<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>Hi,</div><div><br></div>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.<div><br><div dir="ltr">Nicolas Cadieux<div><a href="https://gitlab.com/njacadieux">https://gitlab.com/njacadieux</a></div></div><div dir="ltr"><br><blockquote type="cite">Le 20 avr. 2021 à 21:32, David Strip <qgis-user@stripfamily.net> a écrit :<br><br></blockquote></div><blockquote type="cite"><div dir="ltr">
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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 <a moz-do-not-send="true" href="https://www.usgs.gov/core-science-systems/eros/topochange/science/accuracy-assessment-elevation-data?qt-science_center_objects=0#qt-science_center_objects">link</a>,
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. <br>
TL:DR - the SRTM value is probably better than your GPS reading.<br>
<br>
On 4/20/2021 5:45 PM, jean Lukusa wrote:
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<div dir="auto">Hi all .
<div dir="auto">I hope you are doing fine.</div>
<div dir="auto">I have experienced an issue:</div>
<div dir="auto">The elevation taken in the field with gps
garmin64 is 623 m but the one given by SRTM download from Qgis
is 612m.</div>
<div dir="auto">How can I interprete this gap knowing that SRTM
resolution is 30 meters.<br>
<br>
<div data-smartmail="gmail_signature" dir="auto">Jean Lukusa</div>
</div>
</div>
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<br>
<br>
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