SRTM Frequently Asked Questions

Introduction

The Shuttle Radar Topography Mission (SRTM) obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth. SRTM consisted of a specially modified radar system that flew onboard the Space Shuttle Endeavour during an 11-day mission in February of 2000. SRTM is an international project spearheaded by the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA).

Technical Questions & Answers

  1. In what software can I open the GLCF's SRTM data?
  2. Are the SRTM-UTM files in the exact same coordinates as the GeoCover Landsat imagery?
  3. Why do I have negative values in the SRTM data?
  4. Why wasn't cubic convolution interpolation used instead of nearest-neighbor?
  5. Why does the GLCF distribute SRTM data in GeoTIFF instead of USGS DEM format or ArcGRID format?
  6. How can GeoTIFF formatted SRTM data be used in 3-D modeling?
  7. As a DEM user, how will using elevation data in GeoTIFF format be different?

1. In what software can I open the GLCF's SRTM data?
Any software that has decent support for the GeoTIFF 1.0 standard, such as ENVI, Imagine, ArcGIS, ArcView, PCI, etc.

2. Are the SRTM-UTM files in the exact same coordinates as the GeoCover Landsat imagery?
They match very well basically, but there are some differences of 30-150m in Meso-America, Eastern Africa, and Western Canada. However, the file sizes are different. The SRTM-UTM dataset has a buffer of 7.5-km around the WRS-2 tile. Also, All the GeoCover Landsat images are in UTM zone North, while the SRTM-UTM data are in UTM zone North only if the region lies in the Northern Hemisphere. For SRTM-UTM data in the Southern Hemisphere, UTM zone South was used. Most GeoTIFF-ready software can handle this seamlessly.

3. Why do I have negative values in the SRTM data?
This first edition of GLCF SRTM data is not what the USGS labels as "Finished". "Unfinished" SRTM datasets have missing values. JPL used value -32768 for missing pixels, which the GLCF maintains.

4. Why wasn't cubic convolution interpolation used instead of nearest-neighbor?
During the projection from geographic to UTM coordinates, the mid-high latitudes have much overlap between scenes, and, therefore, much need for interpolation. When a lot of missing values (-32768) are present during a cubic convolution resampling in these areas the result would be less useful than a nearest neighbor resampling. Thus a nearest-neighbor resampling was adopted.

5. Why does the GLCF distribute SRTM data in GeoTIFF instead of USGS DEM format or ArcGRID format?
All GLCF SRTM is distributed using 16-bit GeoTIFF files. It has several advantages when compared to the DEM format or the GRID format, with which some modelers may be more familiar.

  • GeoTIFF is the actual standard of raster geospatial data that can be used across different software platforms. The GLCF aims to provide interoperable datasets.
  • Second, each elevation point takes 2 bytes to store in a 16-bit GeoTIFF file, while taking about 20 bytes to store in a USGS DEM format file.
  • Third, the GeoTIFF file contains all the information within the single file, but the ArcGRID format needs several files to contain data and geographic information. Therefore, providing SRTM data in the form of GeoTIFF lets the user to download, manipulate and store data faster and more efficiently.

6. How can GeoTIFF formatted SRTM data be used in 3-D modeling?
For the 3 types of most widespread applications using elevation data, GeoTIFF can be used as the following paths:

  • For 3-D fly-throughs using elevation data and surface imagery, many software can take GeoTIFF directly, such as ERDAS Imagine.
  • For Hydrological/Civil Engineering modeling, many applications such as Mike11, take solely USGS DEM format. In these instances, it is recommended that GeoTIFF files be converted to DEM format, which much spatial software can accomplish.
  • Some 3-D GIS applications accept GeoTIFF directly while others may require conversion to DEM or ArcGRID formats.

7. As a DEM user, how will using elevation data in GeoTIFF format be different?
Both formats give the user an array of elevation data, so basically there is no difference. However, it may be useful to refer to the USGS DEM format specifications and the GeoTIFF specifications. Since most software do not explain the process for conversion from DEM to GeoTIFF or vice versa it is important to synchronize the datum and projections of all data.

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