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).
To examine the effects of GCPs chosen from swath edges
on DEM quality and geodetic volume change, we identified
points in the central area of one swath which also lay close
to the edge of an adjacent and overlapping swath (within the
outer 5% of the swath width, or 40m of the swath edge). It
was then possible to alter the coordinates of GCPs from those
of the closest lidar point in the centre swath to those of the
closest lidar point from the edge of the overlapping swath.
A total of 50 points identifiable in both the lidar data and
aerial photos were selected, of which 20 were additionally
identified along overlapping swath edge zones.