[GIS] GPS Elevation height

elevationgps

I want to calculate the height of an object based on nearest ground surface. I have GPS coordinates and GPS elevation(which I assume is ellipsoid height) as an input in WGS84 coordinate system. I read through some basics and it seems I have to convert this ellipsoid height to orthometric height which I guess is roughly equivalent to Above Mean Sea Level (AMSL), correct?

I downloaded NOAA's VDatum for ellipsoid height conversion.. Attaching screenshot herewith.

I'm not 100% sure if 122.6m ellipsoid height for GPS location 47.662511,-122.152067 is correct or not?

Please also let me know if I want to get accurate height am I following correct approach?

I may have to again convert NAVD88 to AMSL for more accuracy right?

Best Answer

There's not really a way to tell unless you can find some reference data to confirm what either the NAVD88 / approx MSL / height relative to geoid OR the ellipsoidal height is.

One way is to find a topographic map with elevation contours for the area to get a general idea. That would give you NAVD88 or NGVD29 elevations.

I pulled some NGS (National Geodetic Survey) control mark datasheets. There's nothing exactly at the location so I was pulling datasheets for marks that were within a few minutes away (several kilometers). Their NAVD88 elevations were around 15 - 25 m. The separation between the ellipsoid surface and the geoid/NAVD88 surface is around 23 m, which you can see in the VDATUM results.

Several searches to datasheets are available at the NGS Survey Marks and Datasheets page. I used the radial search and you'll want to look for "any vertical control." Note that the searches sometimes want postive-west longitude values and/or DMS (degrees minutes seconds) rather than decimal degree values.

GPS "elevation" should be either ellipsoidal (that's natively calculated), but many GPS devices do have access to a geoid model and can convert to elevations / gravity-related heights.

I would suggest looking through your data to see if you can match up something close to a survey mark to compare the z values.

Note: GPS height values are not as accurate as the 'horizontal' (latitude-longitude) values due to the geometry of the GPS satellite constellation. If the GPS data wasn't RTK'ed (real-time kinematic) or post-processed, it could be up to a few meters off at least.

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[GIS] How to get an elevation profile for a gps track

For a local solution, GRASS can be scripted to do this:

# extract raster values at our points
# use cubic convolution for interpolation between DEM locations
v.drape in=my_pts out=pts_srtm_elev type=point rast=srtm_dem method=cubic

I ran an extended version of this for one of my use cases and performance of v.drape was no issue at all.

[GIS] Is it better to take the elevation of a GPS unit or that of a Digital Elevation Model

Short answer, use the DEM.

Handheld GPS units of the type you mention (consumer grade), can potentially use two different methods to figure elevation - the calculated GPS position or an internal barometric altimeter. Because of the way GPS works (explained at the page mkennedy originally linked to in a comment, and elsewhere), without high-grade, corrected systems, the barometric method is typically more accurate than GPS. The next step up from Consumer is Differential, which according to that page has a vertical accuracy of 2-3x horizontal, and above that is Survey Grade which can get down to mm level depending on mode and range factors. Differential and Survey do use GPS calculation rather than barometric.

However the barometric method still has issues - readings can be affected by weather as the air pressure and temperature changes. You're unlikely to get consistently detailed or accurate elevation readings. I use a Garmin 60CSx and could provide multiple GPS track examples of this; I have tracks that are loops yet show a 200ft vertical difference between the start and end points even though they're the same place, as well as multiple versions of the same track with completely different elevations throughout. Some documentation on their Edge units puts vertical accuracy with calibrated barometric readings at +/-50 feet.

A couple of related links from Garmin's FAQ:

And there are many other places on the web you can find information/discussions about GPS calculated vs barometric altimeter accuracy for elevation readings, particularly related to handheld consumer units. Bottom line, every unit will return different values, and even the same unit will on different trips.

On the other hand, the DEM is basically a mapped value you're comparing the route against. You will have inaccuracies based on both the horizontal and vertical resolution of the DEM (so higher res is better), but it will be consistent from track to track (assuming said tracks are within a given horizontal tolerance of course). A DEM or map with contours or any other such data source is basically fixed - the values are what they were when measured/interpolated. But each GPS track is going to be whatever the unit recorded at whatever time, so it will differ from the map value at the same point. This is readily apparent on GPS units which are capable of displaying map data; you can see the elevation the map says you are at along with what the altimeter and potentially even GPS calculated elevation values are (and all three will probably differ).

Best Answer

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[GIS] How to get an elevation profile for a gps track

[GIS] Is it better to take the elevation of a GPS unit or that of a Digital Elevation Model

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