shrug-l: NAD 83 to NAD83 HARN

[Sykes, John]

OK, I saw your second message.  I've been there too.
 
About my first post, I forgot to mention two things.  First, conversion from
NAD27 to NAD83 is not exact.  The software uses a best-fit conversion using a
grid developed by the National Geodetic Service, so errors may be in the 10
to 20 cm range depending on where you are (not in Florida however, our grid
is quite tight).  NAD83 to HARN also depends on the State's network of CORS
stations (again, quite good in Florida).  There is a new iteration of the
HARN in the mill, which will be coming out in February 2007.  Supposedly,
this would reduce the distance error from a point in Key West to a point in
Seattle, Washington to about +/- 2 cm.
 
Unlike the NAD conversions, the conversion from Lat-Long to FDEP Albers
coordinates IS an exact mathematical relationship.  So it's accuracy is
limited only by the mathematical precision of the computer.  Oh, by the way.
I forgot to mention in my last post, Double Precision = 64 bits, Triple
Precision = 80 bits and Quad Precision = 128 bits in an Intel-based PC, or
about 15, 18 and 30 significant digits, respectively.

-- John 

 

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Thomas, Jim
Sent: Thursday, May 04, 2006 11:14 AM
To: shrug-l at lists.dep.state.fl.us
Subject: shrug-l: NAD 83 to NAD83 HARN


Shruggers,
 
Is this true?
 
"To project data from State Plane NAD 27 West to NAD 83 HARN takes two (2)
transformations."
 
I'm dealing with data referenced as FLSP-W NAD83 HARN and it's not overlaying
my NAD27 data.  Could it be that ArcMap is incorrectly projecting on the fly?
If I change the reference system from NAD83 HARN to NAD83, it overlays
properly.  Am I missing something?
 
Jim T.
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