[Qgis-user] (no subject) (elevation question)

Tue Apr 20 19:23:43 PDT 2021

Hi,

David gave you a very good answer. Your looking at a 11 meter difference.  That is not much.  The first thing would be to look at the gps measurement.  (Any trees? Good satellite constellation?  Any obstacles?). At best, your looking at a 6 to 10 meter error  vertically. SRTMs are affected by trees.  That would make the SRTM higher (not your case).  Last thing to look at the the differences between the gps geiod model (a model of the average sea level).  SRTM use EGM96.  Your GPS probably uses something a bit more modern. You can easily get a 5 meter error between two Geiods.

Nicolas Cadieux
https://gitlab.com/njacadieux

> Le 20 avr. 2021 à 21:32, David Strip <qgis-user at stripfamily.net> a écrit :
> 
>  The 30m resolution of SRTM data refers to the spatial resolution, not the resolution of the elevation reading. A number of factors come into play when determining the vertical accuracy of SRTM data, but according to this link, the RMS error for SRTM vertical measurements is 3.56M. Because of the configuration of the satellites used to calculate elevation with a GPS, the vertical resolution is significantly worse than the horizontal resolution. Further complicating matters, many Garmin units contain a barometric altimeter which is preferred over the GPS reading by the software. However, unless you calibrated the sensor at a known elevation before starting your readings, you have no idea how accurate it is. 
> TL:DR - the SRTM value is probably better than your GPS reading.
> 
>> On 4/20/2021 5:45 PM, jean Lukusa wrote:
>> Hi all .
>> I hope you are doing fine.
>> I have experienced an issue:
>> The elevation taken in the field with gps garmin64 is 623 m but the one given by SRTM download from Qgis is 612m.
>> How can I interprete this gap knowing that SRTM resolution is 30 meters.
>> 
>> Jean Lukusa
> 
> 
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