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I was doing some hiking today and climbed into a gully of sorts. It seemed pretty deep and I'm curious how deep relative to the surrounding land. It'd be cool if I could just plug in arbitrary GPS coordinates into a website and have it spit the elevation back out to me. But alas I know of no such website.

I think in theory GPS is able to figure out the elevation that the sensor is at but the program I was using to get the coordinates was Google Maps, which doesn't let you get the elevation, in-so-far as I know.

I could install another app and go back and I might eventually do that but it'd still be cool to know before hand. And plus, sometimes knowing could be good before going there for planning purposes.

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    Some GPS receivers include a barometer, allowing more precise elevation measurements than GPS alone (I can see mine change by 1 metre when moving it between waist level and boot level). They can usually be set to use the (noisy, but long-term stable) GPS altitude to apply correction to the barometer (which drifts long-term due to meteorological pressure changes). This is all assuming that you can physically reach both top and bottom of the feature, of course (or dangle the device into it). You're unlikely to get a DEM as precise as you'd need for this.
    – Toby Speight
    Commented Apr 25, 2021 at 11:04
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    In the UK, the Ordnance Survey provides elevation data on a grid of 50 m free of charge, and to a 5 m grid for account holders. They also provide programming APIs to be able to create your own system based on their data.
    – Weather Vane
    Commented Apr 26, 2021 at 6:23

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CalTopo will give you an elevation when you mouse over or enter coordinates in the search box at the top of the website. This feature seems to be location dependent and works well in the USA and is spotty internationally. This solution requires a cellular connection. Consider a GNSS application that will show elevation on your phone or better still - download a free topographic map using an application like AvenzaPDF maps. Learn to read contour lines and figure out the gully depths that way.

Be warned. These CalTopo elevation values (and your GNSS elevations) are based on the WGS84 ellipsoid. These elevation values will differ from USGS topographic maps. Historical USGS topo maps will reference the NGVD29 vertical datum. Newer USGS topographic maps will reference the NAVD88 vertical datum. Depending on your location there may be considerable differences in the elevation values for your location. Elevation data and topographic maps from other countries may use other vertical datums to express elevation values.

10 meter spatial resolution USGS surface models have been freely available for decades. Outside of the USA the 30 m SRTM is freely available.
Modern surface model generation techniques rely on LiDAR data. Many USA Federal, State, Local, and Tribal governments freely distribute 3 ft spatial resolution surface models with z value errors in the centimeters. Here is the Washington State DNR LiDAR portal for your viewing pleasure. Check out the difference between the base map (SRTM) data and the LiDAR surfaces! Outside of the USA finding free surface model data is much more difficult.

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  • Given the difference in ellipsoids/datums, if you want an absolute height (as opposed to a gully depth) presumably the best way is to correct using the nearest peak height
    – Chris H
    Commented Apr 30, 2021 at 14:02

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