Altimeter way off

This is from Garmin Support.  Maybe trying the soak will help.  I use Dawn dish soap which will also help with oils and any lime or mineral deposits without harming your watch.  It worked wonders on my 935.  Best of luck.

Other factors to consider:

Some factors that might impact the function of the altimeter/barometer might include rapidly moving air entering the sensor ports while participating in high speed activities; climate controlled buildings which can have slightly different pressure indoors compared to the ambient pressure outside; and soap, dirt, or salts from sweat which can clog the sensor ports. For the latter, you can try to clean the ports by soaking the watch in warm water with a little mild detergent.

NOTE: Never insert anything into the openings as it could damage the sensor inside.

I did the 1200ft vert hike this morning. Checked Elevation at the top(466 meters ) and it was 357 Meters lower. That's 300ft difference. I calibrated with DEM and it came up as 127 meters ( way way out). Calibrated with GPS, it was the closest at 450 meters. I calibrated manually to get the correct 466.

Goint to try TMK17 suggestion. I've already had issue connection to the PC due to build up on the terminals so his suggestion could do the trick.

Were you near to a steep hillside with a vertical drop to 127 meters when you calibrated with DEM.

The DEM part has me worried, because we should generally expect that to fit the landscape. But of course we can only trust it if we use it in a landscape which is so flat that a small GPS error doesn't throw us up or down a steep hill.

Within 2 meters of a drop off but only about 50 meters. 

I'm going to do a 850 meter vert hike tomorrow. I'll use climb pro for the first time. The peak is on a big area with no drop offs so I'll test all calibration methods again then. Will let you know the results.

Ok this is where I'm at. Did the hike today, top of mountain was 846 meters. Checked DEM it was 515 meters. Something seriously wrong there. Then switched to calibrate with GPS 844 meters. I can live with that.

Next plan is do the soak in water method overnight and test again tomorrow. Process of elimination.

The soak in water will only help if you have problems with your ambient pressure sensor hardware.

But the DEM problem is obviously a software/data problem, not a hardware problem. It would seem that the DEM map in the watch is way off for your area, or the watch uses the data from the map wrongly (a rogue miles to km conversion, perhaps, though that would make little sense?).

I don't know if there is any way of systematically checking DEM data against a map with good, known altitudes.

I think your Right Allan. I didn't bother with the water soak. Did the local 400 meter vert hike yesterday. half way up I calibrated the altimeter with GPS. got to the top and it was 3 meters out. Huge improvement.

I have a feeling it hasn't been calibrating continuously during a activity. I'm on Firmware 7.60 atm. Don't know if changing to the beta is going to do anything. What's your view?

Regarding the beta, I can only say that I am running 8.01, and I haven't experienced any new problems, compared to 7.60, and I haven't heard of any either. I don't know if it will solve your problem, but I would consider it a "risk free" attempt.

Regarding the altitude:
I think you should start out by more systematically mapping the difference between GPS altitude, DEM altitude and real altitude at a few different locations in flat(ish) terrain, but at different altitudes. If GPS and real altitude agrees, and DEM is way off, then Garmin will have something to explain.

If you want to know if continuous autocalibration is taking place, I think that you can test it by doing this, but I haven't tested it myself:

1. Force your watch into altimeter mode. This should lock your barometer value to a constant reading, as long as the altimeter isn't recalibrated.
   
   
2. Start an activity with altimeter auto calibration during activity set to "At Start".
   
   
3. Put the Barometric Pressure data field on a data screen in the activity. Note its reading after starting the activity.
   
   
4. Do the activity for some hours and wait for an altitude error to build up (best to do this on a day where a large change in barometric pressure is expected). The barometer reading should be unchanged during this. This step is only to verify that the watch is really staying in Altimeter Mode within an activity, which I don't know for sure.
   
   
5. While you are still in the activity, do a forced altimeter calibration. Do not calibrate the barometer. Then look at the barometer screen again and verify that the barometer reading changed as a result of the altimeter calibration.
   
   
6. Start an activity with altimeter auto calibration during activity set to "Continuous".
   
   
7. Again keep a look to the barometer reading during your activity. If it stays fixed while you build up an altimeter error, your auto calibration during the activity is probably not working. If it changes, and you still get large altimeter errors, the watch is actually doing auto calibrations, but perhaps with wrong DEM data.

All of the above assumes that the ambient pressure sensor is working correctly.