shrug-l: WGS84 vs NAD83 and data transformation

Sykes, John John.Sykes at dep.state.fl.us
Wed Jun 7 14:47:54 EDT 2006


Actually, that is a good argument for post-processing GPS data (which you
have to do to get to sub-centimeter accuracy).  I believe that you can
provide the raw GPS data and NGS will reference it to CORS current positions
and convert it into NAD83(HARN) for you.

Precision IS an issue, as I stated in an e-mail to Eric.  In the USGS
Professional Paper on projections, the author makes the argument for
quadruple precision (128 bits) in the computations to avoid precision
problems.  I developed a FORTRAN 95 program to convert Lat/Long NAD83(HARN)
to FDEP Alber's NAD83(HARN) that carries 18 significant digits (roughly
triple precision - 80 bits), but, as I said to Eric, I really only trust
about 15 (e.g., 3 guard digits), which still makes the conversions accurate
to better than 1 μm.  Unfortunately, going from Lat/Long to Alber's has an
exact solution, unprojecting the data (going from Alber's to Lat/Long) has
some problems and is not explicitly solvable (you can use an infinite series
expansion or an iterative approach).  Now, going from NAD27 to NAD83(86) or
NAD82(HARN) is a different story!

-- John

-----Original Message-----
From: Eric_Songer at URSCorp.com [mailto:Eric_Songer at URSCorp.com] 
Sent: Wednesday, June 07, 2006 2:26 PM
To: Shawn Lewers
Cc: 'Brandt, Holli'; Sykes, John; shrug-l at lists.dep.state.fl.us;
shrug-l-bounces at lists.dep.state.fl.us
Subject: RE: shrug-l: WGS84 vs NAD83 and data transformation

My 2 Cents,

No, I don't think so, but you will need to be very careful when you are using
data that is this accurate.  We all take liberties when the error is 5-10
meters, maybe even 3-5 meters, but when you are overlaying 1 meter data on a
6 inch resolution image of unknown spatial accuracy these differences become
apparent, and data may appear "wrong" when it is really stored in a different
projection/coordinate system.

There may be a way to do this already, but I think a more robust way in
Arcmap is needed to account for the variations of the different "adjustments"
made to NAD83 by the various states.  If there were a way to assign NAD83
1996 adjustment to the dataset's projection file, I think that would work.
We messed around with it and moved on to creating our own projection and
incorporating an offset that corresponded to the difference between NAD83 and
the adjustment.  This was for Texas and Arkansas and the offset was as much
as 2 feet for X or Y.  I can't remember exactly.

I think the precision issue is a red herring.  Something somewhere has to
make that calculation.  Maybe somebody can help out here,and tell me I'm
wrong, but if the projection/coordinate system is set in the unit, then the
unit has to calculate the conversion.  Maybe it stores higher precision
numbers, or maybe I don't understand how that math works.

As for FL HARN vs. NAD83.  You can take the published coordinates for
benchmarks in both coordinate systems and subtract them in a spreadsheet.
These differences don't matter until you are zoomed in to around 1:1200
absolute scale, I think.

I like this thread and forum.  If somebody is annoyed by this and wants it
off topic, or me to shut up. just let me know.

Eric Songer
URS Corporation
1625 Summit Lake Drive
Tallahassee, FL 32317
Direct: 850.402.6327
Main: 800.842.9671 ext. 327
eric_songer at urscorp.com


 

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             "Shawn Lewers"                                                
             <SWL2727 at mailer.f                                             
             su.edu>                                                    To 
                                       <Eric_Songer at urscorp.com>, "'Sykes, 
             06/07/2006 12:12          John'" <John.Sykes at dep.state.fl.us> 
             PM                                                         cc 
                                       "'Brandt, Holli'"                   
                                       <brandth at tesorocorp.com>,           
                                       <shrug-l-bounces at lists.dep.state.fl 
                                       .us>,                               
                                       <shrug-l at lists.dep.state.fl.us>     
                                                                   Subject 
                                       RE: shrug-l: WGS84 vs NAD83 and     
                                       data transformation                 
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




After reading this I am left with thought that any time you capture GPS data
that is sub centimeter and then you change the projection there is a risk
that the original precision is compromised in some way.  How does the
transformation to the Harn adjustment in Florida State Plane systems impact
this discussion?  It may be necessary to capture all field data with that
datum selected if you want to preserve the high precision????

Shawn W. Lewers
Director College of Social Sciences GIS Labs ESRI License Administrator Erdas
License Administrator
321 Bellamy Building
Florida State University
Tallahassee, Florida 32306
850-644-8373
Fax 850-645-4923

-----Original Message-----
From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of
Eric_Songer at URSCorp.com
Sent: Wednesday, June 07, 2006 12:01 PM
To: Sykes, John
Cc: Brandt, Holli; shrug-l-bounces at lists.dep.state.fl.us;
shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: WGS84 vs NAD83 and data transformation

5) Finally, State Plane Coordinate systems are a planar system (hint, look at
the name).  Other coordinate systems are usually spherical or, better yet,
ellipsoidal, so the conversion from SPCs to Geographical Coordinates, for
example, are done by a "Best Fit" data method.  The errors from this
conversion can be in the 10 - 20 cm range at worst (in a state like Florida,
which has many fit points, it is much better).

Whoa.  Wait a minute.

SPC's are coordinate systems, but they are derived using map projections.
The following is from ESRI's documentation.

The State Plane Coordinate System (SPCS) is a coordinate system designed for
mapping the United States. It was developed in the 1930s by the U.S.
Coast and Geodetic Survey to provide a common reference system to surveyors
and mappers. The goal was to design a conformal mapping system for the
country with a maximum scale distortion of 1 part in 10,000, then considered
the limit of surveying accuracy.

Three conformal projections were chosen: the Lambert Conformal Conic for
states that are longer in the east-west direction, such as Tennessee and
Kentucky, the Transverse Mercator projection for states that are longer in
the north-south direction, such as Illinois and Vermont, and the Oblique
Mercator projection for the panhandle of Alaska, because it is neither
predominantly north nor south, but at an angle.

To maintain an accuracy of 1 part in 10,000, it was necessary to divide many
states into zones. Each zone has its own central meridian or standard
parallels to maintain the desired level of accuracy. The boundaries of these
zones follow county boundaries. Smaller states such as Connecticut require
only one zone, whereas Alaska is composed of ten zones and uses all three
projections.


Eric Songer
URS Corporation
1625 Summit Lake Drive
Tallahassee, FL 32317
Direct: 850.402.6327
Main: 800.842.9671 ext. 327
eric_songer at urscorp.com




 This e-mail and any attachments are confidential. If you receive this

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