In my Site information section I see
Elevation: Unknown
**Antenna location above ground level: **6 meters (Update)
I can update my antenna location above ground level but where do I set the Elevation?
They take the ground elevation off Google Earth when you set the location. So, you only need to enter the elevation above ground ( needed for MLAT accuracy).
I added my antenna height and reset my location. It’s still at unknown. Guess it will find me.
And it did, I am at 63 meters
New options … so, being my antenna is 6 feet tall, do you measure from the ground to the bottom or the top of the antenna?
When you select update it shows to the bottom of the antenna.
As long as you’re approximately correct it doesn’t really matter if you’re out by a bit. The main thing is to distinguish 1m AGL from 100m AGL 
But I can’t read pictures! 
I see it now.
I did wonder what this was but figured it out. 97M Elevation!
As by “Antenna Height” it’s the height from the floor to the bottom of your antenna.
For example on a two storey house and say your antenna was on the floor above the ground floor (2nd floor) then the ground to the 2nd floor is about 3Meters.
For an exactly height, use a GPS.
Good idea. I can stick my Bad Elf at the bottom and get an exact position. If the pre-filled elevation is off, I can use the antenna height as a correction factor! hehheheh.
This works for most – but not all – GPS receivers and it’s probably a good idea to double check.
As an example, my handheld Garmin eTrex 20 and my Garmin GPS 18x LVC receiver (both reasonably modern receivers) report the elevation relative to MSL. However, my Trimble Thunderbolt timing receiver (quite a bit older) reports the elevation relative to the WGS84 ellipsoid. In Bern, Switzerland the difference between the MSL and WGS84 elevations is around 50m.
Unfortunately, which system is used for elevation is not documented well in the manuals for most GPS receivers. Most modern receivers report the elevation relative to MSL, but the internal lookup table for WGS84-MSL conversion can be somewhat coarse.
If one needs better accuracy or wants to double check if their receiver is reporting elevation relative to the WGS84 ellipsoid or MSL, one can always use this handy page: just enter a set of coordinates (or street address, or any other location information that Google Maps understands) and it will output the elevation of the ground at that point in both meters and feet relative to MSL.
Alternatively, if one prefers going outside, there’s often survey markers left by the government. Be aware that altitudes have been reported in various units and scales over time, so be sure to verify which is being used for that particular marker.
For example, the Swiss use their own coordinate system from 1903 and report mean sea level relative to a marker in Lake Geneva, which is in turn defined relative to the mean sea level in Marseilles, France. The government agency responsible for mapping and surveying offers a utility for converting between the Swiss grid and WGS84 while this site allows one to convert elevation at a specific point between the WGS84 ellipsoid and EGM96 geoid (which approximates MSL). The difference in elevation at the nearby benchmark between the Swiss system, EGM96, and the Google Location API is within 1 meter. The elevation reported by my eTrex 20 is the same within the estimated error (~5 meters) so it’s clear the GPS receiver is reporting the altitude relative to MSL.
I’m sure that similar methods can be done in other countries.
On a related note, your suggestion to “use a GPS” sparked several hours of fascinating research into the various mapping systems and standards used in several countries. Thank you.
On the server side it’s all ECEF coordinates, so at least there’s no ambiguity there. Actually at the moment it assumes that elevations are height-above-ellipsoid when converting the receiver location to ECEF, which is just wrong because the web interface is set up to capture height-above-geoid. Eventually I’ll teach the server about EGM96 or a similar geoid model to get a little more accuracy. Also there’s an unresolved thing with transponder altitudes being pressure altitudes …
The other errors in the system are large enough (dongle timing error is ~150m) that it doesn’t really make all that much difference at the moment; and the mlat solver is fairly insensitive to elevation errors unless the aircraft is right on top of you. It’s still nice to capture the data accurately though, since then it’s useful once the other bits get fixed.
edit: it actually turned out to be pretty easy to change, so the servers are using EGM96 now. So MSL really is MSL again