Tools and Links

Tools for gpx files

I've put together a number of tools for displaying tracks, elevation profiles and statistics (distance, max, min and accumulated altitude, etc.) for gpx files recorded on GPS devices. The altitudes recorded in the gpx file can also be compared with those obtained from satellite data (see below).

Comparison of elevations for GPS tracks in GPX format

Altitudes recorded by GPS devices may be considered to be accurate to +/- 23 meters and maybe worse if there are obstructions in the vicinity (e.g overhanging cliffs) - see for example this web page. The horizontal accuracy is much greater than the vertical accuracy for reasons of geometry, see for example this web page for a discussion.

The altitudes recorded by GPS devices can be compared with world-wide (or almost) elevation data from either satellite or topographic sources. The programs on this web site for doing this are limited to files recorded in the GPX format.

Elevation Datasets

There are a number of publicly available databases of elevation data covering a large proportion of the globe. Following is a short description of those I know of.

NASA SRTM3 dataset

From the SRTM home page:
"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."

The data files are available from the USGS web site. These are binary files, each of which covers 1º of arc in latitude and longitude, with each datapoint separated by 3" of arc (approximately 90m). Elevations are coded on 2 bytes in big-endian order. The naming of the files is of the form [NS]nn[EW]mmm.hgt where nn is the latitude and mmm the longitude.

CGIAR-CSI dataset

The NASA/USGS data contains 'no-data' holes, especially in mountainous areas. CGIAR-CSI members have processed the NASA SRTM3 data to fill in these holes and correct other innacuracies, and the improved data is available from the CGIAR-CSI web site.

The CGIAR-CSI data has the same 3" resolution as the SRTM3 data, and is available in two formats:

ASCII format with each file covering 5º of latitude and longitude. Each file has 6001 rows and 6001 columns. This format is not very convenient when processing a significant number of trackpoints in a GPX file, since the lines in the file have to read sequentially, although one can resort to tricks to avoid re-reading the file many times.
GeoTiff format, again with each file covering 5º of latitude and longitude. Being unfamiliar with the GeoTiff format, I was very happy when I came across the work of Bob Osola, who has made available PHP code for SRTM & Ordnance Survey Elevation Data. However, I found a couple of errors in Bob's code, which only affect regions east of Greenwich, and the code on this web site has been corrected for these errors.

Jonathan de Ferranti datasets

Jonathan de Ferranti's Viewfinder Panoramas web site includes a page describing elevation datasets for mountainous regions around the world. This page includes SRTM-derived DEMs (Digital Elevation Models) "in which the no-data void areas have been filled from the best available alternative sources. They are much more accurate than those created by interpolation." For the Alps, the sources include local 1:25000 and 1:50000 topographic data. The data files are in the same format as the USGS SRTM3 data files, but there are also versions with 1" resolution, and it is these files which are used by the program linked to this web page.