DIGITISING TABlE CALIBRATION USING ClOSE RANGE
PHOTOGRAMMETRY
Kevin Jones and Basil Pathe
Sehool of Surveying
Queensland University of Teehnology
BRISBANE QUEENSLAND 4000
Commission V
ABSTRACT
Throughout the surveying and mapping industry there is a proliferation of digitising tables being used to aequire data for Geographie Information
Systems. In many work situations the digitising tables receive only a eursory ealibration, if they reeeive one at all. Users generally rely on GIS
system software to make the appropriate eorreetions to the data when fitting to eontrol. To obtain the most reliable and accurate information,
however, all phases of a system should be calibrated. The paper describes a method of calibration using close range photogrammetry via the
bundle reduction teehnique and discusses the results of tests carried out.
KEVWORDS:
Calibration, Cartographic, Close-range, GIS/LlS, Photogrammetry
1.
4.
INTRODUCTION
Two sets of digitised coordinates of the grid intersections were
acquired for this project. For convenience they were labelled Dig 7
and Dig 10. A description of the data sets follows below.
The question of digitiser calibration arose while the authors were
developing a data acquisition system using manually digitised aerial
photographs. As the method depended on digitised coordinates, it
was essential that the accuracy of the equipment was verified. Of
course, this is in keeping with the sound and practical expectations
that every piece of equipment used for precise seientific measurement
must be calibrated to provide full information on the accuracy
attributes of the data.
4.1
The precision/repeatability of the measurements was 0.06mm (one
sigma).
This paper is a description of the project and its outcome.
Dig 10 Data Set
4.2
DIGITISER CALIBRATION
The Y.:{ coordinates of this data set are the means of 10 eonsecutive
measurements over eaeh intersection of the grid. That is, eaeh grid
intersection was measured 10 times, with the cursor being moved off
the intersection after each individual measurement. When 10
measurements had been made on a particular grid intersection, the
cursor was nioved to the next interseetion, and the procedure
repeated. The whole grid was measured once.
The digitiser being used was an ARISTOGRID table digitiser MODEL
GRT108 with a measuring area of 915mm x 1220mm, and a quoted
resolution!repeatability of 0.025mm (1/1000 inch). The measuring
process uses the principle of absolute coordinates. The sensor can
be lowered or raised from the digitising surface at any time without
affecting measurement aceuracy. We had never experienced any
problems with this digitiser and in fact it had a 'fee I' of reliability and
accuracy. There were, however, other digitisers within the University
which had exhibited a gradual creep in 'size' over a number of years.
The precision/repeatability of the measurements was 0.04mm (one
sigma).
The procedure proposed was to manually digitise a reference grid,
use elose range photogrammetry to coordinate the reference grid,
and then relate the digitised coordinates to the photogrammetric
coordinates. At this stage the reference grid was more of a
convenience as a set of targets and transfer medium than as a
calibration grid. The antieipation was that the photogrammetric
coordinates would agree with the reference grid coordinates.
3.
Dig 7 Data Set
The Y.:{ coordinates of the data set are the arithmetic mean of 7
sequential sets of measurements over the grid. That is, after a grid
intersection was measured the cursor was moved to the next
intersection, and then the next, until all grid intersections were
measured. Then the whole grid was measured again, for a total of 7
times. The arithmetic mean of each grid intersection was determined,
giving one set of data.
At the time, the School of Surveying had just acquired a Rollel 6006
metric camera. As all the tools, including analytical plotter and
software, were conveniently available, it was decided to use close
range photogrammetry to calibrate, or verify, the calibration of the
digitiser being used.
2.
DIGITISER COORDINATES OF GRID INTERSECTIONS
5.
5.1
PHOTOGRAMMETRIC COORDINATES OF GRID
INTERSECTIONS
Outline
The coordinates of the grid intersections were determined
photogrammetrically by the bundie method. The reference grid was
fixed to the digitising table with tape and three one metre long scales
helped keep it flat as weil as providing control for the project. The
scales were positioned so as not to obscure any of the grid
intersections.
REFERENCE GRID
A reference grid on film was purchased from a Queensland
Government Department. It was a 1cm grid with an overall size of 1m
x 1m wh ich did not exactly coincide with the digitiser measuring area
It was decided to carry out the calibration of the digitiser at 100mm
Intervals even though it extended beyond the recommended
measuring area along one side; a total of 121 points. The 100mm
grid intersections were high-lighted with marking pen to avoid
misidentification.
5.2
Photography
Photography was taken with a Rollei 6006 metric camera with a 50mm
lens using black and white film. Lighting was normal room lighting
consisting of standard fluorescent lights. Reflections obscuring the
217
grid intersections were not a problem, except in only a couple of
instances. Nine regularly spaced camera stations in classical
configuration approximately 2 metres from the digitiser were used to
acquire full coverage over the grid (refer to Diagram A). The camera
was mounted on astand, and coverage confirmed before exposing
the film. The film was processed through a public commercial
laboratory.
the plane of the digitiser) was about 0.2mm. Sigma nought for image
measurements was 5 microns, which was considered satisfactory for
this project.
6.
The digitised coordinates (Dig 7 and Dig 10) were shifted and rotated,
but not scaled, to fit the nominal values (even 100mm) of the
reference grid. The X and Y differences between the transformed
coordinates and the nominal grid values were determined and 0.1 mm
contours of the separate X and Y differences plotted. These results
are shown in Figures 1 to 4.
r
As can be seen from the plots, the two manual digitiser results are
similar to each other, but differ from the reference grid by a
considerable amount; some half a millimetre in both X and Y
directions. At this stage it appeared there was a simple uniform scale
error in the digitiser.
0
Next the photogrammetric coordinates of the intersection points were
compared with the nominal values of the reference grid. 0.1 mm
contours of the X and Y differences are shown in Figures 5 and 6.
The differences are similar to the manual digitiser results; some half
a millimetre in both X and Y directions.
Dlgltiser
1
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ANALYSIS OF COORDINATES
Contours of the X and Y differences between the manual digitiser
coordinates and the photogrammetric coordinates are shown in
Figures 9 to 12. As could be expected, the results are in agreement
with the differences, and more or less consistent with the
photogrammetric accuracy of 0.1 mm.
~
Camera station
and directlon
The scale tor the photogrammetric coordinates was derived
independently from metal scales and not from the reference grid. The
conclusion to be drawn is that the reference grid contains a scale
error.
Finally, the nominal grid coordinates were conformally transformed to
fit the photogrammetric coordinates. The scale of the grid was
calculated as 1:0.9993. Contours of the X and Y differences are
shown in Figures 7 and 8.
At this stage the projeet appeared to change from one of digitiser
calibration to one of reference grid calibration.
7.
CONCLUSIONS AND RECOMMENDATIONS
The projeet was successful in that we verified to 0.1 millimetre, the
measuring accuracy of our digitising system.
However, the projeet highlighted a problem with our ·standard"
reference grid. Had we adopted the reference grid as correct then
we would have mistakenly thought the digitiser to be in error.
Diagram A: Camera-Digitiser Geometry
5.3
A solution to the calibration problem for a large table digitiser would
be for the manufacturer to mark a grid on the table itself in addition
to boundary marks, similar to the grid etched on a photo carrier of an
analytical plotter. The calibration of the permanent grid could then be
done efficiently and conveniently by close-range photogrammetry.
Image Coordinates
Image coordinates of the grid intersections were measured on a Zeiss
Planicomp C1 00 analytical plotter using the black and white negatives.
Lens distortion, but not film unflatness, was allowed tor during the
measuring procedure.
5.4
8.
Bundle Adjustment
ACKNOWLEDGMENTS
The authors wish to thank Mr Andrew O'Dempsey of the Redland
Shire Council, Queensland, for his input while he was a research
associate with the Queensland University of Technology.
Grid intersection coordinates were determined by bundle technique
using the image measurements of the nine photographs. The
adjustment was constrained by holding the central grid point fixed
together with one other point for "azimuth". The digitising table was
assumed to be reasonably flat and a number of height points were
loosely held. Scale control was provided through measurement on
three one-metre scales.
9.
REFERENCES
Burrough, P.A., 1986. Principles of Geographicallnformation Systems
for Land Resources Assessment. Clarendon, Oxford.
5.5
Rollin, J.R., 1986. A Method of Assessing the Accuracy of
Cartographic Digitising Tables. The Cartographic Journal Vol.23 NO.2.
The result of the bundle adjustment was a set of coordinates tor the
grid intersections. The one sigma level of accuracy for X'{ (in the
plane of the digitiser) was about 0.1 mm while for Z (perpendicular to
Slama, C.C. (Ed), 1980. Manual of Photogrammetry 4th Edition.
American Society of Photogrammetry, Falls Church, USA.
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