Issue 52

Using Shuttle Radar Topography Mission (SRTM) Data in Surfer

Introduction

The Shuttle Radar Topography Mission (SRTM) collected high-resolution elevation data during the February 2000 flight of the Space Shuttle Endeavour. The data are available for download and can be used in Surfer. Surfer does not read the data files directly, but the data are available in two formats that can be modified for use in Surfer - BIL and HGT.

Artist's representation of the Space Shuttle Endeavour and SRTM in space.
(German Aerospace Center)

Convert SRTM BIL File to GRD with Bil2Grd3.bas

The BIL data are available from the USGS Seamless Data Distribution Center.

The following instructions for downloading BIL data are current as of October 2004.

1. Click on the View and Order Data Sets link for the United States Viewer or the International Viewer data. The National Map Seamless Data Distribution Viewer takes some time to launch.
2. Zoom in to the desired area, choose the Download tool, and drag a rectangle to select an area to download. The USGS SDDS-E Data Extraction Summary Page pops up in a new window.
3. For SRTM data, click on the Modify Data Request button. 
4. Click on the check box to the left of SRTM 30m or SRTM 90m.
5. In the Data Format column, click on the dropdown list to change to the BIL format.
6. Scroll to the bottom of the page and click on the Save Changes & Return to Summary button.
7. Click on the Download link to the right of the SRTM description. The USGS Raster Extraction Request Status window pops up while the file is created.
8. When the extraction is complete, the Enter name of file to save to ... dialog prompts for the name and directory to save the file. The default name is eight digits, with the ZIP extension. You may wish to change it to a more meaningful name, but append the original name to the end for future reference, e.g. ConiferCoSRTM30m_61463845.ZIP.
9. Close the USGS Raster Extraction Request Status window by clicking the close icon in the upper right corner of the window.
10. Close the USGS SDDS-E Data Extraction Summary Page.
11. Open the downloaded ZIP file, and extract the BIL, HDR, STX, and BLW files. Note that the file names use the same eight digits as the original ZIP file name. You may want to rename these files to more meaningful names, or extract them to a separate directory with a descriptive name.

Surfer 9 and 10 can import BIL files directly in place of a grid file. Anywhere a grid file is used, you can select and use the BIL file.

For Surfer 8, the BIL must be converted to a grid file. The Surfer script Bil2Grd3.bas converts the BIL file to a Surfer GRD file. It is available for download from our site athttp://www.goldensoftware.com/public/scripts/surfer8/bil2grd3.bas.

Open and run the script in the Surfer Scripter program. Specify the BIL file name. The script reads the associated HDR, STX, and BLW files to create a GRD file to use in Surfer.

The script has created the output grid file.

Surface map created with SRTM BIL data converted to the Surfer GRD format with Bil2Grd3.bas

Convert SRTM HGT File to DEM Format

The HGT data are available for download from the USGS FTP site in SRTM1 and SRTM3 formats.

The SRTM HGT format is similar to the GTOPO30 DEM format supported by Surfer. To load the file in Surfer, rename the HGT file to DEM, and create HDR and STX files. You can create the HDR and STX files in a text editor, like Notepad, and simply save or rename them with the HDR or STX file extension instead of TXT.

HDR files have different formats for 1 arc-sec / 30m data and 3 arc-sec / 90m data.

HDR File - STRM-1 (1 arc-sec / 30m)

BYTEORDER M  
LAYOUT BIL  
NROWS 3601  
NCOLS 3601  
NBANDS 1  
NBITS 16  
BANDROWBYTES 7202  
TOTALROWBYTES 7202  
BANDGAPBYTES 0  
NODATA -32768  
ULXMAP -105  
ULYMAP 41  
XDIM 0.000277777777  
YDIM 0.000277777777

HDR File - SRTM-3 (3 arc-sec / 90m)

BYTEORDER M  
LAYOUT BIL  
NROWS 1201  
NCOLS 1201  
NBANDS 1  
NBITS 16  
BANDROWBYTES 2402  
TOTALROWBYTES 2402 
BANDGAPBYTES 0  
NODATA -32768  
ULXMAP -114  
ULYMAP 36  
XDIM 0.000833333333333  
YDIM 0.000833333333333

The ULXMAP and ULYMAP are the coordinates of the center of the grid cell in the upper left corner of the map.

The STX file (statistics) consists of a 1, then the minimum, maximum, mean, and standard deviation of the Z values.

STX file

1 0 1000 500 100

Surfer uses the minimum and maximum, so enter those values in the STX file if you know them. If you don't know the minimum and maximum, enter any reasonable values, and use Grid | Convert to convert to a GRD file. Surfer will calculate the correct Z minimum and maximum values and save them in the GRD file.

Surface map of SRTM HGT data 39°N 105°W.

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