Elevation data wrong for first couple of seconds

How long between getting a GPS lock and starting the recording?  If you are starting recording very soon after getting GPS lock it is likely the altitude values haven't stabilised yet.

I wait about as long as I ever did with my Edge 510.  Probably about 3-5 minutes.  Never had this problem with that device.  I've had other problems with that device, but not this one.

The first elevation data point is off. The second data point is reasonable.  It happens nearly (if not every) ride.

There's one more thing i was going to try today.  If I find anything else, I'll update.

It's probably not my idea then.  To be honest I wouldn't worry about it.

Do you have your start point recorded with the correct elevation as a location?

When I start my ride, I press the "Start" button, a message pops up straight away, "calibrating elevation" and I have the correct elevation the moment that I start. Without the correct elevation and just relying on GPS, it may take a few seconds to calculate your elevation.

If your not starting from home etc, and instead somewhere different, just use the garmin to record your start point and save it as a location and it should record as well the elevation or manually input it if you know it. Then when your about to start your ride, it will work as I mention above.

it almost seems like the very act of pushing the button lowers the pressure and the first value recorded is quite off from the others.
I don't really care if my precise elevation is recorded as long as it's relatively accurate throughout the ride.  The profile should be accurate.  Eg. I use the relative elevation gain/loss as a parameter in calculating my power.  1200W power spikes in the first second of EVERY ride is annoying.

Having said that:
I have a home entry entered and I do get the 'Calibrating Elevation' (or whatever it exactly says) message.  It does seem better when starting from this location, but even still, it's like the act of pushing the button lowers the pressure in the device or something.  Also, the first data point recorded does not match the Elevation data shown on the head unit at the time.

You get a message pop up saying something like GPS ready.  Also, if you look at the map page there is a question mark on the location marker while it is looking for satellites.  This goes away when you have GPS lock.