As far as I know, pretty much all surveys carried out over the years have the south east corner of G1 at some 10cm or less under 60m above sea level. As they all kind of agree - some might have 59.94m ASL, others 59.97m, and still others 59.95m - you can't really go wrong with 59.96m ASL. THat SE corner is the tricky thing though...do all surveys identify the same mark upon which to place their reflectors and staffs to measure to?
Surveys use different approaches when focusing on one point, as they come to that point from different directions and different survey marks spread out across the landscape. There could 4 main ways of coming to the SE corner from different directions and different main survey marks used by government surveyors for all kinds of surveys around the plateau area. These are the main marks used for road and building surveying and have been used so often that their exact position and HASL is accepted to probably within a tenth of a millimeter. Those 4 main marks can be used from probably 20 other main marks in different directions away from the plateau, and as all can be sighted to, all agree with each other about exact positions and HASL. When it comes to construction anywhere, exact boundaries need to be known so future contests about where something actually is do not mean extra expense to someone for having to either take something down or pay someone for adjustments to be made. All survey marks throughout the entire city can be used to verify each and every one of them to within a tenth of a mm for position and height above sea level.
How extra important is knowing HASL for anything? Aviation.
I don't think you can go wrong with basing things away from G1 according to 59.96m above sea level. Of course, you will get just a couple of posters here will side track away from your topic and take this little post all over the place because they just love to argue even 1 inch...and one particular poster doesn't like sea level as something to measure from lol You know which ones are interested in your topic and which just drink too much and need to argue because they are all alone at their home and are bored and boring.