity, Sloane Square, London: the detail to the right was taken from the same image to illustrate the amount of detail visible. The
facetted towers were also photographed and rectified separately.
Rectified photography provides a
cost effective method for recording a high
level of detail on relatively flat structures
and objects such as building facades, floors,
stained glass windows and wall paintings.
It can be used to produce a highly versatile ‘drawing’ which can be taken on site and
easily annotated, read and understood by the
many different specialists involved in building
conservation. By its very nature it provides
a detailed photographic record of the subject
at a point in time. The resulting images are
also often impressive and can be used for
publicity and fund-raising. It must, however,
be remembered that the technique is only
really effective for relatively flat structures
and objects. If a similar image is needed of,
for example a vault or 3D line work, more
complex techniques such as orthophotography
and traditional photogrammetric stereo
plotting can be used.
Producing rectified images can be a
relatively simple process using a compact
digital camera and a tape measure or a
more specialist process using high quality
distortion-free cameras, measured control
points and dedicated software. It can also
be combined with other measured survey
information such as line work. Whichever
method is chosen, rectified photographs can
communicate a great deal more information
than most traditional line work drawings; a
picture speaks a thousand words.
WHAT IS RECTIFIED PHOTOGRAPHY?
In its most basic form it is a photograph ‘with
the image plane of the camera approximately
parallel to the principal plane of the object
and then printed to scale’ (Bryan et al 2007).
In other words the effects of distortion by
perspective are removed and the image scaled
upon one or more principal planes of the
subject. Measurements taken off the rectified
image are only accurate on these scaled
planes. A good example would be the front
elevation of a typical Georgian terrace town
house. Once the perspective is removed, the
image is scaled on the flat brickwork of the
facade. Measurements between window cuts
would be accurate. Measurements taken on
the projecting cornices would be too big, those
on a recessed fan light too small. These single
images can be formed into mosaic composites.
HOW IS IT PRODUCED?
Until recently rectified images were produced
in the darkroom. The advent of higher quality
and more affordable digital cameras has made
the techniques easier and more flexible.
When rectified photography is
commissioned, first decide what scale the final
output will be. The greater the scale, the more
detail will be visible but more images will be
required. More photos equals higher costs,
which may be unnecessary for the work in
To some degree, every photograph contains
distortions: most obviously perspective
and barrel distortion. Some of these can be
removed by the use of high quality ‘shifted’
lenses. Each picture should be taken square
on to the subject and if possible the effects
of perspective removed by shifting the lens
accordingly. Good even lighting conditions are
desirable to reduce shadows to a minimum or
eliminate them altogether as they represent
a loss of information, and can result in
extra work for either the surveyor at the
time or possibly an end-user. However, ideal
conditions are unattainable, and the
images may be slightly oblique and/or tilted.
This results in more images being taken on
site and will certainly result in more work in
|Measured survey drawing and integrated rectified image of the stone floor of the nave of Staunton Harold Church: the detail to the right is taken from the same image,
illustrating the amount of detail recorded.
The choice of camera will depend on
the quality and end-use of the image required.
More expensive tripod-mounted professional
cameras, the dedicated software that
accompanies them and high quality lenses will
produce impressive images with amazing
resolution. The example of a church floor
mosaic, above, illustrates the point: the user
can see the whole area or zoom in to assess
the condition of one individual stone slab.
However, perfectly usable images of simple
subjects can also be produced with a compact
digital camera. The resolution will not be as
good and there will be more distortion, but if
the end result is suitable for its use then there
is no need for a more expensive solution.
The measurements or ‘control’ needed
to scale and fit single images together into
a mosaic can be collected in different ways.
Taped dimensions may be fine across grave
slabs, but for the elevation of a cathedral,
electronic distance measurement (using
reflectorless EDM) may be the only practical
solution. Four points are required for
removing distortion and to scale the image.
Photographs can be rectified in different
software packages including Photoshop or
more dedicated programs like Phidias or
Photoplan. The final images are often imported
into CAD software, and borders, text and
annotations may be added before printing.
HOW IS IT USED?
The cost of producing rectified elevations is
relatively low compared to other aspects of
conservation work and can save a great deal
of time for other specialists. The images give
a good overall picture of the subject before
being obscured by scaffolding, and a surveyor,
architect or archeologist can annotate the
elevation once scaffolding is erected.
|Rectified image of Holy Trinity, Anslow, Staffordshire,
including specification annotations added for the
conservation and repair by Alan Gardner Associates
Areas of surface requiring repair or
conservation can easily be measured and
calculated from the images. These should be
accurate enough for a schedule, pricing and
on site repair. For example, rubble-built walls
can be costly to have drawn stone by stone,
but individual stones will be clearly visible
in a rectified photograph and areas requiring
repointing, consolidation or replacement can
be marked up on the image and measured.
Buildings archaeology phasing information
can also be added either to the same image or
to another copy.
The images provide a less subjective record
than a drawing, as the use of line and shade to
indicate features relies on interpretation and
experience. A rectified image of an elevation,
floor or wall painting will show not just
the outline of a stone block or area of lime
plaster but clearly the damage to the block or
fragment of wall painting on the plaster. The process is non-contact and is ideal for
fragile surfaces or structures.
The same images can also be enhanced
for visualization purposes. What, for example,
would that streetscape of early Victorian
terrace houses look like if all the mismatching
or uPVC front doors and windows were
replaced through a grant from a townscape
heritage initiative? Similar imagery can be
used for interactive computer-based site
interpretation or reconstructions.
This cost-effective survey method can
be used to great effect on different subjects,
providing a great deal of information for
many different specialists and the general
public. While it has limitations, it has been
proven by many people to be another valuable
way of recording and surveying in the
conservation of historic buildings. As with
any other method, including laser scanning
or stereo photography, it should only be used
where appropriate, and it should be avoided
if another easier technique can be used to
similar effect. It does, however, produce a
massive amount of clearly understandable,
easily readable information.
- DP Andrews et al, ‘Photographic Survey of
Mosaic and Tiled Floors – a Methodology’, New Perspectives to Save Cultural Heritage,
CIPA 19th International Symposium, 2003
- D Ayala et al, Minimum requirement for
metric use of non-metric photographic
documentation, University of Bath (report), 2003
- JEO Brennan, ‘The use of rectified
photography at the Painted Hall’,
Conservation News, issue 87, 2003
- P Bryan et al ‘Digital rectification techniques
for architectural and archaeological
presentation’, Photogrammetric Record,
16 (93), 1999
- CM Clark, Informed Conservation: Understanding Historic Buildings and their Landscapes for Conservation, English
Heritage, London, 2001
The National Trust,
Rodney Melville, Richard Stiles, Alan Gardner,
Alan Gardner Associates, Dan Martin, Biscoe
Craig Hall and the Church of the Holy Trinity,
Sloane Square, London.
article is reproduced from The Building Conservation Directory, 2008
JAMES BRENNAN is a chartered surveyor who
first trained as an archaeologist. He is now
the director of James Brennan Associates specialising in surveying,
photographing and assessing the historic environment.
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