NED data shows different elevations in GM 12 and 13

stitchenal

I have been downloading and processing National Elevation Data from the USGS seamless server for a couple of years. I just started processing some of the data using GM 13. I find that the same NED 1/9 arc ArcGRID data shows different elevations in GM 13 than in 12. Attached are two screen shots showing the difference. This is the specific area shown using a custom shader:
http://extract.cr.usgs.gov/Website/distreq/RequestSummary.jsp?PR=0&CU=Native&ZX=-1.0&ZY=-1.0&ML=COM&MD=DL&AL=41,40.75,-75.75,-76&CS=100&PL=NED01HZ,ND901HZ&orig=SMLSVWR
DEM_GM12.pngDEM_GM13.png

global_mapper

The problem is that the data doesn't explicitly provide any elevation units (the PRJ.adf file lists the Zunits as 'NO', which means none provided). In v12 Arc Binary Grids with no elevation units would default to meters, but in the latest v13 builds they now default to the ground units of the projection, if linear, but in this case the ground units are degrees so the default elevation units are based on your elevation display/export units setting on the Vertical Options tab of the Configuration dialog. If you have that set to Statute units then the files will come in using Feet as the elevation units when the units aren't specified. You can correct the elevation units interpretation to Meters on the Alter Elevations tab of the Options dialog.

Thanks,

Mike
Global Mapper Guru
gmsupport@bluemarblegeo.com
http://www.globalmapper.com