Fig. 1
From: The challenges of estimating the distribution of flight heights from telemetry or altimetry data
Illustration of the difference between true and recorded flight height. a True flight height above ground (htrue), and true elevation above ellipsoid (ztrue). b Adding the five sources of error, with circled numbers referring to headings in “Review of the sources of error”. DEM stands for digital elevation model. c Two tracks with the same amount of error. The bird of track 1 is flying high so all the recorded flight height data remain positive despite the errors. The bird of track 2 is flying low, so some of the recorded data fall below the digital elevation model