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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:
Your GPS device does not log altitude, or you had poor satellite reception when you recorded the track.
Your GPS device does log altitude, but it's not very accurate.
You drew a track using the drawing tools in Google Earth or a similar application, like GPS Visualizer's Sandbox.
You have a KML file that came from Google Maps or Google Earth's "driving directions" feature.
You created a route in Google Maps and have the URL of that route.
You have an NMEA log file that contains only "GPRMC" sentences, not "GPGGA."
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 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 (described in the table below). Complete copies of the SRTM3 and USGS NED databases, as well as a large number of SRTM1 and ASTER tiles, are stored on GPS Visualizer's server — that's more than 250GB of raw data.
Note that the elevation-adding feature will erase any existing altitude data (for example, from a GPS) that might already be in your file. Often, this is desirable; profiles made with 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 SRTM data, and that's unavoidable. If GPS Visualizer runs into one of these, it will not overwrite those elevations in your input data.)
DEM data sources
The U.S. Geological Survey's excellent National Elevation Dataset covers the United States, Canada, and Mexico. 1 arc-second (~30-meter) horizontal resolution is available in most areas (NED1). For Alaska, 60-meter data is also available (NED2).
NASA's SRTM1 database (from the Space Shuttle Radar Topography Mission) has a resolution of about 30 meters. Previously, SRTM1 data was only available for the United States (and was not very useful because NED1 is generally better), but recently NASA has released SRTM1 tiles for the entire world. GPS Visualizer's server does not contain the full global dataset, but it does contain all of Europe, Japan, South Korea, Taiwan, and New Zealand, as well as parts of Australia and South Africa. Additional locations may be installed upon request.
Austria's OpenDataPortal hosts a number of high-quality digital elevation files from Western Europe which have been copied to GPS Visualizer's server. Most of the files are a combination of SRTM1 and local LIDAR and/or survey data, and they have a horizontal resolution of 1 arc-second (about 30 meters). In most locations, they are slightly more accurate than SRTM1 files.
The ASTER GDEM is a joint product of NASA and the Japanese Ministry of Economy, Trade, and Industry (METI). ASTER has very good 30-meter coverage of the entire world, but the awkward download process and the massive size of the database make it impractical to store all of it on GPSVisualizer.com. ASTER files for Iceland are available, however, since none of the other 30-meter databases cover that area. (As with SRTM1 data, additional files can be added upon request.)
NASA's SRTM3 dataset includes 90-meter resolution for the entire world. SRTM3 data normally only goes up to about 60° latitude, but thanks to the efforts of the volunteers at Viewfinder Panoramas, GPS Visualizer has the polar regions covered as well.
Solution #2: Google Maps API
The Google Maps API is able to return elevations for points anywhere in the world; these are usually (but not always) the same elevations you'd see in Google Earth. Google's data comes from a variety of sources and is sometimes (but not always) more accurate than the SRTM databases.