off-the-shelf close-range photogrammetric software for cultural

International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXVIII, Part 5
Commission V Symposium, Newcastle upon Tyne, UK. 2010
OFF-THE-SHELF CLOSE-RANGE PHOTOGRAMMETRIC SOFTWARE FOR
CULTURAL HERITAGE DOCUMENTATION AT STONEHENGE
A. M. Wojtas a, *
a
Completed while at Warsaw University of Technology, Poland – agnieszka.wojtas@yahoo.pl
Commission V, WG V/2
KEY WORDS: Close range photogrammetry, camera calibration, cultural heritage, PI-3000, Stonehenge
ABSTRACT:
This paper is a summary of a student internship with English Heritage's Photogrammetric Unit. During this placement affordable
solutions for heritage close-range photogrammetric measurement were examined. Topcon's Image Surveying Station PI-3000
photogrammetric software was used on a project to record carvings found on one of the stones at Stonehenge. Analysis of this
carving (and others) is essential in order to understand Stonehenge's history as well as to help to conserve its condition. An original
survey of this carving was carried out in 1967 (Atkinson, 1968) providing context against which this survey using PI-3000 could be
compared to in the future. In preparation for this work a comparison of camera calibration techniques (using PI-3000 and
PhotoModeler) was carried out.
Creating a 3D model using the photogrammetric method based on a bundle adjustment requires knowledge of camera calibration
parameters. Photogrammetric camera calibration parameters allow for the recovery of the central perspective bundle. They can be
determined during a self calibration process if using appropriate software. It was decided to assess the camera calibration provided by
PI-3000. Therefore, to test the software, a Canon IXUS 900Ti was calibrated using PI-3000 and PhotoModeler. The paper presents the
results of these calibrations and compares the two software packages.
PI-3000 was then used to document a single carving found on inner face of the 53rd stone of Stonehenge. The outcome could be
compared with the survey performed in 1967. Around ten images were collected with a calibrated Cannon A640 camera. Scale was
introduced to the scene using a ruler laid against the stone. PI-3000 was then used to produce a meshed surface of the carving. Results
will be presented.
The paper concludes that off-the-shelf photogrammetric equipment and software could be utilized in commercial projects to provide
heritage recording. However, principles of close range photogrammetry still needed to be understood in order to produce high quality
results.
According to some theories its stone's alignment derives from
religious or astronomical origins (English-Heritage, 2010).
1. INTRODUCTION
This paper reports on work undertaken during an internship with
English Heritage's Photogrammetry Unit in 2007. The principal
aim was twofold. Firstly, to investigate the calibration of nonmetric cameras using Topcon's PI-3000 'Image Surveying
Station' software and EOS System's PhotoModeler. Secondly, it
was to use these cameras to generate a DSM (Digital Surface
Model) of carvings visible on the fifty third stone at Stonehenge
at a contour interval of 0,5 mm. These carvings were discovered
by R. J. C. Atkinson (reported by Bryan and Clowes, 1997) in
the early 1950s and later recorded photogrammetrically using
analogue methods (Atkinson, 1968).
Detailed survey and analysis is necessary to understand
Stonehenge's historic roots as well as to help to protect the site.
Three dimensional reconstruction plays a significant role in this
research and in the protection of other ancient cultural sites
(Zheng et al., 2008). Rapidly improving and increasingly
available survey techniques suited to the measurement of
archaeological digs and stone carvings, such as those found at
Stonehenge, now allow people to obtain more information
about this great 5,000 year old monument, as well as about
people living during the time when Stonehenge was erected.
1.1 Surface measurement for Cultural Heritage
The collection of surface information, which could be used for
the monitoring of surface condition, is one of two important
issues concerning the three dimensional reconstruction of small
objects. The second is texture mapping of the surface, which
could provide a photorealistic three dimensional model.
Accurate measurement and a photorealistic surface model are
Stonehenge, a UNESCO World Heritage Site, is located on
Salisbury Plain in Wiltshire (Southern England). It dates from
prehistoric times being raised between 3,000 BC and 1,600 BC.
* Author now works for the Forestry Commission, Bristol, UK.
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International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXVIII, Part 5
Commission V Symposium, Newcastle upon Tyne, UK. 2010
The accuracy of a digital photogrammetric survey is composed
of a number of factors:
•
Higher resolution imagery means features can be
more precisely located.
•
A more accurate reconstruction of the light bundle is
possible if the parameters of the camera are known
(camera calibration – see section 2.1 below).
•
Intersection angles of 90 degrees are optimal, but
could have a negative impact on image matching
performance in some cases.
•
Multiple images per feature (above the minimum of
two) will improve accuracy if they strengthen the
image network.
•
Illumination angles and object radiometric texture
should be sufficient to allow the application of digital
image measurement techniques, improving the
likelihood of sub pixel measurement.
•
In most cases, even when using fast shutter speeds,
the camera should be situated on a tripod to guarantee
stability and limit the effects of vibration on image
sharpness.
necessary for an object's reconstruction process. The required
accuracy of a survey is generally a function of an object's/site's
size, but in the case of small carvings, which are likely to
experience erosion over a number of years, the resolution must
be around 1-2 mm, while the accuracy should be 0,5 mm or
better (Remondino and Menna and, 2008).
1.2 Close range measurement techniques
One of the many approaches now available to measure a small
surface is the application of close range photogrammetry
techniques (Martos et al., 2008) using digital technology. A
close range photogrammetric survey is recognised as a
measurement executed by a camera situated up to 300 m away
from an object (Wolf, 1983).
1.3 Image capture
Digital close range photogrammetry, in contrast to analogue
photogrammetric methods and other more complex and
expensive solutions, is now relatively easy to apply (Castro et
al., 2004). Its ease of use arises from improvements in the
performance of off-the-shelf digital cameras, increasing facilities
for storing and transferring image data, the ease of image
capture (without any need to use paper prints or digitising
films), as well as the decreasing price of digital cameras. This is
complemented by increasingly easier to use software for camera
calibration and measurement, such as PI-3000 (Topcon, 2010),
PhotoModeler (PhotoModeler, 2010) and others.
To provide the image network with scale, a known distance,
visible on all images, needs to be known. It is recognised that
the simplest and least expensive method of providing such
control is to employ a single scale bar, which allows data to be
extracted and output to a known scale (Bryan and Chandler,
2008).
1.4 The process for surface generation
Therefore, close range photogrammetry now allows the
recovery of three dimensional measurements with a high
accuracy and it can be utilized by qualified photogrammetrists,
and non-expert users. The collection of images can be very
quick, limiting the time required for fieldwork. However,
traditional photogrammetric theory still needs to be employed.
For example, there is a need to ensure an accurate knowledge of
image geometry.
Following collection of the images, software is used to orientate
the images by measuring corresponding points. This preliminary
process needs to be completed before the collection of three
dimensional measurements and orthophoto generation.
1.5 Off-the-shelf photogrammetric software
There are now many software packages that could be used to
execute image orientation, surface measurement and three
dimensional modelling with very little user interaction or
previous knowledge of photogrammetry. During this internship
Topcon's PI-3000 was applied. PI-3000 has been applied to the
measurement of small objects, historic monuments and
buildings (Bryan and Chandler, 2008).
For the reconstruction of an object it is necessary to measure a
number of three dimensional points on its surface. The most
common way of achieving this, for surfaces in a single plane, is
to use stereo-pairs, traditionally with a 55% - 60% overlap, to
provide a stereoscopic view. Although for small objects one
stereo-pair is often sufficient, with the advent of digital image
matching techniques multiple stereo-models can now be used to
obtain high accuracy results. Zheng et al. (2008) note that
application of multiple stereo-models with short baselines
guarantees continuity of the feature on the images as well as
precision of the intersection. While Wackrow and Chandler
(2008) note that convergent imagery eradicates the impact of a
slightly inaccurate lens model, which improves accuracy leading
to recommend the acquisition of mildly convergent stereoimagery (Bryan and Chandler, 2008).
PI-3000 is designed for deriving three dimensional
measurements from stereo-pairs. It also allows the generation of
orthophotos and three dimensional models. Measurements are
collected in stereoscopic mode, from two images taken with
different angles – left and right, presenting the object of interest.
It is also possible to execute the survey and object in
monoscopic view. PI-3000 is quoted by Topcon as having the
capability to extract coordinates for well defined points with 0.4
mm accuracy for imagery taken from 10 m distance from an
object (Topcon, 2010).
For the purpose of photogrammetric survey, a selection of
commercial digital cameras could be utilized. A consumer grade
digital camera will often be supplied with an auto-focus lens and
a telescopic lens. Chandler et al. (2007) provide guidances on
using such cameras: “By adopting the widest zoom setting and
maintaining a camera-object distance > 0.5 m, both the
largest object coverage is provided and the required camera
calibration procedures are simplified”.
2. CAMERA CALIBRATION
2.1 The importance of camera calibration
For an accurate photogrammetric survey it is necessary to
determine key parameters of the camera in use. These are
required by the basic photogrammetric approach that uses
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International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXVIII, Part 5
Commission V Symposium, Newcastle upon Tyne, UK. 2010
intersection based on the bundle method for determining the
relative and absolute orientation of images and estimates point
coordinates.
It was decided to compare the results of a camera calibration
procedure using PI 3000 and PhotoModeler in order to
understood the consistency between the packages and improve
confidence in their use. A Canon IXUS 900 was selected for this
test, being a good example of a high resolution consumer grade
digital camera.
In order to determine the camera's true parameters it is essential
to undertake a camera calibration process, generally before
performing the main survey. Not knowing those parameters
could lead to inaccurate measurements or even not obtaining
any solution at all. Camera calibration parameters may also be
determined in a self calibration process while executing the
survey if using the appropriate software. In that case additional
adjustment points and more images should be collected.
A tripod was used to ensure image stability. The camera was
placed in front of the test field so as to image all of the control
points on the chart clearly. The same chart was used for both
software packages. The chart was flat and had 165 control points
marked in a regular pattern. A number of images were collected
(Figure 1) following the guidelines for each software package.
The parameters derived from a camera calibration are:
•
The principal distance (PD)
•
Principal point offset (x0, y0)
•
3 parameters for radial distortion (K1, K2 K3)
•
2 parameters for tangential distortion (P1, P2)
•
Terms for affinity and non-orthogonality
Although camera calibration is not a complicated task using offthe-shelf software, attention to it is required.
Figure 1 – Examples of images collected during the calibration
process.
2.2 Camera preparation for calibration
When compared with metric cameras, the distortion of
commercial off-the-shelf camera lenses can vary greatly due to a
constantly changing principal distance. Therefore, a camera
requires calibration for a given principal distance. Generally, for
an acceptable range of object sizes and distances, the best focal
length setting is the wide angle one. Automated camera features
should be deactivated as far as possible, and the camera's auto
focus should also be turned off.
The key difference between the two procedures was the image
network recommended by the software. For PI-3000, the chart
was placed on a stand and five images of it were collected from
different angles (front, left, right, top and bottom). Whereas for
PhotoModeler, the chart was placed on the floor and two images
were collected from each of the chart's four sides (the camera
was rolled by 90 degrees at each location). In both cases image
capture was quick and straightforward and could easily be
followed by a non-expert user.
2.3 The camera calibration processes in PI-3000 and
PhotoModeler
The results of the calibration procedure are presented in Table 2.
PI-3000 (through the PI-Calib extension) and PhotoModeler
offer automatic image measurement procedures for calibrating
cameras. During this step, crosses on a grid are extracted
automatically, as control points. The results of the image
measurement process are fed into a bundle adjustment in order
to solve for the exterior and camera calibration parameters. The
approaches required by each of these packages are compared in
Table 1.
PI - 3000
Parameter
PhotoModeler
Image points occupying large part of a chart.
Minimum of 5 images.
Minimum of 8
(multiple angles).
Requires an estimated focal
length.
Estimates focal length itself.
PD (mm)
7,8065
7,7984
X0 (mm)
3,5234
3,5084
Y0 (mm)
2,7221
2,7203
K
1
2,43E-003
2,47E-003
K
2
-2,94E-005
-3,22E-005
P
1
-6,87E-005
-2,58E-005
P
2
1,32E-005
-2,18E-005
3. SURVEY OF STONEHENGE
Tripod necessary.
Manual measurement of
four control points required.
PhotoModeler
Table. 2 – A comparison of PI-3000 and PhotoModeler camera
calibration results.
images
Fully automatic process –
program
measures
all
control points itself.
PI-3000
3.1 Field work
For the purposes of the survey at Stonehenge a Canon A 640
was made available. The camera was calibrated using PI 3000
prior to the survey taking place.
The aim of the survey was to measure the shape of the carving
(often interpreted to be a dagger) found on the 53rd stone of
Stonehenge and to extract a surface model with contours at
Table. 1 – A comparison of PI-3000 and PhotoModeler camera
calibration guidelines.
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Commission V Symposium, Newcastle upon Tyne, UK. 2010
intervals of 0,5 mm. Previously, it had been measured using
analogue methods (Atkinson, 1968) .
Field work at Stonehenge started early in the morning, with
bright illumination (Figure 2). A scale was introduced to the
scene by placing a ruler at the bottom of the stone, and the
carving of interest was photographed with the Canon A640. This
provided the survey with the known distance required during
further processing in PI-3000. Cameras were placed on a tripod
in order to ensure image stability. A number of images were
collected, provide multiple stereo-pairs. Thanks to early start, the
field work was carried out in around 30 minutes before other
field projects began.
Figure 4 – Wireframe output following the surface extraction.
4. SUMMARY, CONCLUSIONS AND FURTHER WORK
4.1 Summary
The use of images obtained with digital non-metric cameras to
the three-dimensional photogrammetric measurement of objects
is currently of great interest (Rodriguez et al., 2008).
This paper has outlined work to record a carving on one of the
stones at Stonehenge. It has discussed the use of low cost digital
cameras and off-the-shelf software for photogrammetric
recording. It has briefly compared the results of camera
calibration using PI-3000 and PhotoModeler and described the
main survey. The final output has been presented.
Figure 2 – Fieldwork at Stonehenge.
3.2 Surface extraction and results
4.2 Conclusions
The images were loaded into a new PI-3000 project and the
previously determined camera calibration parameters were
added. The ruler in each of the images was manually measured
and the known distanced entered. Common points in the images
were also identified and the PI-3000 solution was executed. The
result, without further user interaction, was a three-dimensional
surface, with an optional texture map. The process was
completed in a matter of minutes and the results are shown in
Figure 3 and Figure 4.
PI-3000 has a very user-friendly interface and, along with
PhotoModeler, is relatively low cost when compared to other
specialist photogrammetric systems. These packages make
camera calibration straightforward, and the results, obtained
during a like for like comparison following the manufacturer's
guidelines, are comparable.
The output of the survey was a realistic and detailed model of
the carving in question. This further supports the use off-theshelf digital cameras for such measurement tasks, which is likely
to lead to a reduction in the cost for the survey of such features,
and an increase in the number of projects applying threedimensional recording.
However, knowledge of basic photogrammetric principles is still
required to understand the results, even if such results are often
hidden from the non-expert user. Likewise, knowledge and
experience would be required to overcome any problems arising
– highlighting the need for the specialist skills of the metric
survey professional.
4.3 Further work
Although the manufacturer claims that PI-3000 could provide
an accuracy of 0.4 mm at 10 m stand-off distance, this paper
has not assessed the final accuracy of the survey. Instead, it
has concentrated on the process used to generate the model.
An assessment against a control object or survey of a known
accuracy of least 0.04 mm is required to do this.
Figure 3 – Textured output following the surface extraction.
If required the surface model could be exported into commonly
used data types by other software, such as for example DXF or
VRML.
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Commission V Symposium, Newcastle upon Tyne, UK. 2010
Topcon, 2010. PI-3000.
http://www.topcon-positioning.eu/index.asp?
pageid=ead8cb2a181e4afbbab9f9b7b1161cde. Last accessed 30th
April 2010.
Similarly, comparison with the 1967 survey was not carried
out as the results were not available digitally at the time. If
this were to be done, the lack of common control is likely to
require the use of surface matching techniques. These would
also be required to compare future surveys.
Wackrow, R., Chandler, J.H., 2008. A convergent image
configuration for DEM extraction that minimises the systematic
effects caused by an inaccurate lens model, Photogrammetric
Record, 23(121): 6-18.
5. ACKNOWLEDGEMENTS
The author gratefully acknowledges the assistance of Paul Bryan
and the rest of the former Metric Survey Team at English
Heritage.
Wolf, P.R., 1983. Elements of Photogrammetry. pp. 477 – 513.
Zheng, J., Yuan, W., QingHong, S., 2008. Automatic
Reconstruction for Small Archaeology Based on Close-Range
Photogrammetry, The International Archives of the
Photogrammetry, Remote Sensing and Spatial Information
Sciences, Vol. XXXVII. Part B5.
Support from the Life, Earth and Environmental Sciences
Standing Committee (LESC) of the European Science
Foundation made this poster/oral presentation possible
(www.esf.org/lesc).
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