shrug-l: NAD 83 to NAD83 HARN

Sykes, John John.Sykes at dep.state.fl.us
Thu May 4 11:23:29 EDT 2006


I don't know about the internal workings of ArcView, but if you use Corpscon,
this is correct.  Corpscon will first convert from NAD27 to NAD83, then from
NAD83 to NAD83 HARN.  If you are using NAD27 State Plane Coordinates (SPCs),
this is further complicated by the fact that there was a major shift in the
NAD27 SPCs to NAD83 SPCs, then if you go to Lat-Long, there is another
conversion, and so on and so on . . .
 
I have found out (through trial and error - mostly error) that if software is
not set up to Quad Precision*, mathematical errors get into the picture as
well.  Right now, I am playing around with projecting Lat-Long to FDEP Albers
using a FORTRAN program I wrote.  The results using Double Precision vs.
Triple Precision* are in the centimeter range.  My compiler doesn't have Quad
Precision, but I wish it did.

-- John 

* Triple Precision is actually 80 bit, or about 18 significant digits,
utilizing the built-in math coprocessor in the Intel chips.


  _____  

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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