Using SRTM Elevation Data

Introduction

This page explains and demonstrates with a working script how to obtain and read Shuttle Radar Topography Mission SRTM elevation data for use in an on-line map or similar geo-app.

Obtaining And Understanding SRTM Data

Before play there is work, getting the data.  Although free, and tolerably well documented, the number of small files that have to be downloaded, as opposed to just a few big ones, turns obtaining SRTM data into something of a chore.  Here are the most important links concerning SRTM data:

• Historical notes about the mission and its data (unfortunately, many of the most interesting links on this page are now broken):
       http://www2.jpl.nasa.gov/srtm/
• The data documentation can be obtained from here (now only available from the Wayback Machine):
       http://dds.cr.usgs.gov/srtm/version2_1/Documentation/
• The 1 arc second v3 data tiles can be obtained from here (may be restricted outside of US) (now only available from the Wayback Machine):
       http://e4ftl01.cr.usgs.gov/SRTM/SRTMGL1.003/2000.02.11/
• The 3 arc second v2 data tiles can be obtained from here (now only available from the Wayback Machine):
       http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/

SRTM data comes in several resolutions, but only the two most detailed are covered here, 1 arc second² (approximately 30m² at the equator) and 3 arc second² (~90m²).  For both, the data is held in files called tiles, one for every degree² of longitude v latitude where there is some land (areas that are all sea have no tiles).  For the 1" data, each tile contains 3601×3601 pixels, for the 3" data, each tile contains 1201×1201 pixels.  Note in each case the extra row and column, which allow the rows and columns at the edges of each tile to repeat the corresponding data from neighbouring tiles, thus making it easier programmatically to scan from tile to tile.  Each pixel consists of a two-byte binary value in big-endian order (high byte followed by low byte), where -32768 (0x80,0x00) represents a data void, any other value a genuine elevation relative to the WGS84/EGM96 geoid, for which average global sea-level is approximately zero.  Note also that within each tile the data is stored in row-major order (x longitude within y latitude), and, like an image, from the top of the tile to the bottom.

Obviously, you must download enough of the actual data to cover your area of interest, but, due to the density of it, you will probably want to stop at that, as server space is usually a constraint.

UK Data

This section gives metadata for the SRTM tiles covering all of the British Isles, by which is meant England, Scotland including all the remote islands, Wales, all of Ireland, The Isle Of Man, and The Channel Islands, but excluding Rockall in tile N57W014:

Geographical Layout of SRTM Tiles Covering The British Isles

	11W	10W	9W	8W	7W	6W	5W	4W	3W	2W	1W	0	1E	2E	3E
60N*									•	•	•
59N					•	•	•	•	•	•
58N				•	•	•	•	•	•
57N			•	•	•	•	•	•	•	•
56N				•	•	•	•	•	•
55N			•	•	•	•	•	•	•	•
54N	•	•	•	•	•	•	•	•	•	•	•
53N	•	•	•	•	•	•	•	•	•	•	•	•
52N	•	•	•	•	•	•	•	•	•	•	•	•	•
51N	•	•	•	•		•	•	•	•	•	•	•	•	•	•
50N						•	•	•	•	•	•	•	•	•	•
49N					•	•			•	•	•	•	•	•	•

*  SRTM data gives out at 60°N, which unfortunately for us in the UK is half-way up The Shetlands.  The 1 arc second data doesn't include 60N tiles at all (hence it has 3 fewer), while the 3 arc second data includes them, but their data is incomplete and incompletely processed to remove noise.  See the section 60°N below for further details.

SRTM BI Tile Map

SRTM British Isles Tile Metadata

	1 arc second2 (~30m2)	3 arc second2 (~90m2)
Unzipped (MB)	2,547	291
Zipped (MB)	334	59
Data Tiles	103	106
Tile List	SRTM_UK_1as_Tile_List.txt	SRTM_UK_3as_Tile_List.txt

60°N

There are no SRTM 1" tiles for latitude 60°N, while the 3" tiles have no valid data north of latitude 60.385°N, and the sea south of that line contains noise which in other tiles lying within the main SRTM band of coverage would have been removed, and further this noise is also present in the top, shared, line of pixels of those tiles from 59°N which have neighbours to the north at 60°N.  Thus, in the UK, the five land tiles (one is all sea) that cover The Shetlands are affected: N59W002, N59W003, N60W001, N60W002, N60W003.  For most purposes in this area, raw SRTM data would be unusable, and alternative data, for example from OS's Terrain 50, must be used instead, either as from source, or else to construct replacement SRTM tiles.  To illustrate, the affected tiles are here converted into images  -  that on the left is raw SRTM, that in the middle is made from tiles created from Terrain 50, while that on the right compares the two:

The two numbered sections shown below demonstrate what this does to profiles returned by the script:

Section 1 between (60.25N,1.25W) and (60.5N,1.25W) demonstrates the biggest problem, the total loss of data north of 60.385N.  The first profile is from raw SRTM, the second from tiles made from Terrain 50:

Section 2 between (60N,1.325W) and (60.385N,1.325W) demonstrates the second problem, the noisy sea.  Again, the first profile is from raw SRTM, the second from tiles made from Terrain 50.  Although after averaging the image doesn't show it that clearly, the noise in the first gives points in the sea that are up to 8m below sea-level:

Replacement Shetland tiles made from OS Terrain 50 data: SRTM3-OST50-Shetlands.zip

World Data

Example Script

This Python script is designed to run as a standard CGI script on a webserver.

By default, it uses SRTM 3" (90m²) tiles, but can be altered to use 1" (30m²) tiles by changing all references in the following lines 163/167ff from SRTM3 to SRTM1:
   # Relative path to tiles
   Path = SRTM3Path
   # Unzipped file name template
   FName = SRTM3FileName
   # Zipped file name template
   ZName = SRTM3ZipName
   # Tile width (sample points)
   X = SRTM3Cols
   # Tile height (sample points)
   Y = SRTM3Rows
   # Angular resolution (radians)
   ARes = SRTM3AngRes


By default, either set of tiles, or the zips containing them, are expected to be in a sub-directory named SRTM beneath that where the script is located.  Debugging output is HTML5 compliant and requires two other files, an HTML template, which can vary slightly from script to script and therefore is named after the associated script to distinguish it from others, and an invariant CSS stylesheet, by default named Minimal.css.

Obviously, these defaults can be changed to suit individual requirements, but the script (for the SRTM data and template paths) and/or the template (for the stylesheet path) then must be edited accordingly.

The script is self-documented.  Instructions for use are included as leading comments, and also are displayed in response to errors in calling it.

If using this script, and scarcity of available server or web space so demands, the tiles can be left zipped, as, when looking for a tile, the script will look first for an unzipped version of it, but if that is not found it will then look for a zipped version.  Inevitably however, leaving the data in its original zips will slow things down a little.

A profile point is returned for each intermediate SRTM cell up to 100 (the empirically determined maximum realistically displayable using a URL to the Google Charts API to draw the profile).  If a profile path crosses more than 100 cells, then each of the 100 points returned is the average, or maximum if given the mx switch, of the cells crossed by the path that are closest to that sample point  -  that is, the profile returned is derived from all the cells it crosses, not just a sample of them.

The profiles returned have been shown to follow a geodesic to within 1m up to about 70km, further has not been tested.

Server CGI

Displaying The Profile

Unfortunately, the only convenient way that I know of displaying a profile, the Google Static Image Charts API, has been deprecated since 2012.  As per this discussion in their forum, they have made soothing noises, but these remain unofficial, and there is still no definite word on any possible replacement.  If it were to go, this wouldn't be the first time that they've broken my work in this manner, and, as I don't suppose it will be the last, ideally what is needed is some sort of standard library that can be installed on one's own site, but I don't know of anything remotely comparable to the ease of just feeding Google a suitable URL.

However, at the time of writing in November 2021, it's still running(!), so here's how to use the service.  It really is as simple as taking the URL returned by either Python script, replacing ~Width~ and ~Height~ by suitable dimensions (pixels), ensuring that the two when multiplied together come to less than Google's limit of 300,000, and then making the result the src attribute of an img tag in your web-page.  Here's a demo, which, incidentally, also demonstrates the result of remaking the Shetland tiles  -  previously this profile had an entirely spurious cliff in the terrain at the point where the original SRTM data ended …

<div id="ProfDiv"></div>
<script>
  <!--
  var imgWidth = 650;
  var imgHeight = 250;
  var profData;
  
  function callBack( data )
    {
    profData = data;
    profData.url = profData.url.replace( "~Width~", "" + imgWidth + "" ).replace( "~Height~", "" + imgHeight + "" );
    var img = document.createElement( "img" );
    img.id = "ProfImg"
    img.style.width = imgWidth + "px"
    img.style.height = imgHeight + "px"
    img.src = profData.url
    document.getElementById("ProfDiv").appendChild( img );
    }
  //-->
</script>
<script src="http://example.com/cgi-bin/SRTM.py?x1=-1.317&y1=60.243&x2=-1.359&y2=60.394&cb=callBack&pr=true"></script>