[Proj] How to transform between NAD83 HARN/HPGN and WGS84

[Aaron Friesen]

Landon, et al.

> -----Original Message-----
> This has been a great discussion. I'll try to respond to some of your
> further comments and questions below.

I'm glad you enjoy my pain.  ;^)

> Aaron wrote: " This example will do quite nicely, I think.  The NAD83
> flavors are not
> intermixed throughout the entire US, but are used as local variants,
> such as HARN Colorado, HARN Virginia, CORS96 in various places, etc."
> 
> That is what I meant. :]
> 
> Aaron wrote: " I will neither confirm nor deny ;^), but lets assume
> Trimble GeoXH/ProXH
> with post-processing, wink, wink, nudge, nudge."
> 
> Roger that. I suspect I will be seeing more of your type of question as
> more GIS users begin to obtain sub-foot positions with the new GPS
> receivers Trimble is putting out. The problem is where starting to get
> GIS grade positions that want to play in a survey-grade world. :]
>
> Aaron wrote: " I'm not necessarily needing a single formula.  Multiple
> formulas based
> on region would be viable, it that even is possible."
> 
> I don't think this will even be possible. There is only one way that I
> know of to get from GPS to NAD83. That is by surveying NGS or other
> survey control in the area that has record (established) NAD83
> coordinates. You have to remember that surveyors don't convert WGS84
> positions directly to NAD83 either. GPS is just a measuring stick that
> allows us to calculate NAD83 coordinates from existing control
> monuments. You could do the same thing with conventional (optical)
> survey instruments if you wanted to spend the time. In fact, before GPS
> this is how NGS used to establish horizontal control.
> 
> Aaron wrote: " Clarifying the "across the US" piece, I have data in
> lots of places across the US, but it is multiple sets, not necessarily 
> needing to mesh into a single data set."
> 
> I don't think this is going to matter in the end game.
> 
> Aaron wrote: "Isn't the CORS data generated from GPS readings of some
> sort?  How do they make the translation?  Or do they do something 
> like start with an assumed known location and simply monitor its 
> deviation from the original location?"
> 
> That is exactly how they do it! :]

I suspected, but wasn't sure.

> They use existing NAD83 control to establish NAD83 coordinates on the
> CORS. I get the raw GPS observations from the CORS in WGS84, but the
> record coordinates are usually always NAD83. In this sense, the CORS is
> just acting as one end of the measuring stick. :] What I'm really
> interested in is the vector (or baseline) from that CORS to whatever
> point I'm measuring on the other end.
> 
> Your next question will probably be "how did they come up with the
> original NAD83 positions?"
> 
> Don't quote me on this, but I think it involved the following:
> 
> Large horizontal control networks created from terrestrial observations
> using conventional (optical) surveying measurements, astronomical
> observations (to determine latitude and longitude), and gravity
> measurements (to determine the deflection of the vertical).
> 
> This NAD83 network was later adjusted using GPS measurements. But
> again, GPS was just serving as a measuring stick, even in this 
> process.
> 
> So in some sense NAD83 is really defined by a network of existing
> surveying monuments located on the ground. There are mathematical
> parameters (like the dimensions of the ellipsoid) that are used in
> calculating the NAD83 positions of those monuments, but the monuments
> are the most important thing.
> 
> If you aren't incorporating the measurement of NAD83 monuments into
> your own measurements somehow, you aren't getting NAD83 positions.

I will note this.

> Aaron wrote: " That may actually be the answer, but I need to be able
> be able to reference reliable, if not definitive, sources on that 
> answer.
> 
> Thanks again for your assistance and expertise,"
> 
> I must admit I am getting way out of my own area of expertise. You
> might try contacting the NGS for a "reliable and definitive" answer 
> to your question. I can put you in touch with the NGS director for 
> California if you want.

I will pass on this suggestion and offer to the higher-ups.

Thank you,

Aaron