This article applies to Photo Ephemeris Web 2.0.44 or later
Photo Ephemeris Web uses multiple different elevation data sources to obtain the height above sea level displayed within the app. These are:
- Google Elevation Service
Each of these data sources consists of a digital elevation model (DEM) that provides height above sea level for some region of the earth's surface.
SRTM3 and SRTM1 are both derived from the Shuttle Radar Topography Mission. The '3' and '1' refer to the resolution of the data, i.e. 3- and 1- arc seconds respectively, which are approximately equivalent to 90m (98 yds) and 30m (32 yds). 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 dataset covers land areas between 60 degrees north and 56 degrees south.
AsterGDEM derives from the Advanced Spaceborne Thermal Emission and Reflection Radiometer, another 1-arc second model. At the time of writing, the underlying source data is using v2 of AsterGDEM.
GTOPO30 is a 30-arc second model developed by the US Geological Survey. It is significantly lower resolution than the others, but provides coverage at extreme latitudes not covered by alternate DEMs.
Google Elevation Service is a proprietary elevation model operated by Google. It synthesizes multiple data sources to provide the best accuracy, and calculated interpolated values where necessary. Its resolution is, as best we can tell, better than 1-arc second.
Which should I use?
The list of available elevation services depends on the latitude of the map pins and your subscription level. A suitable default service is always selected automatically.
PRO subscribers can override this selection by choosing from the dropdown above the map next to the displayed elevation:
Note: Google Elevation is only available when the app displays a Google Map.
Generally, SRTM3 or Google will provide the best results for general use. SRTM1 has higher resolution, but the dataset also includes some gaps and artefacts. AsterGDEM covers a wider range of latitudes than SRTM, and it or Google are usually the best choices beyond 60N or 56S (e.g. Iceland).
At extreme latitudes GTOPO30 or Google may be the only available choice. The low resolution of GTOPO30 makes it unsuitable for any applications requiring even moderate accuracy (e.g. you cannot use this to estimate when the sun will drop behind a mountain peak).