shrug-l: WGS84 vs NAD83 and data transformation

Eric_Songer at URSCorp.com Eric_Songer at URSCorp.com
Wed Jun 7 14:25:45 EDT 2006


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




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