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

[Aaron Friesen]

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