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

[Landon Blake]

Aaron,

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

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! :]

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. 

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.

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: Friday, February 20, 2009 3:34 PM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] How to transform between NAD83 HARN/HPGN and WGS84

Landon,

> -----Original Message-----
> A quick Google search of the term "NAD83 WGS84" turned up this
> interesting link:
> 
> http://forums.groundspeak.com/GC/index.php?showtopic=97337
> 
> There is quite a long discussion on your very question. I skimmed some
> of the postings, and I think the answer to your question is not a
> simple one. I am going to study the postings in more detail, and I may

> check with some other surveyors on the topic. I'm also going to
forward 
> this message to my boss, who is a geodesy junkie.

Thank you for the pointer.  I have only quickly perused the info so far,
but it is very interesting and definitely on topic.

Rob/Geocacher (apparently now on the banned list on that forum) makes
the overly simplistic statement that one should just "use the datum that
is right for the job".  Unfortunately, this is also not the answer,
since the only cost-effective means to collect much of this type of
data is via GPS, but for these activites that result from government
edicts, the result still needs to be in a NAD83 variant.

> I'm finding it hard to understand the practical circumstances
> surrounding your question. Let me take a stab at a scenario that I
> think is close to your own scenario, and then I will ask some
questions:
> 
> You've been provided data on all of the municipal wells in the United
> States for cities with a population over 250,000 people. The position
> of the wells in this data is (supposedly) sub-foot, and it is provided
> (supposedly) in flavors of NAD83, including state plane coordinates.

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.

> You now need to go out and (1) verify the position of wells that are
> already contained in the data set and (2) add new positions for
> municipal wells for cities with a population over (let's say) 50,000.

The scenario still sounds good enough for this discussion.

> Here is my first question:
> 
> What type of receiver are you using to obtain new sub-foot positions,
> and on what datum are these positions based?
> 
> Let's say you're using a new super duper GIS grade GPS receiver from
> Trimble that get's sub-foot post-processed accuracy. This means that
> you've got positions that are precise to less than a foot on the WGS84
> datum.

I will neither confirm nor deny ;^), but lets assume Trimble GeoXH/ProXH
with post-processing, wink, wink, nudge, nudge.  

> Now you want to convert those very precise WGS84 coordinates to very
> precise NAD83 coordinates.

Yup.

> I think you are screwed. There is no single formula conversion between
> NAD83 and WGS84, based on the little bit of information that I have
> read.

I'm not necessarily needing a single formula.  Multiple formulas based
on region would be viable, it that even is possible.

> If you really want a data set that covers positions throughout the
> United States and maintains an overall sub-foot accuracy, I think you
> will have to survey every feature in your data set from scratch. This
> would give you a new dataset with sub-foot WGS84 positions.
> 
> I think the only way to establish NAD83 positions is to survey in
local
> NAD83 control, or to use NAD83 CORS data in your post-processing. This
> would be very time consuming, if not near impossible, for data all
> across the United States. We have a hard time getting NAD 83 data to
> fit closer than a foot from one side of California to the other. All
types
> of problems like ground subsidence and plate tectonics come into play.

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.  

Pardon my display of ignorance...

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?

> The root cause of your problem: It may not be as easy to turn sub-foot
> WGS84 positions into sub-foot NAD83 positions as one may first think.

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,

Aaron

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