CIRCULAR & LINEAR ERROR
Circular error 90% (CE90) is the distance within which 90% of the points in the elevation model fall from their actual location in a horizontal or XY direction. Linear error 90% (LE90) is measured the same as is CE90 only it assesses vertical (Z) accuracy.
GROUND CONTROL POINTS
Ground control points are locations that are visible in the imagery used to create your elevation model with a known latitude, longitude and elevation. They should have an accuracy equal to the pixel size of the input dataset or better. We prefer to use points with an accuracy that is at least half the pixel size.