What is the configuration of your altimeter settings? See Setup > Altimeter. And are you looking at the GPS Elevation data field or the Elevation data field? The latter shows the value that the GPSMAP uses for actual elevation.
Note that GPS elevation is not very accurate (+- 400 feet) due to the way GPS works. Therefore the GPSMAP uses the barometer in combination with GPS elevation to determine Elevation.
And if you are at a known elevation, you can always manually calibrate to that elevation.
This Garmin support article may give you some more insights on elevation accuracy: support.garmin.com/.../
The stated accuracy is for horizontal positioning (location on earth) with GNSS. Vertical positioning (elevation) by GNSS is around 1.5 - 2.0 less accurate than horizontal accuracy.
Although WGS84 can also be used to refer to elevation, on the GPSMAP 67 it is used as coordinate system (afaik).
The GPSMAP will use both the elevation as measured by GNSS and the atmospheric pressure as measured by the barometer. Based on this and the changes in atmospheric pressure a more accurate elevation is calculated. The way calibration works is explained in this article: Altimeter Auto Calibration for Outdoor Handhelds and Watches.
Unfortunately not. Other Garmin devices (watches, cycling computers) do have this feature but it's not available on the GPSMAP 67. On the other hand I have always found the elevation of the GPSMAP 67 quite accurate, within a couple of meters of the actual elevation.
I think elevation measurement is always approximate, even what is stated on topo maps is lose assumption. There is no absolute "sea level" point by the sea, as a result there is no absolute barometric pressure at the given sea level. The weather stations people /GIS take a place near the sea and call it "sea level" and whatever the BA is becomes the reference.
well the geoid height is defined, no problem with that.
The only small problem is how to check it with suitable method. Common satellite positioning is mainly designed to deliver x and y information on the surface, just slightly less reliable on the z.
There is no barometric pressure defined for anything, as such value is simply meterological value depending on the amount of air being above a given point at given time. If the is low pressure situation at a given point, the amount of air and thus pressure will be different then when the is high pressure .
If someone is depending on elevation value measured above any point, he needs to calibrate the device just before the measurements take place. Like an airplane, will check / calibrate its device either when on ground or according to advise from the ATC on the airport.
When at cruising level, standard values are used, as in this situation not every feet is relevant.
So no, just some 'weather people' will no decide what they just like. They know their position and height, therefore they are able to measure atmospheric pressure at that point.
"well the geoid height is defined, no problem with that."
No, that value is assumed, because all parameters that *define* "sea level" are variables. As someone else said tieds can change the sea level significantly. A 10 meter tied(which is not uncommon) alone will change the BP by 1-2 atm if we were to measure it, that is 10-20meters of inaccuracy. But to keep the peace, we accept the assumed values. That is why accuracy of elevation by GPS should be taken with a grain of salt and forgiveness....
