shrug-l: WGS84 vs NAD83 and data transformation

Mayo, Michele L. Michele.Mayo at dep.state.fl.us
Wed Jun 7 13:02:37 EDT 2006


Shawn,

You're right, any coordinate transformation introduces some amount of
positional error, so it's always a good idea to keep the processing to a
minimum (and document it in the metadata).  But the issue with DGPS is that
the coordinates have to be captured using a global datum because the
satellites are, well, global.  They're located all over the earth, so they
have to use a common reference system that's valid for any or all of them, so
we're obliged start with WGS84 and then transform to a more local coordinate
system.

Michele

Michele Mayo, GISP
Florida DEP,Bureau of 
Beaches and Coastal Systems
850.413.7776
michele.mayo at dep.state.fl.us


-----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 Shawn Lewers
Sent: Wednesday, June 07, 2006 12:13 PM
To: Eric_Songer at URSCorp.com; Sykes, John
Cc: 'Brandt, Holli'; 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

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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