shrug-l: Surveyors, AARRGGHH!!!

[Tim Spivey]

John -
If you have an ArcEditor license, I would do a Spatial Adjustment if you
know where the CAD file should fall on the ground and have good
reference points.
I don't even bother with projections for the CAD surveys we get, just
the Spatial Adjustment tools.
The tools may be available at the ArcMap level, I'm not sure.
 
Tim

________________________________

From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Sykes, John
Sent: Thursday, July 19, 2007 2:26 PM
To: shrug-L at lists.dep.state.fl.us
Subject: shrug-l: Surveyors, AARRGGHH!!!



I just received an AutoCAD file of a survey produced in the late 1980's.
Additional field work was done in 2004 and this revised survey is the
CAD file I have.

I assumed the survey work was done in State Plane Coordinates NAD83, but
when I created a projection file for this survey it ended up about 30
miles west of the site when I let the file project on the fly!

So I went back and created a new projection file, this time establishing
the coordinates as State Plane Coordinates (SPC) NAD27.  When I imported
the CAD drawing I immediately saw that the CAD drawing was coming out
southwest of where it should be by about 50 ft.  I realized that the
surveyor apparently had taken a SPC NAD27 file and offset it by the
correction factor from NAD27 to NAD83 without taking into account the
difference in the offset that also exists between the two projections (I
have been told that this 30 mile offset was added to the SPCs to make if
obvious when the wrong projection was being used).

The problem is that, if I reproject the SPC NAD27 file to SPC NAD83, it
adds the offset between the two datums back in again!!!  Short of
getting the surveyor to correct their file (not likely since the job was
finished in 2004) how do I get this error corrected???

By the way, I tried telling the PRJ file that the coordinates were SPC
NAD27 and the datum was NAD83, but that didn't work.  Also, I found that
the reprojection tool in the toolbox doesn't work for CAD files.

Of course, ultimately I'm trying to get this whole thing into FDEP
Albers for my map. 

-- John 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.dep.state.fl.us/pipermail/shrug-l/attachments/20070719/6b0879a8/attachment.html