There is a very new information about elevations from PGC:
12-17 - Gifting, Labs and Elevations
We have been fiddling with the elevation on and off for a few years now. Actually, we have probably spent too much time on it. There aren't any sources which are 100% correct, and in fact, most sources have quite big cons in some area. Google's elevation services seem to be the best, which we have used. But a few years back they added a cost to it that wasn't feasible, not even worth considering. Since then we have been trying several different approaches.
Earlier this year we started to use a source called Mapzen. It's actually a merge of several other sources. The idea is to use the source that's best in the area it's best in. We have slowly been converting our data from our old, to data received from this system. After 1-2 months of progress we found out that there were some real discrepancies in many of the zones where differen sub-sources were close to each other. Sometimes values way higher than Mount Everest were returned in areas that were close to sea level. We then made some fail-safes in our end, and restarted processing data.
About now we are back when we were as furthest, with about 30% of our data converted to the new elevation system. We will keep converting data slowly and by time we will catch up, hopefully with a better end-result.
Note that the elevation data probably never will be perfect. We are using sources available to get as good data as we can. It's not feasible for us to manually review and update elevation data for millions of positions on the earth. Due to that we have decided that we don't make manual exceptions at all. However, cases which are wildly off will be investigated, analyzed and hopefully countered.