ASSUMED COORDINATES TO STATE PLANE 

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GQSURVEYOR
Posts: 1

Joined: 1/6/2009
Topic  ASSUMED COORDINATES TO STATE PLANE       Flag »  Reply »
I HAVE A JOB THAT WAS SURVEYED WITH ASSUMED COORDINATES AND A MAGNETIC OBSERVATION. GIVEN THE SCOPE OF THE PROJECT I DECIDED TO APPLY STATE PLANE COORDINATES TO TWO OF MY CONTROL POINTS.  USING A RAPID STATIC BASE AND ROVER
POST-PROCESSING THE DIFFERENCE IN STATE PLANE COORDINATES YIELDED 0.01' ON A 1400' BASELINE.  MY QUESTION IS IS THERE A SCALE FACTOR TO BE APPLIED TO MY EXISTING COORDINATES OR WILL TRANSLATING AND ROTATING MY EXISTING POINTS TO THE NEWLY AQUIRED STATE PLANE BE SUITABLE.
  Tuesday, January 06, 2009 at 10:53:51 AM
Isleno
Posts: 42
Location: Gonzales, La USA

Joined: 10/20/2008
Reply  Re: ASSUMED COORDINATES TO STATE PLANE Flag »  Reply »
The extent at which the scale factor affects the distances between points is a function of the location of the project in relation to the zones of 1.0 scale factor.  For many survey projects, the scale factor is very close to 1.0 and can be ignored.  Be certain that multiple observations are made on the control points.  Values derrived from a single session lack sufficient redundency to warrent reliance.
  Tuesday, January 06, 2009 at 3:26:39 PM
Jack Chiles
Posts: 3

Joined: 2/13/2009
Reply  Re: ASSUMED COORDINATES TO STATE PLANE Flag »  Reply »
State Plane Coordinates are expressed as grid coordinates. Unfortunately, one cannot use these coordinates as on-the-ground coordinates. One must apply a Combined Scale Factor to those coordinates to effectively employ them onsite. Here in South Central Zone Texas, that means a difference of about 0.13 feet in every thousand feet one traverses. When publishing coordinates, one should always give sufficient metadata to explain your results. That data might include Combined Scale Factor, Zone, Control Points used (with published NGS coordinates, if possible), Adjustment date (if any), date surveyed, etc. I would avoid using local assumed coordinates, if at all possible.
  Friday, February 13, 2009 at 3:01:04 PM
Barry
Posts: 4

Joined: 2/20/2009
Reply  Re: ASSUMED COORDINATES TO STATE PLANE Flag »  Reply »
Please see my post in the GIS: Outside the Boundaries section:

http://www.profsurv.com/forum/Professional%20Surveyor%20Magazine%20Discussion/GIS-Outside-the-Boundaries/COGO-Grid-Scale-Factor-3100.aspx

What I am stumped on, regarding the TX State Plane coordinate system is how to find the Grid-Scale Factor for each position to apply, in order to convert from ground to grid coordinates.  In particular, I do not know how to obtain the R value (Mapping radius at a particular latitude), based on the NOAA Manual NOS NGS 5, "State Plane Coordinate System of 1983".
  Monday, March 02, 2009 at 9:58:50 AM
Isleno
Posts: 42
Location: Gonzales, La USA

Joined: 10/20/2008
Reply  Re: ASSUMED COORDINATES TO STATE PLANE Flag »  Reply »
Download CORPSCON6 from the internet.  It is free.  Enter the SPC and covert to Geodetic.  The program will give you the scale factors.  It is not necessary to vary the scale factor for every point.  A scale factor for the center of the project can be used as long as your not going over five miles or so, dependent upon the projection system.
  Tuesday, March 03, 2009 at 1:54:49 PM
Barry
Posts: 4

Joined: 2/20/2009
Reply  Re: ASSUMED COORDINATES TO STATE PLANE Flag »  Reply »
I have tried the use of the grid scale factor output from Corpscon, Isleno, but did not like the comparison to some data obtained from a licensed surveyor when applying the grid scale factor from Corpscon, as opposed to what the surveyor's data contained.  I would use Corpscon for some work, but would really prefer to have the formula to generate the grid scale factor as well, to compare to Corpscon's output values.
  Wednesday, March 04, 2009 at 10:39:57 AM
Isleno
Posts: 42
Location: Gonzales, La USA

Joined: 10/20/2008
Reply  Re: ASSUMED COORDINATES TO STATE PLANE Flag »  Reply »
Barry:
    I'm at a loss as to why something as miniscule as scale factor would be a problem.  Most survey data for distances that are less than a thousand feet are full of other measurement errors that exceed scale factor.  However, NGS has a tool box on its web site with programs that will do the job.  If that fails, there are some old Technical Reports that are available from NGS that have the appropriate formulae so the computations can be made by hand.  It is a fun exercise.
  Wednesday, March 04, 2009 at 11:19:12 AM

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