I have a couple of questions regarding grid-to-ground conversion with the Topcon GR-3 Receivers, using the FC-200 data collector.
1. I am working in mine grid systems that may extend 6.5 miles east-west. I put my control on State Plane Coordinates by tying into NGS stations and checking with OPUS solutions. To convert to the mine grid systems on the FC-200 data collector, Configuration / Coord System, I use the "Origin Pt" Option. I input the mine coordinates of a control point and the azimuth of a line in both coordinate systems and the data collector computes the combined Grid-to-Ground Factor at my chosen control point.
So far this method has worked fine, but I am wondering if I can use a combined factor that I compute that may fit the entire site a little better. When I try this using the "Scale Factor" method, I need to input North and East offsets. When I test this method against the "Origin Point" method, using the same Combined Factor and Coordinate offsets, I get different results. It seems like the offsets used in the "Scale Factor" method are the offsets of the origin of the coordinate system rather than the offsets of a particular control point.
Can anyone clear this up for me?
2. Regarding the "Localization" method of Grid-to-Ground conversion, I have a question: Since this is a coordinate transformation, as long as the coordinate system you are localizing to is self-consistent, you will not detect any problems. This might happen if the ground coordinates were derived from State Plane Coordinates using one combined factor that may not be appropriate for the local area.
Any thoughts on this?
Having worked with “Mine Grids” since the mid-1970s (some of them dating back well into the 19th Century), I can tell you flat out, that a “Custom Projection” (maybe an LDP, maybe NOT) is both the path of least resistance, AND the MOST accurate/precise solution.
Exactly how “easy” that would be, depends on several conditions unique to each “mine grid.”
How close to geodetic North is it aligned?
How close to the surface of the Earth is it expressed?
And last but NOT least, how spatially consistent (precise) is it?
MOST (but not all by any means) Mine Grids are aligned pretty close to geodetic North (usually via astronomic observations), and scaled based on a measured Base-Line (or two) somewhere on the project. This is VERY common on “old” mine grids, but somewhat less so on “modern” mine grids.
I have seen rotations in excess of 40 DEGREES, and linear distances expressed at “sea level” or even some “level” within a particular SHAFT (like the 750 foot level BELOW the collar of such and such mine)! There are also some “modern” mine grids wherein folks have mixed apples and oranges to the point that NOTHING really fits worth a crap!
My modus operandi has always been to get geodetic coordinates on as MANY extant “mine grid stations” as possible, and then go from there. That way you can analyze BOTH the orientation and the scale, as well as get a feel for the spatial integrity of the existing [mine grid] coordinate estimates.
In my experience, MOST mine grids are pretty darn GOOD, but I have seen some real stinkers too!
I realize that this doesn't really answer your question, but that's my 2-bits worth.
Loyal
Thats easy...
Just ask one of the seven dwarfs...they have lots of mine expirence!