Converting DEMs

Malcolm Williamson malcolm at
Fri Oct 10 10:58:37 EDT 1997

According to _Digital_Elevation_Models_-_Data_Users_Guide_5_ (USGS, 
1993), NAD83 and WGS84 can be considered functionally identical for the 
conterminous United States. Therefor, you should be able to load the 
1-degree DEM in the normal fashion to a Location established with a NAD83 
datum in UTM. However, that probably still won't help you too much, as I 
bet your 7.5 minute data is in NAD27. You can output a sites file from 
your raster data and use the NAD2NAD program that comes with PROJ (the 
program from the USGS that GRASS uses for coordinate transformations). 
You can also download the contributed r.proj program and look at what 
modifications would be required by it to do datum transformations, again 
using NAD2NAD. THe other alternative is to look at other software 
packages for datum converstion.
Malcolm D. Williamson - GIS Specialist           E-mail: malcolm at
Center for Advanced Spatial Technologies      Telephone: (501) 575-6159
Ozark Rm. 12                                        Fax: (501) 575-5218 
University of Arkansas              
Fayetteville, AR 72701

On Thu, 9 Oct 1997, J. Brian Hughes wrote:

> I am trying to figure out how to use 1.0-degree Digital Elevation
> Map (DEM) data to fill in the gaps in 7.5-minute DEM data.  This
> involves converting coordinates from a latitude/longitude coordinate
> system using the WGS 72 or WGS 84 datums to the UTM coordinate system
> using the NAD 27 or NAD 83 datums.  Can GRASS help me to do this?
> Has anyone out there done coordinate conversions involving these
> coordinate systems?  I presume that if I can successfully get the data
> into an acceptable form to be loaded into a GRASS database, then I can
> save the information out to a file in a new coordinate system format.
> Is this correct?  Any input will be greatly appreciated.
> J. Brian Hughes

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