In: Wagner W., Szekely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
4. ISSUES 
The experience of using lidar data through these projects has 
also revealed a number of issues relating to the data that need to 
be understood by anyone intending to use lidar for 
archaeological survey. These include the question of resolution 
and data formats, as well as issues related to using the data in 
day-to-day survey, ranging from producing viable hard copy 
printouts for use in the field, to mapping with fully interactive 
raster surfaces in CAD and GIS. 
4.1 Data capture 
One of the key elements that needs to be understood when using 
lidar data is the question of resolution. This is a twofold issue; it 
relates not only to the actual final resolution at which the data is 
displayed, something that limits the size of the features that can 
be seen and recorded, much in the same way as for other image- 
based data, such as satellite or standard aerial photography. It 
also relates to the original resolution of the data defined by the 
number of hits within a square metre and the footprint of the 
laser beam when it strikes a surface. A combination of these 
two elements can mean that although an average of one point 
per metre is nominally collected, it is possible for small features 
to be entirely missed by the survey and not recorded. A good 
example of this, illustrated in the guidance note, is shown by 
the bluestones at Stonehenge in that the lidar data captured at 
one hit per square metre does not appear to show several of the 
bluestones (Crutchley & Crow 2009, pi6). 
Whilst it is unnecessary for the user to understand everything 
about the techniques used during data capture and initial 
processing, it is important to be aware of the stages of 
processing the data have been put through, as these can result in 
data artefacts that can be misleading. These issues are discussed 
in further detail in the guidance note. 
4.2 Interpretation 
Lidar data is primarily a record of x,y,z coordinates that provide 
a view of the land surface. In order to differentiate between 
archaeological features created by human interaction with the 
landscape centuries or millennia ago and the remains of modem 
agricultural or other practices this data requires interpretation. 
There are various aids to viewing the data to aid the 
interpretation process. 
The main product of lidar data tends to be imagery viewed as 
hill-shaded images. These appear similar to vertical 
photographs of earthworks lit by low sunlight, so the analysis of 
lidar for the identification and characterisation of 
archaeological sites requires similar skills as those applied to air 
photo interpretation, for example the ability to recognise slight 
earthwork banks or ditches based on their appearance with 
reference to shadows and highlights, while filtering out features 
due to modem agricultural practices, geology and data 
processing artefacts. 
Figure 3 - Feature misinterpretation - lidar derived 
image (lidar © Mendip Hills AONB; source, 
Cambridge University ULM (April 2006)). 
This can be demonstrated by an example from the guidance 
note that shows the potential confusion between features of 
archaeological interest and those of modem origin. The feature 
seen in the bottom centre-left of figure 3 gives every 
appearance of being a burial mound, being of a similar size and 
shape to other known barrows in the vicinity here on the 
Mendip Hills. Using not only all available mapping data, but 
also analysing all readily available aerial photographs, both 
traditional and digital, simultaneously with the lidar data, will 
help to ensure correct interpretation, as in this case where the 
evidence from aerial photographs and mapping (Fig 4) reveals 
that the feature is in fact the site of a covered reservoir. 
Figure 4 - Feature misinterpretation - aerial 
photograph showing the true nature of the feature 
(photo PGA_ST5050_2006-04-30_part. Licensed to 
English Heritage for PGA, through Next 
Perspectives™; OS background map © Crown 
Copyright. All rights reserved. English Heritage 
100019088. 2009). 
4.3 Usage 
Whilst lidar data is remarkably useful and has generated great 
interest in archaeological circles, it is not necessarily inherently 
user friendly and many organisations who might wish to make