Yes Minecraft can be good exampla of how to have huge maps with low memory consumption.
But From what I understand whtemple1959 is trying to make real time strategy game similar to OpenTTD. For RTS you need to have whole map loaded at all times.
Google-Maps do contain height information but it is only available through Google Earth application and not Online Maps.
I belive Google Maps do have such acuracy.
If he is trying to make somekinda remake of OpenTTD he will probably alow player to modify terrain as it is posible in OpenTTD.
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