making sense of field data + a plat
All these years I have done research and recon and then handed off the field data. Someone else made the drawing and worried about how it fit the plat. Now for the first time ever, I am making drawings, and from someone else’s field work.
(Getting the LSIT has been a surprisingly legitimizing career move. I got a job! I’m happy as a clam, details someday if they keep me a while. Meanwhile I am being subbed out to do calcs for half the surveyors in town, & learning that everyone has their own way of doing things.)
I have a 1950s steel-tape plat. It closes flat.
I have a handful of RTK shots on a control points, state plane grid with elevations, but no geoid applied. Coordinates, not vectors.
I have some total station data, one setup of radial ties in two faces to all the RTK points plus some other found stuff in relation to the RTK points. That crew hates me now, because when they couldn’t find cased mons and then the GPS stopped getting fixes, they were ready to drive away. They recorded the gun’s idea of horizontal distances and did not keep zeniths or rod heights.
It’s fitting a little better now that I had star*net apply a project average elevation (300 ft.) but I am still wondering about applying a geoid separation, another 77 feet roughly. Now the total station data fits the RTK surprisingly well, 0.04′ almost everywhere, but also reveals a couple of RTK misses, a tenth here and there. Those are, of course, on 2 of the mons we want to use. (If I hadn’t insisted on the additional TS work, they would be using those RTK shots to stake from!) With the total station data correcting the RTK shots, those mons are still off the plat calcs by a tenth or more.
We have a found unpunched railroad spike, and found unmarked PK that match well for inversed distance but miss by a tenth in 500 feet for bearing. We also have a rebar & cap set by a local surveyor a tenth from a calculated plat lot corner nearby. Most plats in the area used RR spikes as original mons. So, could be original, could be a replacement spike. It winds up that the unmarked PK is dead on if you regard the RR spike as 0.07 S and the found R&C as 0.07′ N
My instructions for this particular map are to hold the (grid) RTK shots on the mons and either scale and rotate the plat to it, or hold the mon closest to the lot we are staking and rotate to the other without scaling, leaving the slop at the other end of the plat. “We’re not prorating and we’re not gonna break the record.” Offset those mons by 30′ to get the ROW lines, the front corners, etc.
What bugs me about this as a standard procedure is the slop in combining the state plane coordinates with the ground distances. Also seems like the 60 year old plat would be ground distances, so the grid stuff should be fluffed up to match. By whatever luck, the horizontal distances match the GPS shots pretty well once everything is reduced to grid, or fluffed up to ground.
We are very close to the state plane meridian, and 0-500 feet elevation. It must be a fluke of our location that it is a standard local practice to mix and match state plane coordinates and ground distances. What’s a few hundredths? Oh, by the way, where are those extra and/or missing tenths creeping in?
I monkeyed with it til it fit & the crew went to stake it. Hopefully they found something already there that will give an additional point or two to hold. One more setup would have gotten the section corner that is the NW corner of the plat. I’m tempted to drive out and get that myself.
As I said, I am completely new at this aspect of surveying. I know I am fussing over a tenth or so, and that in boundary work, sometimes the numbers are just a tool. On the other hand, I want to get the procedure improved to the point that it is at least defensible as far as doing the right thing with grid-to-ground and elevations. Any clues appreciated.
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