Grid to ground conversion?

I am running a large (for me) traverse of about 15,000 linear feet.   I have 4 pairs of GPS along the way.  My plan is to hold one of the GPS points (near the center of the project) grid position and scale the other GPS points to ground from there, by averaging the CGF provided by OPUS. 

What's the best way to apply this factor?  SHould I use the grid azimuths and apply the computed scale factor on that same azimuth, for each point?  Or should I scale the Northing and Easting by the same computed factor?  

Any help would be great!

A couple of IMPORTANT questions first:

 

Is this a “route traverse” or a “loop traverse?”

 

What is the total [topographic] RELIEF involved?

 

What is your distortion budget?

 

I should ask (but won't...because I don't want to start WWIII this morning) WHY you are using SPC (or UTM) to start with, IF you want to be “on the ground” when you are done! Besides which, I probably already know the answer to that one.

 

:)

Loyal  

Thanks for replying.  

This will be a closed loop traverse.

I am not sure what you mean by distortion budget.  

I am using SPC to tie the site relative to other sites in the vicinity.  

I am running this loop 3D, if that matters.  

Will Star*Net do the conversion?

 

Okay...

 

The “distortion budget” can be defined as simply the ACCEPTABLE magnitude of distortion within the final adjusted solution. Basically, the relationship of the final grid distances (assuming perfection thereof) and the actual ground distances. Unless you are working in a very flat terrain, this distortion can be anything from a few ppm to well over 100ppm.

 

If you have [say] 400 feet of vertical relief in your project, and you use the average height for your developed surface, then you can see as much as ~10ppm grid to ground distortion within your traverse (even AFTER it is adjusted). This would be acceptable for MOST practical applications. Once you get much past that threshold, it becomes noticeable to most careful surveyors, although I have found that even 30ppm falls within the warm-n-fuzzy of many folks, and 30ppm is acceptable for most construction projects too.

 

Any georeferenced coordinate projection will “tie to site relative to other sites in the vicinity,” so that's pretty much a matter of personal preference.

 

The traverse WILL have to be run in “3D” in any case. To do otherwise precludes the proper computation of vertical convergency, and will degrade the final results based on the amount of total vertical relief in the traverse. More relief, more degradation.

 

Once you have DEFINED a georeferenced Coordinate System for your Traverse, Star*Net will do all of the heavy lifting for you, including “moving” (expanding/contracting) each horizontal distance to the developed surface of your projection. Star*Net supports just about any formal mapping projection (I think), and can certainly handle any SPC, UTM, or LDP. I haven't used Star*Net in many years now, and I never used it on MODIFIED SPC, so I don't know how well that technique is supported.

 

IMO a Low Distortion Projection (LDP) specifically tailored to your project would be both the most accurate solution, and the path of least resistance, but that's just my personal preference. OPUS has already generated and returned all of the fancy data that you would need to generate the projection parameters (constants), and any decent coordinate transformation software (like Blue Marble's Geographic Calculator) will transform the LDP back and forth to any other georeferenced coordinate system (SPC/UTM/LDP) in the blink of an eye.

 

Loyal 

 

It is NOT my intention to lobby for an LDP here, and if you would like to post (or Email to LDOGEO at AOL dot COM) the OPUS solutions, I will compute BOTH methods for you (an LDP and a suggested modified SPC) and post (or privately Email) the results for your review.

 

My question about TOTAL relief is VERY important, and without that information, it's impossible to analyze your specific situation.

 

Loyal 

I hope that you both  continue the discussion here so others can learn and benefit.  The project desribed has many typical elements that must be considered by 'reluctant geodesists' struggling with grid-ground issues.  

Loyal-- Thanks for all of your help.  You know way too much about what you're talking about.  I will send you an email.

I spent many years working for a surveyor that did things "quick and dirty" and now that I am on my own I am trying to do things the right way.  I want to be a valuable member of my surveying community and I sincerely appreciate the advice!

I, too, used to work for someone that would get the furthest points on the site and just snap to the midpoint of the GPS and the field measurements. I would really like to get my hands on some textbook or user manual that can show step by step how to do a PROPER conversion. Does anyone have any suggestions?

If I'm understanding right.
You want ground (easily mea with tape, uncorr EDM, etc) coord with SPCS control.
Remember only one point on the job can have accurate SPCS "looking" values and still be correct if expanding/contracting to "ground"

Once you "abandon" straight out SPCS coordinates, make sure you adopt values that won't be easily confused with actual SPCS values by casual observation.
ex.
N= 2,123,456.789   E=6,123,456.789   SPCS
change to
N=   123,456.789   E=  123,456.789   Yours

Depending on type of projection, wheather you are going E-W or N-S can have a large impact on grid-ground as does elevation change previously mentioned.

Working in fairly flat terrain and only a few hundred feet above sea level, I just ignored ground and worked grid.
Most of the time the error is meaningless.
I always wished that all construction plans be in grid. Then using nearby SPCS control points always keeps things within reasonable tolerances.