How accurate and usable is GDEM? A statistical assessment of GDEM using LiDAR data

A methodological framework for assessing accuracy of the GDEM product is described using four small case studies in areas of variable relief and forect canopy (Booschord in the Nether- lands, Calabria in Italy, Fishcamp in USA and Zlatibor in Serbia). Focus is put on evaluating the true accuracy of ASTER GDEM us- ing LiDAR data aggregated to 30 m resolution. Three aspects of ac- curacy have been evaluated: (a) absolute accuracy of elevations (goodness of fit between true and GDEM elevations), (2) accuracy of stream networks (goodness of fit for buffer distance maps for stream networks), and (3) accuracy of surface roughness paramet- ers (goodness of representation of nugget variation and residual er- rors). Results show that GDEM seems to be of little use in areas of low relief (st.dev. .995 could be used as the threshold level for a satisfactory fit between LiDAR and GDEM (this R-square corresponds to RMSE of .60 seems to be satisfactory. Analysis of the short-range variability allows determination of the effective grid cell size that more closely matches the true surface roughness. These results support previous work that indicates that a more suit- able grid cell size for GDEM v1 is about 90 m.