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

[Landon Blake]

Aaron,

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.

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.

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.

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. 

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

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.

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.

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.


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 11:31 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] How to transform between NAD83 HARN/HPGN and WGS84

Thank you all for your initial responses...

===== Short follow-up =====

A short follow-up question for those that don't want to read my
long-winded follow-up below:

Do either NADCON or CorpsCon provide datum transforms between a
given NAD83 and WGS84?  The websites seem to indicate that they
are only for transforms among various NAD datums and I do not
know of a NAD83 variant that is truly assumed to be equivalent
to WGS84/ITRF00.  Please correct me if I am wrong on that.

Thanks again,

Aaron

===== Long-winded follow-up that you may want to ignore =====

Hopefully, you do not mind continuing the thread.  I am responding
to multiple people in this message, hoping to keep the thread more
coherent.

Due to non-disclosure limitations, my original description of my needs
only indicated Colorado as a location under NAD83 HARN, thinking I might
be able to extrapolate responses to a more general solution.  I
apologize
if I wasted your time with my original query.

Let me expand a bit and re-emphasize one point that seems to not have
been addressed (i.e., conversion to/from WGS84).

I will be working with data originating in various parts of the US.
The data represents various assets in the field where sub-foot accuracy 
is important (or claimed to be important).  The first assumption is that

existing data has the accuracy it claims to have.  The second assumption

is that existing data is in the datum it claims to be in.  Both of these

assumptions may be incorrect, but need to be assumed to be true for this

discussion.

For existing data, I need to be able to go to the field and locate that 
asset within the combined error of the original reading and the current 
reading from the GPS device used to locate the asset.  For new assets, I

need to be able to take a reading and add that information to the data
set.  
All GPS field readings for this discussion can be assumed to have
sub-foot 
accuracy.  If they do not, that is an issue between me and the GPS 
manufacturer claiming such accuracy.  In any case, we are still talking
sub-foot mapping grade GPS, not sub-centimeter survey grade.

The data starts out in various projections (some in State Plane, some
in UTM, some living its entire life as lat/long).  I would also like to
assume that any projection does not add significant error.  Yes, another
assumption that may or may not be valid, but for now...

The problem is that these various data-sets are in various NAD83
incarnations,
and to take the data into the field or bring back new data from the
field, one
or more datum transforms are necessary, getting it to WGS84 and back
retaining
its original sub-foot accuracy.

The transform to WGS84 seems to be the point not directly addressed.  At
least
I did not see it that way.  For sub-foot work, the old assumption of
NAD83==WGS84
doesn't do it.

> From: C. Mugnier
> 
> Such questions regarding geodetic-accuracy transformations in Colorado
> are best directed to the U.S. National Geodetic Survey.  NADCON indeed
> does have NAD83 HARNHPGN files, and can provide the necessary
> operations either on-line or through a download.  The same goes for
> current versions of CorpsCon.  Both solutions are not commercial and
> both are free.
> 
> Go to www.NOAA.NGS.gov and you'll get your problem solved for free
> courtesy of the U.S. Government.  A Google search will get you to the
> current version of CorpsCon.

Both NADCON and CorpsCon descriptions talk of conversion between various
incarnations of NAD83, but neither seems to address the transform
to/from
WGS84/ITRF00.

> From: Landon Blake
> 
> When you say NAD83 HARN are you using Colorado State Plane
Coordinates?
> Or are you dealing with lat/long values?

All lat/long WGS84 in the field.

> I don't think there isn't a huge difference between NAD83 and WGS84,
> but
> I'd think Proj would have a conversion between the two.

Depending on the NAD83 incarnation, NAD83 to WGS84 can differ by several
meters.  Much of the Proj documentation implies that Proj does a null
transformation for NAD83 to WGS84 by default.  That may or may not be
true, but the documentation seems to imply it.

With the +towgs84 switch, I might be able to over-ride that, but I'm not
sure this is sufficient to meet my sub-foot requirements and cannot find
any information on what those parameters might be for a given transform.

> If you are dealing with state plane coordinates a local surveyor can
> probably point you to the conversion formulas that you need. Or you
> might check with the state land surveying association in Colorado.
> 
> If you get stuck, let me know. I can bounce a message off a couple of
> land surveying message boards with Colorado Surveyors listening.

I'm hoping to find a single conversion package (or at least a small
set) to address this problem in the general case.

> I'd be careful using something like Corpscon for data that is sub-foot
> accuracy and needs to stay that way.
> 
> We wouldn't use that program or any like it to convert data that needs
> to be that accurate. I don't know what the purpose of Aaron's data
> conversion is, but he might need to be careful.
> 
> Transforming the location of gopher holes using Corpscon is one thing.
> Transforming building corner positions, or the alignment of a new
> pipeline with Corpscon is another thing entirely.

My needs are closer to the pipeline example than the gopher hole one.

> From: Rich Greenwood
> 
> Corpscon (http://crunch.tec.army.mil/software/corpscon/corpscon.html)
> based on NADCON is a good, free, authoritative tool when working in
> the US.

(Sorry for the repeat) But neither Corpscon nor NADCON seem to be able
to get me to WGS84.

> From: Landon Blake
>
> I was recommending that one consider the purpose of the data
> transformation before deciding to use ANY transformation package. If I
> was building a nuclear reactor I would want to perform field surveys
to
> tie into local survey control.
> 
> Aaron is probably fine using CopsCon. I just get nervous when I hear
> "sub-foot" accuracy. Usually when you are dealing with coordinates at
> that level of accuracy you are leaving the GIS arena and entering an
> arena that demands field work to reference local control on the
ground.

Rest assured, no nuclear reactors are in the works.  I understand what
you are saying.  Many times, I deal with highly-precise but very
inaccurate
data, with GPS readings recorded down to 7 or more decimal places from a
consumer grade device with only 30 meter accuracy.

Thanks again,

Aaron
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