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Source: https://outdoors.stackexchange.com/q/22893
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

<p>A barometric altimeter is supposed to be more accurate than a GPS altimeter in the relative sense (total ascent, total descent, during stable weather), but may have considerable error in the absolute sense (more than 100 metre error if pressure has changed a lot since last calibration).  The GPS altimeter may have an error of perhaps 3–4 times the horizontal error, which for positioning may be much better than the barometric altimeter; however, the GPS altimeter is noisy such that tracks are noisy and "total ascent" is a fiction.  See also <a href="https://gis.stackexchange.com/q/333252/4904"><em>Random and systematic error of GPS or barometer based altitude measurements</em></a>.</p>

<p>I've personally found that for accurate <em>positioning</em>, the GPS altitude (on my GPSmap62 which has no barometric altimeter, or on the GPSMAP66c which does have one) is often more accurate than the barometric altimeter.  Is there a combination of settings that will improve the accuracy of the barometric altimeter?  Alternately, can I disable it in favour of GPS-measured altitude?</p>

<p>Some online sources about GPS altitude error agree with me:</p>

<p>From <a href="https://geoawesomeness.com/accurate-altimeter-gps-watch/" rel="nofollow noreferrer">geoawesomeness</a>:</p>

<blockquote>
  <p>The general rule of the thumb is that vertical error is three times the horizontal error. </p>
</blockquote>

<p>From <a href="http://www.borgeltinstruments.com/GPSvsPressurealtitude.pdf" rel="nofollow noreferrer">borgelt instruments</a>:</p>

<blockquote>
  <p>the GPS altitude is the GEOMETRIC altitude above  Mean Sea Level accurate to 10 to 20 meters* and explains from theory and practice that GPS altitude at 10 to 20 meters (35 to 70 feet) error  is superior to Pressure Altitude.</p>
</blockquote>

<p>From <a href="https://www.dcrainmaker.com/2010/05/understanding-sport-device-gps.html" rel="nofollow noreferrer">dcrainmaker.com</a> states (emphasis mine):</p>

<blockquote>
  <p><strong>Barometric altimeters</strong> from a sports standpoint <strong>tend to give you graphs that are ‘shaped’ correctly</strong> – meaning when you go up the graphs show an incline, and when you go down they show a decline.  The challenge though with barometric based devices is that* <strong>*the baseline altitude can sometimes be incorrect</strong>.  This is because normally barometric altimeters are calibrated off of a known barometric pressure reading.  In airplanes for example, this ‘calibration factor’ is constantly being tweaked as the pilot flies the plane.</p>
</blockquote>

<p>Pressure changes can easily lead to barometric altitude error in excess of 100 meter.  My horizontal accuracy is usually around 5 metre, which is consistent with an elevation accuracy in the order of 15-20 metre.</p>

<p>Although I can access the GPS altitude via the satellite screen or frequently calibrate the barometric altitude by the GPS altitude, an automated low-battery solution would be preferable.  I've noticed there is an "auto calibration" option, but I don't understand what it really does.  What combination of settings can I use?</p>