I've been trying to work out how the Vista HCx altimeter is set up and used, the manual is pretty poor.
Here's what I've worked out, from experimentation and web searches. This might be useful to you, or you can tell me what I've got wrong...
The altimeter uses the pressure, it's not possible to turn this off (like the compass) and just use GPS altitude (but there's a workaround).
If you want to know the GPS altitude, then go to the satellites page (the one that appears straight after booting up) and pick "GPS Elevation" from the menu. This just tells you the number at that instant in time, it doesn't record it in the .gpx files.
On the profile display page, there are two different ways to display air pressure (pick them using "Change Data Fields" from the menu). "Ambient Prsr" shows the air pressure where you are, just like a mechanical barometer. "Barometer" tells you what the pressure at sea level is. So "Ambient Prsr" will change as you go up and down, but "Barometer" doesn't.
From the setup menu (main menu, "Setup", "Altimeter") there are two options to pick.
"Barometer mode" is either "Fixed Elevation" or "Variable Elevation". In "Variable Elevation" mode the calculations assume that changes in pressure are caused by changes in elevation. In "Fixed Elevation" mode, it assumes that changes in ambient pressure are caused by changes in the atmosphere.
"Auto Calibration" on or off. If this is on, then it periodically checks the GPS altitude and uses this to change what it thinks air pressure at sea level is. If this is off, then you should manually recalibrate frequently from known actual altitude.
You can manually calibrate using the menu on the profile display page. You can tell it what you know the ambient pressure to be (e.g. off a barometer) this means that the pressure numbers it displays will be correct. More importantly you can tell it what your current altitude is, this allows it to estimate pressure at sea level and therefore altitude given pressure.
If you are cycling (or hiking) and you want to know your elevation, then keep auto calibration on and use variable elevation. Before you leave try to calibrate using the known elevation of your house.
If you are sailing then your altitude will not change much, but you need to know the air pressure to check the weather. So use auto calibration off and fixed elevation. Calibrate altitude to sea level when you get on the boat.
If you are flying in an pressurised aeroplane then you really want to use the GPS elevation. You can simulate this by using auto calibration on, and fixed elevation. In this case fixed elevation tells it that pressure changes are caused by the cabin, not by changes in elevation. The profile display will seem odd -- the final time point (now) will always have the wrong altitude, but this is corrected a few seconds later. The correct values are stored in the .gpx files. The "Barometer" data field is meaningless, and "Ambient Prsr" tells you the cabin pressure -- panic if it suddenly drops!
You would only use auto calibration off and variable elevation if you frequently visit points of known altitude and manually recalibrate at them.
