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

[Landon Blake]

Sounds like you may have found a workable solution Aaron. I hope things
work out.

Landon
Office Phone Number: (209) 946-0268
Cell Phone Number: (209) 992-0658
 
 

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Aaron Friesen
Sent: Monday, February 23, 2009 8:33 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] How to transform between NAD83 HARN/HPGN and WGS84

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

_______________________________________________
Proj mailing list
Proj at lists.maptools.org
http://lists.maptools.org/mailman/listinfo/proj


Warning:
Information provided via electronic media is not guaranteed against defects including translation and transmission errors. If the reader is not the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this information in error, please notify the sender immediately.