[Rockhounds] more thoughts on sharing collecting sites

[DonH]

Alan Silverstein wrote:

>>...the datum depends on which coordinate system you use.  The UTM
>>system has many advantages over the lat/long system, though some
>>prefer one or the other.  The WGS84 datum should be used with lat/long
>>and, for some reason, NAD27 should be used with UTM.
> 
> 
> Eh?  To the best of my knowledge, the choice of datum and the choice of
> format are wholly independent.  (Am I wrong?)


Well yes they are independent, but for some reason--I don't know 
why--when I change TopoUSA to UTM coordinates, it gives me a warning 
saying something about the datum for UTM/UPS is strongly preferred as 
NAD27.  I've also been told this by some older and more experienced 
geologists.  If anyone *does* know why, let us know!

Perhaps it is because, as you said, many of the older USGS maps are 
based on NAD27, and that is also the basis for TopoUSA?  No idea here.

best,
Don