[PROJ] Static/Dynamic Webmapping Problem version 2.0

Greg Troxel gdt at lexort.com
Thu Jul 18 13:10:19 PDT 2019

Nick Mein <nick_mein at trimble.com> writes:

>>> To me, "high precision" means cm level, or better, which is why I've
>>> been saying that there is no such thing as a precise WGS84 coordinate
>>> (unless perhaps you are working for the US military).
>> I don't follow the reference to being part of the military but perhaps
>> there is non-public data.
>> The US NGS seems to publish coordinates for CORS in IGS08 epcoh 2005.0
>>and NAD83(2011) epoch 2010.0.  As far as I can tell WGS84(G1762), IGS08
>>and ITRF08 are essentially the same.
> Maybe I'm being a bit pedantic here. But the trouble is that a coordinate
> that is described as being "WGS84" is generally either not precise (because
> it is derived from broadcast orbits), or not WGS84 (because it is actually
> some in some national datum that some piece of software decided was
> equivalent to WGS84). So yes, if your precise point positioning service
> gives you a ISG08 coordinate, that is essentially the same as a
> WGS84(G1762) or ITRF08 or ITRF14 coordinate. But calling it "WGS84" doesn't
> seem to help anyone.

Totally fair, and now I understand what you mean.

I was separating in my mind, but not saying it, the intrinsic quality of
the datum, and accuracy issues surrounding coordinates obtained in that
datum.  I now realize that there is effectively no path to access WGS84
at a precise level because nobody publishes WGS84 coordinates of CORS
stations, etc.  That explains the "outside DOD" comment too.

Agreed that precise use of GPS is invariably in some other datum.  At
least these days IGSxx ITRFxx and WGS84(recent) are all basically the

I have a hard time finding clear statements about WAAS, but it seems to
be defined to be some ITRF, last I was able to find it.   I had wondered
if it would be in NAD83, given the FAA.   So I now expect to get the
same coordinates out of nav-grade GPS with waas and without (modulo
errors of course).

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