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In the case of Clifford's Tower a Cyrax 2500 laser 
scanner was chosen with a quoted point measurement 
accuracy of —/- 6mm (16). The manufacturer's technical 
specifications of the system were provided as part of the 
method statement. In addition, certification that the 
scanner is in good working order is required. The 
standard testing of scanning instruments is currently an 
area of much research. No standard test is available to 
provide an indication of if the scanner is working as 
required. In this case an array of scanner targets were 
scanned and compared against surveyed measurements. 
The scanner was shown to be operating to within its 
stated precision. To ensure quality control in the future 
use of laser scanning it is essential that the next 
generation of laser scanners are provided with certificates 
after cach (regular) service showing they are operating 
within their stated technical specification. 
3.3 Survey methodology 
In pre-planning a total of four primary scanning stations 
were estimated to provide 90% coverage of the tower. 
This includes areas at high level and required no platform 
to allow access. However, approximately one metre at 
the base of the tower is hidden by the motte when viewed 
from ground level. In order to eliminate these data voids 
a number of fill-in scans were required. The problem 
was compounded by the 40 x 40 field of view afforded by 
the Cyrax 2500. Fewer scans would have been required 
if using a scanner with a 360 degree field of view. In 
order to account for this a standard topographic survey 
was also undertaken in order to ensure a full record of the 
motte. This is given in Figure 3. The outline of the 
tower was created from the scan data, whilst the 
remaining survey was collected using a total station. 
Additional control stations were established on the top of 
the motte where necessary and linked to the pre- 
established site control. Each scan contained at least four 
reflective control targets with an average precision of 2 
mm in X, Y and Z axes. Surface matching of 
overlapping scans was not used. 
  
Figure 3. Topographic Survey with the Tower outline 
recorded using scan data. 
Data voids, due to temporary obstructions such as 
vehicles and pedestrians moving during the scanning the 
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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
scan, were minimised by the selection of appropriate 
scanning stations and survey times. 
  
Figure 4. A scan of the tower from street level. 
A standard form was used to record information such as 
control points names and the weather during each scan. 
With some additional effort such details could be 
recorded with the scan data improving metadata 
management. For the most part the weather remained 
dull and overcast during the survey. The addendum 
prevents scanning from taking in place in weather 
conditions that may affect the quality of the collected 
scan data. However, although light rain did occur on one 
occasion it was not considered sufficient to prevent 
scanning taking place. Additional data to aid 
interpretation was collected throughout the survey. This 
included high resolution photography and sketches. For 
example, on the North East lobe an area of brick work 
that could have been misinterpreted as an error in the 
scan registration was supplemented by additional 
narrative photography. 
  
Figure 5. Potential mis-registration