Barometer setting for altitude compensation

[B92 8NW]

I'm setting up a barometer. It needs to have the altitude at which it is installed in order for it to correctly display the atmospheric pressure at MSL (obviously).

My GPS says I'm 120 metres above sea level. I've set the barometer offset to 120 metres at it's returning an MSL atmospheric pressure of 1013 hpa. All the nearest Melbourne weather stations are reading 1017.5/6/7.

That'd mean I'm actually 150 metres above sea level. Do I trust the GPS altitude is correct?

[Snapper]

The ambient temperature will affect the reading also.

[Utemad]

I don't know how true this is but I was told by a GPS store a few years ago that the altitude on standard GPS units is accurate to 2x the displayed accuracy.

[JDNSW]

If it is windy, the pressure may actually vary by that much. The equivalence of pressure and altitude is somewhat dependent on temperature, but also it is quite possible that 4mb is about the (absolute) accuracy of the barometer! Given an adequate set of satellites, the GPS elevation should be accurate to a couple of metres. I would be inclined to check your elevation from a map - you may have an atlas with adequate contours. If you give your lat/long it is likely that someone here would be able to give you an elevation.

John

[kenleyfred]

With my barometer experience it is watching the trend more than the exact reading that matters. Of course it is nice to have it as accurate as possible though.

[B92 8NW]

It's gotta be faulty:(

Google Earth is giving 92m
Victorinox watch is giving 90m
GPS is giving ~90m (playing ball now)

Old barometer is giving thirty point zero six millimetres of hydrargyrum.
BOM is giving 1018.0
They match up perfectly.

New barometer set for 90m is giving 1009:eek:

[StephenF10]

Why not just set it to MSL pressure and forget about the altitude correction? This is how domestic barometers are normally set up.

Stephen.

[Gillie]

This is an interesting problem. I have tried to use the altitude reading to find a mine site once. The theory being that if I contour around the hills at the correct altitude I should be able to find the mine for which the height above sea level was known. Step one was to go to a known place. For me this was a bridge over a river with a known elevation of 400 m. The Garmin emap showed an elevation of anywhere from 420 to 470m. Therefore it really didn't help and I wouldn't trust the elevation reading but use it as a rough guide.

I have heard you can ring the weather bereau for the pressure but this doesn't help when you live no where near where the reading has taken place.

I reckon you could take the barometre to a site where the pressure is known, and then just take it home and hang it on the wall. I am told this wouldn't work as the elevation would be different. But wouldn't the barometre adjust accordingly? Surly if it set correctly to start with, moving it to a different altitude shouldn't necessitate another calibration................ or would it?

[adonuff]

Some GPS's have the ability to calibrate, I walked down to the beach sea level and did the calibration there. Result was within a few meters next time I tried it out where the elevation was listed on the map at a spot I was visiting.

[Bushie]

I would take the elevation off a map of the area if you have no other means. GPS is notoriously poor for elevation (it can probably be improved if there is some form of post processing of the data).

This explains it better than I could (http://gpsinformation.net/main/altitude.htm)


Martyn

[JDNSW]

It's gotta be faulty:(

Google Earth is giving 92m
Victorinox watch is giving 90m
GPS is giving ~90m (playing ball now)

Old barometer is giving thirty point zero six millimetres of hydrargyrum.
BOM is giving 1018.0
They match up perfectly.

New barometer set for 90m is giving 1009:eek:

See my comment about absolute accuracy of the barometer. You may want to consider returning it, but it is probably a calibration error you can quite easily live with once you know about it.

John