Aerial Archaeology in Ireland - The Heritage Council

Figure 29: Diagram demonstrating the principles of lidar (H. Mangan)
Using an aircraft flying at a constant height, with its exact position being tracked by a high resolution global positioning system, rapid pulses of laser
beams are fired at the ground. The timing of the first, second and third return reflections off vegetation, buildings and the ground measures their height
and position, providing 3-dimensional mapping of sufficient resolution to reveal small undulations in the ground surface. The data can be filtered to
remove first and second return reflections, leaving only the ground surface measurements, and thereby effectively seeing through tree cover
Advantages of the lidar data include its potential to show up very indistinct earthworks which cannot be seen as easily on the
ground. This is assisted by the facility to apply low-level ‘lighting’ of the digital terrain model from any angle and enhancement
of images by exaggerating the vertical scale and using the image intensity data. This does not make ground survey redundant,
but it does make a major difference to how it can be targeted on aspects and areas that need more detailed information
(Figures 28, 35).
A less well-known application of lidar is for mapping the seabed. Fiona Fitzpatrick of the Marine Institute has provided the
following comments. This application of lidar utilises red and green laser light pulses emitted from an aircraft flying at typical
heights of 200m to 400m to determine water depth. The laser pulses reflect from the sea surface and the seabed back into
receiving optical instruments mounted on a stabilised platform. The time difference between the sea surface return and seabed
return is used to determine the water depth. The systems can give detailed bathymetric data (accurate to IHO Order 1) very
quickly over large areas at a sounding density of about 2m to 5m within the survey area. A typical maximum depth penetration
for lidar is about 40-50m in clear water, 20-40m in coastal waters, and less than 20m in more turbid inland waters.
Commercial companies, such as the Tenix LADS (Laser Airborne Depth Sounder) Corporation, 8 claim that the lidar technique is
the fastest, most cost-effective tool for accurate bathymetric survey in coastal waters. They state that it is capable of surveying
complex areas up to 20 times faster than survey ships and at 20% of the cost. Tenix’s case studies include a number of surveys
that have identified a significant number of wrecks off the Florida coast and elsewhere. The LADS system has been audited by
Land Information New Zealand (LINZ). Similarly, Optech’s SHOALS lidar system 9 offers the ability to survey shallow waters,
8 www.tenix.com
9 www.optech.on.ca
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