Importing SRTM Data | Landscapes

Importing SRTM Data

VNS 2

For areas of the world where 90-m terrain data is not available, SRTM-3 offers a low-cost solution. This tutorial covers the import of a single tile, however, the method is most useful when importing many tiles. The VNS 2 DEM Merger allows you to merge and fill data voids in a few simple steps.

Important
The SRTM data used in this tutorial is evaluation data provided for testing in the GIS community. It is not final processed heightfield data ready for primetime. As such, you should read all documentation and understand the limitations of the data before using it for production work. The following method fills in holes in the SRTM data using surrounding data. While this makes the data usable, it introduces errors into the resulting terrain.

Note
3D Nature's free SRTMFill utility is the fastest and easiest way to fill holes in a few SRTM files or if you're using WCS. The following method is most useful when importing many SRTM tiles into VNS 2.

1. This tutorial uses SRTM data available for FTP download at ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/.

2. I'll be using the N00W078.hgt.zip SRTM file downloaded from ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/South_America/. You'll need to adjust the VNS import settings appropriately for your data.

3. Extract the zip to yield the N00W078.hgt file.

4. Your first stop before doing anything with new data is the documentation or metadata. If you go to the root SRTM directory at ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/, you'll find a Documentation directory.

5. The SRTM_Topo.txt file gives us all the information we need to import the SRTM-3 (3 arc-second or about 90 m) data.

Georeference: 1 degreee x 1 degreee; filename is lower left corner

Data format: HGT: signed two-byte integers; Motorola 'big' order

Data voids: -32768

6. Create a new project and import the N00W078.hgt file. The Import Wizard will identify it as a binary file.

7. Accept the LOAD AS window defaults.

8. Accept the OUTPUT FILE TYPE AND NAME defaults.

9. The COORDINATE SYSTEM SUMMARY window will open the Coordinate System Editor, seeking guidance. Go to the System page and choose Geographic - WGS 84 from the System dropdown box.

10. BINARY INPUT SETTINGS window. Choose Two Value Bytes and the High-Low/Motorola/Big Byte Order. These settings came from the SRTM_Topo.txt file.

11. Accept the defaults in the INPUT COLUMNS, ROWS, AND HEADER window.

12. Accept the defaults in the REFERENCE COORDINATES window.

13. HORIZONTAL EXTENTS window. According to the SRTM_Topo.txt file, the filename is the lower left corner of the 1º x 1º block. A little simple math gives us the following extents.

North Lat	1º
West Lon	-78º
East Lon	-77º
South Lat	0º

14. Accept the defaults in the PRE-PROCESSOR SETTINGS window.

15. NULL DATA window. According to the SRTM_Topo.txt file, the data has voids with NULL data
of -32768.

16. Accept the defaults in the VERTICAL EXTENTS window.

17. OUTPUT DEMS window. Make 1 tile for easy merging when importing many tiles.

18. Make this Coordinate System the default.

19. Here's what the imported data looks like in a planimetric camera view.

20. Create a Search Query that finds all enabled DEMs. Select Items Now to check your query. The Database Editor should open with the DEM selected.

21. Add a DEM Merger that uses the Active DEM SQ. Update bounds. Perform Merge.

22. Disable the original DEM in the Database Editor.