Попробуйте задать Z-фактор:
It is recommended to have your data in a projection coordinate system (units meters). However if you choose to run the analysis with a spherical coordinated system you will need to specify an appropriate z-factor for that latitude. If your x,y units are decimal degrees and your z units are meters, some appropriate z-factors for particular latitudes are:
Latitude Z-factor
0 0.00000898
10 0.00000912
20 0.00000956
30 0.00001036
40 0.00001171
50 0.00001395
60 0.00001792
70 0.00002619
80 0.00005156