Find "Missing" Elevations with GPS Visualizer
The problem: Sometimes you have geographic data that consists only of latitudes and longitudes, but you want to know the altitudes as well -- because for example, you want to colorize points by height above sea level, or draw a profile of a track. Here are some common reasons why you might have "flat" or incomplete data:
Solution #1: DEM database
GPS Visualizer's map, profile, and conversion programs now have the ability to instantly add elevation data -- from a DEM (digital elevation model) database -- to any type of GPS file. If you just want to draw a profile, or convert a single data file to plain text or GPX while adding elevation, you can use the fast and simple form right here:
Or, look in GPS Visualizer's various input forms for the menu called "Add DEM elevation data," and choose one of the elevation databases. For the U.S., you can get 30-meter horizontal resolution from NASA's Space Shuttle Radar Topography Mission (SRTM1) or from the U.S. Geological Survey's National Elevation Dataset (NED). For the rest of the world, a 90-meter NASA database (SRTM3) is available; Copies of all three databases -- 90 gigabytes of raw data -- are stored on GPS Visualizer's server.
Note that the elevation-adding feature will erase any existing altitude data (for example, from a GPS) that might already be in your file. Sometimes this is desirable; profiles made with Digital Elevation Model (DEM) data are usually "smoother" looking than GPS, and typically contain fewer gaps or suspicious readings. (Speaking of gaps, there are a few in NASA's data, and that's unavoidable. If GPS Visualizer runs into one of these, it will not overwrite those elevations in your input data. If your data is from the U.S., you may want to try the NED data, which is gap-free.)
One more note: none of the databases available on GPS Visualizer's server cover areas between 60° and 90° north or 56° and 90° south. For those high-latitude areas, see below.
Solution #2: Google Maps API
The Google Maps API is able to return elevations for points anywhere in the world; these are the same elevations you'd see in Google Earth. Google's data comes from a variety of sources and is generally quite accurate — certainly more accurate than the SRTM and NED databases.
Quick single-point DEM database lookup:Coordinates (lat,lon): FIND ELEVATION →