shrug-l: Surveyors, AARRGGHH!!!

Fri Jul 20 18:06:57 EDT 2007

Greetings!
      According to one source:

       "The readjustment of the North American Datum of 1927 (NAD 
27), Old Hawaiian Datum and Puerto Rico Datum to  the North American 
Datum of 1983 (NAD 83 (1986)) in  July 1986 was both a change in 
reference ellipsoid and a  "clean up" of nearly 200 years of 
surveying data held by  NGS. Based on this readjustment and 
redefinition, positions of points can change between 10 - 100 meters, 
in the conterminous United  States, more than 200 meters in Alaska, 
Puerto Rico and the Virgin  Island and in excess of 400 meters in Hawaii.
      Consequently, the shift  between the various datums are not 
uniform across the United States  and there is no single value that 
can be applied to latitudes or to  longitudes based on old datums to 
convert them to NAD 83. "

     I had a piece of property surveyed since the stakes in the 
ground did not make sense.  The surveyor came back with a new set of 
stakes and said that the base point had three iron rods in the ground 
and he used the center of these markers to start his survey.  Since 
it was a corner lot on a county road, I contact the county road 
people and asked what was involved to find where the right-of-way 
ran.  A county survey crew came out and, using a different reference 
point, put in some more stakes to mark the right of way.  I now had 
the stakes at the corners of the lot that were there to start with, 
the stakes put in by the surveyor I hired, and the stakes put in by 
the county road survey crew.  The county's stakes and the ones put in 
by the surveyor I hired were within an inch of each other so I took 
the county's stakes as the corner markers.  Who knows what would have 
happened if GPS had been around back then.

C. Henry Depew
sisu26 at nettally.com

*********************************************

At 11:50 AM 7/20/2007, Eric_Songer at URSCorp.com wrote:

>Ha,
>
>It's common for surveyors data, especially of that vintage, to be 
>measured in feet and referenced to a local coordinate system. In the 
>old days, if it was measured in feet, then it was often called SPC 
>in my experience.
>
>I would make a 2 point world file and bring the CAD drawing back to 
>the space where you want it. Not an image world file, but one for 
>CAD. Check the ArcMap documentation for how to do it. A 2 point 
>world file will scale and rotate the data, which should be all you 
>need to if the data were really surveyed in the first place.
>
>Now what was the purpose of Chapter 472? To protect the public by 
>allowing only Licensed surveyors do mapping, I believe. I don't feel safer....
>
>I believe there is a 10,000 foot offset between SPC NAD27 and SPC 
>NAD83. I can't remember if it is east-west or north-south. Maybe 
>Ronnie is reading and can fill us in on the details. I always like 
>reading his responses. You can take a file of know projection and 
>project it to SPC in both NAD27 and NAD83 and see the difference.
>
>Thanks,
>
>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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>07/19/2007 02:25 PM
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>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 _______________________________________________
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