Fall 2003
   GEO327G/386G: GIS & GPS Applications in Earth Sciences


Software Tips - 9

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  Messages>Labs>Tips>Tip 9    
9. 

How to import GTOPO30 or SRTM30 data into ArcMap (for ArcGIS 9.x)

   
 

N.B. This Tip has been updated for ArcGIS 9.x.  Look here for ArcGIS 8.x.

ArcMap can display GTOPO30 or SRTM30 elevation data directly by simply renaming the file extension from .DEM to .BIL. If access to actual elevation values are needed for analysis, the DEM must be converted to ESRI grid format. Doing so by the technique below converts original cell values from signed 16 bit binary format to ASCII values, which are elevations in meters. Details of the GTOPO30 and SRTM30 data format are available at ftp://edcftp.cr.usgs.gov/pub/data/gtopo30/global/README.TXT and ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/Documentation/SRTM_Topo.txt.  Note that SRTM30 data will require an additional "header file" that can be downloaded here, or written using instructions that are detailed in Tip 17.

To convert GTOPO30 data to ESRI grid format do the following:

1) Navigate to the file with Windows Explorer and replace the .dem file extension with .bil ;

2) Right-click on the .bil file in ArcCatalog, click "Export" then "Raster to Different Format".  Change the default "Output Raster Dataset" file name (the last part of the path that is listed) by deleting the .img extension (no extension here will produce a GRID format file, whereas the default creates an IMG file) AND give the file a name that is less that 14 characters.  If you get a red warning after completing this step, your file name is likely too long, or the path to the file includes folders with spaces or forbidden characters in their names. Leave the remaining field blank and click OK.

3) Open ArcMap, add the Spatial Analyst toolbar, make sure the Spatial Analyst Extension is activated, and add the newly created GRID file;

4) On the ArcGIS Spatial Analyst toolbar, click Spatial Analyst and select "Options".  Set the Working Directory (General tab; your choice), Analysis Extent ("Same As Layer.."), and Cell Size ("Same As Layer..")

5) Read on ...

The conversion of the .bil file to GRID format in step 2 above did not convert the original cell values properly; the original GTOPO30 or SRTM30 cell values were "signed", meaning negative values were permissible, whereas the new GRID values are "unsigned".  Negative, 15-bit DEM values (the 16th bit indicates a plus or minus sign) that were part of the original data were not preserved. An easy (though somewhat obscure) fix can be accomplished using the following formula in the Spatial Analyst Raster Calculator to derive a new grid:

CON([in_grid] >= 32768,[in_grid] - 65536,[in_grid])

where [in_grid] is the input grid name.

This statement replaces all raster values that are greater than or equal to 32768 (i.e. 2E15) with negative numbers that are equal to original value minus 65536 (i.e. 2E16); cell values greater than or equal to 32768 are an artifact of the original conversion, where the 16th bit was interpreted as an integer rather than a negative sign.

The converted, new out_grid will now have the negative values properly represented, and the statistics of the grid should match those listed for the original GTOPO30 or SRTM30 file. For more on the rationale of this technique, see http://www.esri.com/news/arcuser/0700/files/gtopo30.pdf. For details on the use of conditional statements, see the ArcGIS raster calculator Help file.

6) From the Spatial Analyst toolbar dropdown list select "Raster Calculator" and enter the above expression.  The raster calculator is extremely sensitive to syntax; I recommend you copy and paste the above expression into the calculator and then carefully replace "in_grid" with the real file name.

7)The negative values in the new grid created by the raster calculator include "dummy values" of -9999 for areas of ocean. These comprise a "mask" (see the spatial analyst literature) but should properly be set to NO DATA (in any elevation analysis these large negative elevations values would be problematic!). These can be converted to NO DATA using the "Reclassify..." tool in Spatial Analyst or by entering the following expression into the Raster Calculator:

setnull([Calculation] == -9999, [Calculation])

where [Calculation] is the grid created in step 6 above.

8) This will yield another new grid that exists as a temporary dataset. To make it permanent, right click on the file name in the ArcMap Table of Contents, select "Data>Make Permanent..." and give it a proper file name and folder location.

9) The grid from step 8 has cell values that are elevations in meters, but with x and y coordinates in decimal degrees. Hillshade, slope and aspect calculations in Spatial Analyst are only meaningful if horizontal and vertical units are the same. Thus, either the x, y coordinates need to be converted to meters (e.g. project the grid to UTM or another meter-based projection) or z values need conversion to decimal degrees. To convert meters to decimal degrees, use the raster calculator to divide by the number of meters per degree (~111,120; or to mutiply by the inverse, i.e. ~0.0000090):

[Calculation2]*0.0000090

where [Calculation2] is the grid created in step 8 above.

This new grid can now be used to calculate slope, aspect, hillshade, etc. grids with Spatial Analyst. Be sure to retain the grid with elevations in meters for other uses (who understands elevations in decimal degrees?).

A final note... if the only goal is to use the grid made in step 8 to generate slope, aspect or hillshade grids, then an alternative to step 9 is to enter a "z-factor". The z-factor option is available in the Spatial Analyst tools for creating such grids and is simply the factor that relates the z-value (elevation in meters in this case) to the x and y values (decimal degrees in this case).  The z factor in this case is 0.000009.

   

 Last updated September 13, 2017
 Comments and questions to helper@mail.utexas.edu
 Geological Sciences, U. Texas at Austin