Barometric Drift Correction
GPSPosted by Nick Mon, October 22, 2007 12:17:13Funny business this barometric drift. Depending on how my Garmin Edge 305 is feeling it can play a greater or lesser degree in the accuracy of the altitude profiles I get. Barometric drift is caused on altimeters that use atmospheric pressure to record altitude. As you rise in altitude the air pressure drops. It's suprising accurate and responsive - definitely when compared to GPS based altitude measurement - look at the Garmin 305 Edge and Forerunner products; the only difference is that the Edge has barometric measurement. According to some blogs I read the Forerunner should not even claim to have altitude measurement...
Anyway, the only problems with barometric is that you need to know the start altitude and if the weather changes then the altitude appears to change. Nike claim to have solved this by the way with what they call 'Zero Drift Technology', and the Garmin Edge uses GPS readings to set the starting altitude and perhaps keep it accurate once underway.
Getting a good start with the Garmin 305 is essential and that means waiting an additional few minutes after the accuracy claims to be good. Be patient. Also when your friends have disappeared up the road without you. Even then I've still seen the result being somewhat out as can be seen below:
It's a circular bike ride of about 40 minutes and the pressure was dropping as some dark clouds rolled in. I probably did not wait long enough when starting as it shows an altitude of -33 metres. Even in Holland we do not have any points less than about -7m and I know that I started at a dizzying 1 metre below sea level. Upon my return the reading was -12 metres. The intermediate data is however pretty good, clearly showing the motorway flyovers that consitute the biggest hills in this part of the world. 3D Route Builder luckily allows an easy correction of this data and does so against time if timestamp data is available.
Select the fixed point and the point to move - in this case the last point is fixed and the first point needs moved up 22 metres. Then shift the complete route up relative to time:
Finally move the complete route up 11 metres.
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