SNAP: A SYSTEM FOR NON-METRIC ARCHITECTURAL PHOTOGRAMMETRY 
Petros Patias" and. Dimitrios Rossikopoulos** 
The Aristotle University of Thessaloniki 
; School of Rural and Surveying Engineering 
Department of Cadastre, Photogrammetry and Cartography 
LE 
Department of Geodesy and Surveying 
ISPRS Commission V 
ABSTRACT 
Architectural Photogrammetry can be based on inexpensive machinery like non-metric cameras, small photogrammetric 
instruments or even. digitizers, a small computer and inexpensive output devices. SNAP offers a solution to architects, archa- 
eologists and surveying engineers for the capturing, processing, documentation and archiving of accurate information concern- 
ing monuments, sites and structures of special interest. 
Data can be input to SNAP either by a small photogrammetric instrument or by a digitizer. The photogrammetric processing is 
based on photographs captured by amateur cameras, although it can take advantage of the geometrical stability of metric ca- 
meras, if they exist. The photogrammetric adjustment is a self-calibrating bundle adjustment with photo-variant additional pa- 
rameters and the user can optionally use surveying observations of ground control points coordinates, measured distances 
and angles or enforce constraints like known distances, parallel lines, perpendicular lines, points on circle or arc etc. The output 
is driven to DGN (Intergraph) or DXF (Autodesk) format for use by popular graphic editors. 
KEY WORDS: Non-metric, Architectural Photogrammetry, Photo-variant parameters, Combined adjustment 
1. PHOTOGRAMMETRIC DOCUMENTATION OF 
MONUMENTS AND SITES 
Despite the efforts of many years, the recording of the 
majority of buildings, monuments, structures and sites of 
our architectural heritage is far behind schedule. 
Photogrammetry can offer quick, accurate and economical 
solutions to this urgent demand. The equipment to be used 
and the procedures to be followed has been very much refi- 
ned over the past years, so that the required cost and the 
user experience is kept to minimum levels (Waldhäusl, 
1987). In order to achieve this the use of low-cost compu- 
ter-based photogrammetric instrumentation and sophistica- 
ted and universally applicable procedures (have or) are 
been developed. 
1.1 Equipment 
Analytical Photogrammetry is not any more bounded by 
the restrictions characterizing the good old Analog 
Photogrammetry. As far Architectural Photogrammetry 
concerns even the requirement of universal analytical plot- 
ters is relaxed, giving ground to smaller analytical systems, 
or even to digitizers and mouses as input devices. 
Of course metric cameras have a lot of advantages like 
interior stability and large format but their cost, discomfort 
in use and discontinuity in development leads more and 
more to the use of semi- and non-metric cameras 
(Waldhäusl et.al., 1988 and 1989). 
1.2 Procedures 
Since the time that Analytical Photogrammetry replaced 
the opto-mechanical rods with analytical relations, there 
are no more restrictions on photo configurations, while at 
the same time the development of universal software made 
possible. In the case of non-metric cameras the less rigou- 
rous DLT method has been widely used, whereas adjust- 
ment of bundles can offer solutions to a wider range of 
applications including both metric and non-metric photo- 
graphs. 
The error burden introduced by the use of non-metric pho- 
tographs (especially that due to film distortion) is quite dif- 
ferent from that of metric photos. Therefore the use of 
self-calibration including photo-variant parameters has 
been looked into more closely. 
While it is known that in order to compensate for systema- 
tic errors, redundant control and extensive experience is 
required, the confinement to as little control and 
experience as possible is highly desirable. This is due to the 
fact that many disciplines other than surveyors (eg. 
architects, archaeologists, historians, geographers) are now 
involved in documenting historical sites. These people know 
very little about surveying measurements and normally 
lack surveying equipment. 
2. THEORETICAL ASPECTS ON 
PHOTOGRAMMETRIC DATA PROCESSING - THE 
SNAP APPROACH 
Summarizing the state-of-the-art in equipment, procedures 
and requirements of Architectural Photogrammetry, we 
should note that: 
- The data are collected with analog metric, semi-metric 
and non-metric cameras as well as digital cameras and 
scanned photos. 
- The control can be an extensive number of geodetic coor- 
dinates or simply a few taped distances, just enough to de- 
fine a reference frame (Waldhausl and Peipe, 1990). 
- The measurements of photo coordinates are performed 
with an analytical plotter, a smaller analytical photogram- 
metric instrument, a digitizer or a mouse. 
- The pre-processing of the data includes incorporation of 
the camera calibration report (if it exists) and correction of 
lens and film distortion. 
- The adjustment of the measurements is based on bundle 
adjustment with the use of additional parameters for the 
compensation of the remaining systematic errors. These 
parameters are either photo-variant (for non-metric pho- 
tos) or photo-invariant (for metric photos).