These data provided additional information with which to
identify coincident points between the aerial images and the
lidar elevation data, in particular when proximal to areas of
very high (e.g. light snow surfaces) or low (e.g. ponded liquid
water) laser reflectivity. Using these methods we were able to
locate a total of 50 GCPs throughout the images comprising
the photo block (Fig. 4). Three-dimensional coordinates
extracted from the nearest raw lidar point to the GCP marker
location were assigned to each relevant control point when
measured in the aerial photographs. As some authors have
reported lidar errors to increase with off-nadir scan angle
(e.g. Baltsavias, 1999a), we selected marker locations as
close as possible to the centre of each of the nine swaths
comprising the full dataset. GCP extraction was therefore
limited to a zone 200m either side of the swath centre,
representing 25% of the full swath width either side of the
nadir point (assuming an average width of 783 m).