This lidar–lidar differencing was considered to be
a benchmark measurement and was used to compare the
performance of lidar-controlled photogrammetric models
for the measurement of glacier volume change. Glacier
boundaries were delineated with binary masks for both years,
and we subtracted 2005 lidar elevations from each of the
2003 models. Ice-volume changes were calculated using
pixel summation of difference DEMs (e.g. Etzelm¨ uller and
others, 1993). Total volume change (ΔV) was obtained by
summing the i pixel values (hi2003 − hi2005) between each
difference DEM contained within the larger glacier surface,
A (2003), and multiplying by the area lp
2 represented by each
pixel (where lp is the grid spacing) expressed as:
ΔV = lp
2
A
(hi2003 − hi2005). (1)
Mean volume change ΔV averaged over the glacier
surface was calculated by dividing ΔV by area A. ΔV was
then divided by the time between epochs to calculate mean
annual volume changes (ΔV/Δt).