Okay, here is an excellent example of why using a reproducible bearing basis is exceptionally poor practice. Late last year, I retraced a survey made about twenty years earlier by a surveyor who had the audacity to report the bearings of lines as determined by his survey in relation to Grid North of the Texas Coordinate System of 1927 as determined by solar observations. The bearing basis note in the metes and bounds description of the 500.03 acre tract he surveyed read:
BEARINGS OF LINES refer to GRID NORTH of the Texas Plane Coordinate Sytem (South Central Zone, NAD 27) as computed from solar observations.
Yeah, he was using SOLAR OBSERVATIONS to determine grid North. Probably took him at least twenty minutes of field time, too, and I'll bet he used a one-second theodolite to make the solar observations.
So, fast forward about 20 years and that's me out there hitting his same monuments with GPS equipment, no solar observations, and I'm finding some major discrepancies in the actual grid bearings of the lines compared to what he found twenty years earlier without GPS equipment, just conventional theodolite and total station.
[pre]
1993 Survey 2012 Survey
N71°28’38”E, 226.54 ft. N71°28'36"E, 226.53 ft.
N60°19’54”E, 137.38 ft. N60°19'42"E, 137.38 ft.
N46°21’52”E, 208.81 ft. N46°22'00"E, 208.81 ft.
N26°47’23”E, 348.92 ft. N26°47'24"E, 348.94 ft.
N26°44’31”E, 410.87 ft. N26°44'30"E, 410.88 ft.
N35°05’29”E, 473.99 ft. N35°05'24"E, 474.00 ft.
[/pre]
As you can see, on that line that was 137.38 ft. in length, the 1993 surveyor's bearing was in error by 0°00'12". That's a 0.007 ft. difference in the relative positions of the endpoints. Perry Williams in New Hampshire would be giving up and heading home with that sort of a brain-bustingly difficult problem to solve.
So, don't let anyone tell you that using an independently reproducible bearing basis is good for your sex life. It isn't. You'll just stay up all night wondering why stuff fits so well.
Kent:
Appears that you may need to recalibrate your GPS unit. Too much error! 😀
My guess is that the folks who are bothered by the use of a bearing basis that can be independently checked is that along with it comes the means to decide whether a surveyor has gotten the wrong answer or the right answer. I mean, if the last five resurveys of a parcel have basically agreed that the West line of the parcel has a grid bearing within a few seconds of N36-37-06W, that means that the next surveyor can't casually report that the grid bearing of that line is really N36-57-45W without some 'splaining to be done.
But instead of retracing a 1993 survey done with a with a one second theodolite you were retracing a 1983 survey done with a T-16 type theodolite. You might be missing him/her by about 0.025 ft.
O, the horrors.
🙂
You certainly can't argue with the results, as they are closer than you would typically expect to find. I've been following the recent posts on the bearing basis issue and it makes me wonder if I'm missing something here. If you locate the same monuments and the interior angles check, does it really matter what the original was based on? I prefer to use grid north here in Florida, but I never considered it a terrible thing if another surveyor assumed north on a section line and based his bearings off of that for the survey. Either way I can do my best to reproduce what was originally intended to be ran on the ground. Naturally, it is easier if I know what the bearings were originally based upon, but I won't lose sleep if it's not known.
I will second your comment on the bearing basis not improving your sex life. If I came home after a long day at work bragging about using independently reproducible bearings, she would respond with, "Great,good for you... please clean up the kitchen and change the baby's diaper."
Category Error Kent
This thread belongs in the Humor or Irony category.
What the heck does accurate measurement agreement have to do with land surveying in the first place?
Paul in PA
Some folks in the name of humor are a little harsh-myself included. I think the practice offers little or no value to myself or my CUSTOMERS at this point.
1) There's enough/better evidence left to retrace a survey without it. I have all the stake eating demons in my world you do, but probably a little safer control.
2) Our old compass surveys weren't accurate/consistent enough to demonstrate a correlation between my results and the foot steps I followed. Other evidence is better.
3) In my corner of PLSS the largest tracts I've surveyed are ~5280x2640 with the clients preferring to have a 'feel' for the survey they purchased by seeing one side assumed to be N-S or E-W.
The customer does matter, and in my world forcing this on them gives the profession a black-eye more so than the lack of technical expertise we demonstrate by using an assumed bearing.
Maybe in Tejas going with true or grid makes perfect sense, especially when I read about the size of some of the parcels you survey. In my area of the PLSS it does not, and I certainly don't want it forced on me.
Steve
[sarcasm]pretty good for a "one beep" solution...[/sarcasm].
> You certainly can't argue with the results, as they are closer than you would typically expect to find. I've been following the recent posts on the bearing basis issue and it makes me wonder if I'm missing something here. If you locate the same monuments and the interior angles check, does it really matter what the original was based on?
Isn't the main idea that all of the original monuments often won't be in place undisturbed and that on urban parcels it isn't unusual for most of the boundary monuments to be either MIA or suffering from PTSD?
Sloppy, sloppy, sloppy. Get back out there and reshoot those monuments until you get every shot to match your predecessor's work.
:pissed:
In the interest of full disclosure, I meant that I used GPS only to get the bearing basis of my work last year. The monuments were connected by conventional measurements as well and the GPS vectors were adjusted with the conventional angles and distances since that is an easy way to improve both and to orient the survey to grid North.
Texas Positional Tolerance Hardware
Immovable Markers
All of those monuments were in drill holes in limestone bedrock, so it was the somewhat unusual example of monuments that were unlikely to have been moved or shifted, any differences being strictly those in the measurements of the two surveys.
Dollars to Donuts
well, my post was just a light hearted rib/quip not to be taken for anything else.
But
I will bet you dollars to donuts, the s/d lot survey you posted about with the thin air BOB... that s/d would be marred with pincushions at most of the exterior boundary locations.
That how it would be played out here and that is why the contra arguments about a reproducible BOB is weak to me.
and people wonder why pincushions happen...
> In the interest of full disclosure, I meant that I used GPS only to get the bearing basis of my work last year. The monuments were connected by conventional measurements as well and the GPS vectors were adjusted with the conventional angles and distances since that is an easy way to improve both and to orient the survey to grid North.
In the interest of *full* disclosure, wasn't it also your boundary work from 1993 that your were retracing? If that's the case it brings up something I've been kicking back and forth since the independent bearing basis topic came up. It would be very difficult for me to find a single boundary monument from a survey which was supposedly oriented to Grid North or Astronomic North, and re-establish a boundary line with any sufficient length unless I was the one who previously set the monument and established the bearings.
Independent bearings seem to me to be a great practice: one that I'm striving to make a reality on all my surveying projects. But when push comes to shove, I would hope them to be the last remaining evidence of any boundary line that I tried to retrace. It's this latter thought that leads me to believe many of the responses of your posts of late, in particular independent basis of bearings, have missed the essence of what you're trying to convey. Many seem to think that you would take the approach of establishing a bearing from a single monument and rebuilding the world, while giving little thought to any other evidence that may exist to support the location of ancient boundary lines. I think I've read enough of your material over the years to know that's simply not true.
Immovable Markers
> All of those monuments were in drill holes in limestone bedrock, so it was the somewhat unusual example of monuments that were unlikely to have been moved or shifted, any differences being strictly those in the measurements of the two surveys.
> In the interest of *full* disclosure, wasn't it also your boundary work from 1993 that your were retracing?
Hey, I didn't say that I wasn't the surveyor who surveyed that line of the 500.03 acre tract in 1993 that I was following. The point was that the 1993 survey had been oriented to grid North computed from solar observations, and, lo and behold, about twenty years later, a resurvey oriented to grid North by GPS observations gets essentially the same answer.
There wasn't anything particularly difficult or exotic about the solar observations. They were made with a Zeiss Th2, a one-second theodolite, and were made by the hour angle method with the Sun about 25 degrees up, angles taken to the trailing edge, timing with a digital stopwatch and radio time signal, and the observations reduced using Elgin & Knowles's software. I did do the observation on a 3000 ft. leg of the control traverse and used some tricks to reduce the propagation of random angular errors in the traverse, but that was in the nuts and bolts of the surveying, not the basic azimuth determination.
>If that's the case it brings up something I've been kicking back and forth since the independent bearing basis topic came up. It would be very difficult for me to find a single boundary monument from a survey which was supposedly oriented to Grid North or Astronomic North, and re-establish a boundary line with any sufficient length unless I was the one who previously set the monument and established the bearings.
Well, that's why a surveyor ought to be using least squares survey adjustment software and evaluating the uncertainties in the results. On a large traverse, the standard practice I followed was to compute grid azimuth from solar observations at more than one point and use those additional azimuths in the adjustment as well.
> Independent bearings seem to me to be a great practice: one that I'm striving to make a reality on all my surveying projects. But when push comes to shove, I would hope them to be the last remaining evidence of any boundary line that I tried to retrace.
What I think you're forgetting is that as tracts are resurveyed, over time you end up with multiple reports of the bearing of various lines. If they are all on the same basis, independently determined, after three surveys are in essential agreement, the actual bearings can become quite good evidence in my view.
> What I think you're forgetting is that as tracts are resurveyed, over time you end up with multiple reports of the bearing of various lines. If they are all on the same basis, independently determined, after three surveys are in essential agreement, the actual bearings can become quite good evidence in my view.
Overlooking, perhaps. Forgetting, never. 🙂
> > What I think you're forgetting is that as tracts are resurveyed, over time you end up with multiple reports of the bearing of various lines. If they are all on the same basis, independently determined, after three surveys are in essential agreement, the actual bearings can become quite good evidence in my view.
>
> Overlooking, perhaps. Forgetting, never.
As long as there is RTK GPS in use by the usual suspects, it's a safe bet that the rate of convergence of answers is going to be slow as molasses. In that ideal world where GPS users actually know what they're doing, the differences that subsequent resurveys would be able to resolve would be smaller than the movements of many typical survey markers over time, certainly those in expansive clay soils.
Just curious?
Why would surveying in the PLSS mean small surveys?
I imagine I do surveys of a size that even Kent never deals with.
And if you are surveying in the PLSS system then knowing what the bearings are isn't an option it's a requirement.