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.
- 14 Replies
There is only one person who can look over the evidence and closures with you and advise as to procedures and or any more work that might be required, and that’s the PLS that will certify the work. Expect variations on the answers from 2 or more LS’s. Some will think everything is ok. Some will spend time with you to understand how procedures can be improved next time but stamp what you got so far.
A few like me would have reviewed all the preliminary research and analysis and specified the best field procedures PRIOR to the first field visit so you would not have to suffer a mix and match and try to pretend you can prove something with half-azzed whip it out and mix it up GPS float and a few REAL connections.
You need to confirm what the certifying LS requires for his stamp, right or wrong. In a few years of working with one or more LS’s you will eventually know for yourself what you will require when your stamp comes into play.
In a few years of working with one or more LS’s you will eventually know for yourself what you will require when your stamp comes into play.
That’s really the best approach. You really need to have that mentoring LS looking over your shoulder at the CAD display and working the issues out with you. That is how you learn. Right now, you are like that freshman quarterback who has great potential. But the game is not going to “slow down” for you until you get that coaching and experience. It’s unfortunate in a way that you have been farmed out to several clients to solve problems (with many variables) on your own. My hope would be that you are able to find and work with one LS who will mentor you through enough of these projects so that the game will slow down for you.
0.10′ May Be As Good As It Gets
That a taped Plat closes flat shows that the office mathematics were good.
That a monument is within 0.10′ in 500′ shows that required precision of 1:5,000 was met.
Also that a PK and RR Spike are within 1/2 minute of line also shows good procedure in the past.
I take it you mean the actual survey varies from 0′ elevation to 500′ and that 300′ is an average elevation over the project.
Applying the elevation scale factor for 300′ will not gain you 0.01′ in 500′, (1:50,000) so it seems resonable no one cared about it in the past.
Being held by adjacent monuments while remote monuments are within reason puts you in little jeopardy.
Be careful about scaling alledged SPC coordinates given you with little documentation. Odds are great that they will not unscaale your work when they stake it in the field with RTK.
Sounds like you never were in the field with this particular crew and you have no idea even how precise there TS shots were, i.e. plumb bob, rod or better, EDM corrections for atmospheric conditions, ever been to a base line, prism offsets, instrument or data collector scaling from a long past project in the TS or the RTK.
If they are not working to your standards you have no idea if they are working to any standards.
Since you say you are being farmed to reduce various crews field work it is probable each crew or two may have it’s own LS also?
Paul in PA
0.10′ May Be As Good As It Gets
It’s not all that bad. The TS has been to the baseline, it’s honest ground distances recorded on paper with no data collector, and no oddball scaling that I can see. The survey is pretty much flat at 300′ elevation. The distances agree and the worst angle is about 3 seconds.
I suppose after all that narrative my real question is how do YOU match up a ground scale plat and a bunch of state plane RTK points? And how would you if you were at 7000 feet where it mattered and not here in the lowlands?
Right now I have two tasks: 1) more immediately, to calc and draft exactly as they have for decades so they can continue to turn out 2.5 lot surveys per day, year in year out, and 2) to find the floating tenth has been showing up ever since they started using GPS. Right now I am trying to ignore the tenth so I can catch up on the drafting backlog. But, if there is a tenth in the plat, and then a tenth in the field work (RTK) and you want to work “good to a tenth,” is it all good or is it only half as good as it could be?
if you regard the RR spike as 0.07 S and the found R&C as 0.07′ N
Well golly gee, 2 centimeters is about what I would expect from my RTK. So is the spike and rebard really dead nuts on?
First, I’m not trying to be sarcastic, otherwise I would use the “sarcastic” tool.
But, you have a 1950’s plat and your hand-wringing over a tenth?
Let me make a suggestion: Print this post, save it, and after 10 years of experience, re-read what you just wrote.
Making found monuments fit mathematically perfect plats; this is where the rubber meets the road.
Don’t get bogged down over a tenth. Worry about the important stuff.
> First, I’m not trying to be sarcastic, otherwise I would use the “sarcastic” tool.
>
> But, you have a 1950’s plat and your hand-wringing over a tenth?
>
> Let me make a suggestion: Print this post, save it, and after 10 years of experience, re-read what you just wrote.
>
> Making found monuments fit mathematically perfect plats; this is where the rubber meets the road.
>
> Don’t get bogged down over a tenth. Worry about the important stuff.:good:
A few years back I had a pretty sharp PC that worked for me. He was up on all the technology, but I felt he needed a little “tempering” that can only come from exposure to real world data.
Excited about our new RTK setup and data collector he had located a number of existing pins in an existing plat twice. The first job with just a random base set up.
Second, he started a new job in the DC and rotated with a platted bearing and reshot all the pins.
After he downloaded he had two drawings open; each with its own set of points. We were discussing how close some fit and some weren’t such a good fit.
I asked him the distance between two of the pins. He asked which data set. I told him I didn’t care, the distance would be the same…
I could tell by the look on his face he wanted to maybe argue. He futzed around with one drawing and then futzed with the other one. He looked at me confused and, in an amazed voice, told me how far they were apart, and indeed were the same distance apart in both drawings.
I left him with his thoughts, staring at the monitor.
0.10′ May Be As Good As It Gets
It sounds to me, like what you’re really asking, is: what the hell do I show on my drawing?:-S
Like Paden says below, those 2 pins on the ground will always be the same distance apart, no matter what “datum” they are in.
Like others have said; listen to what the signing LS wants. Some like to see: found PK of unknown origin X.xx N and X.xx W, others will be comfortable with a note on the drawing something like:
All monuments were accepted in place, for positions shown, unless otherwise noted.2.5 lot surveys a day in this part of the country is cranking them out, good luck with that…;-)
Cheers,
Dugger
I hope everyone has a great day; I know I will!
If the measurements are 0.1 different from a 1950’s plat, consider buying lottery tickets before your luck runs out. The plat will often have been office adjusted for mathematical fit, but not all are. I’ve scratched my head long and hard over some that didn’t come close to closing.
If the crew’s own measurements don’t agree with themselves by 0.1, somebody needs to find out why.
.
0.10′ May Be As Good As It Gets
my real question is how do YOU match up a ground scale plat and a bunch of state plane RTK points? And how would you if you were at 7000 feet where it mattered and not here in the lowlands?
I don’t! If you want to tie into a plat that was done at surface then work at surface. Just because you are using GPS doesn’t mean you HAVE to be in state plane. I spent YEARS working in state plane without GPS and since I started using it I can’t think of more than two or three projects that I’ve done in state plane.
I suppose if you are close to a 1 scale factor then you may not care. Of course, at 7000 feet that’s not going to happen. But, the elevation factor is only one part of the combined factor. Along the origin longitude for the state plane grid I usually work in the grid factor is .9999375-not a large scale factor, but not 1 either, about .06′ in 1000′. It would actually have a larger effect on the distances than the elevation factor at 300′.
For a subdivision plat that’s probably not a big issue and I guess if it’s important to the company to keep everything on state plane it may not be important enough to deal with. Also, you can work in state plane with a total station-I did it for years.
0.10′ May Be As Good As It Gets
DING DING DING DING we have a winner! What the heck do ya show on the drawing?
re-read above, the differences are not from RTK, they are there after the turned angle/EDM distance.
Does The Above Statement Mean?
You consider RTK to be higher order than total station doubled angle and distance?
If so just pack it up now, step away from the computer and find another line of work.
It appears that you have never created an error budget for all your measuring tools?
Paul in PA
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