PLSS question

Best (easiest) example, would be a nominally North South Line, measured with an EDMI and vertical angles measured with a transit, theodolite, or total station.

Measure the vertical angles BOTH ways, and the SLOPE distance BOTH ways. Mean BOTH values (after converting the slope distances to Chains, and the Vertical angles to + & - values relative to the local horizon), and multiply the Cosine * the slope distance.

Bingo...

Using GPS requires SOFTWARE that isn't usually in your average data collector.

Loyal

It actually means just what it says.

80 chains on the ground means measured on the ground, wherever the ground happens to be. If you are not using an actual chain approved by the Surveyor General, it means that you must follow a means of measurement which most closely follows the instruction originally given for that meridian you are in and replicates the measurement on the ground as if measured by the specified chain. I have never had any problem using RTK GPS and total stations to retrace these lines as I understand the issue. The Surveyor General has always had a solution for you from 1815 on as the results are to be found on the north and west lines of a township, and dealt with as to the Instructions or Manual in effect at the time.

To be more specific, it does not refer to the height above the ellipsoid, the measurement along the ellipsoid, the state plane coordinates, a distance along the geoid, but only what was specified in the original instructions.

I realize that this causes problems for some surveyors today who do not understand the original instructions since they do not involve, and completely ignore, gps and geodetic surveying in the interior of townships. But the instructions and the manual have always recognized that and have directed surveyors in the subdivision of townships, which subdivision has never involved geodetic surveying for most of the PLSS, as the Surveyor General traditionally solved this issue for you at the meridian, range line, and township line level.

> what is the import of the following passage 7-9 2009 Manual
> …” the Public Lands System Datum is based upon measurements at actual average ground elevation along the line.”
> Based upon measure tools(total station,GPS) what are the procedures needed to obtain a retraced measurement that is the same or equivalent to the measurement at actual average ground elevation?
> What kind of distance is the 80 chains that appears on the Official plat? Is it a horizontal distance or a geodetic distance based upon the PLSS parameters?
>
> Thanks for your help

Import...as important? I would see it is important.

Meaning?
I would say it is a geodetic measurement along a constant mean ground elevation.
I would say it is "geodetic" in that the measurements are constantly being adjusted to a mean ground height, and along the curve of the earth.

I know you have taken the CFedS course, so I suspect the question is a bit deeper than I am understanding or that you are asking it as some kind of "test". (?)

Isn't the easy answer to this question, that the distances shown on the plats are horizontal measurements?

and therefore to replicate them you shoot horizontal distances, not scaled and/or computed distances.....

> Isn't the easy answer to this question, that the distances shown on the plats are horizontal measurements?
>
> and therefore to replicate them you shoot horizontal distances, not scaled and/or computed distances.....

Yes, it's all probably close enough. It might be that the point is that if you measure a chain or less at a time, the "horizontal" is constantantly changing, and the mean elevation might also change at each chainage. Therefore the "horizontal" distance measured directly from one point to the other could theoretically be slightly different than the accumulated horizontal distances. However, the difference is probably well within the maximum standard of error.

The "distance" along the ground as someone said, could be considered a "slope" distance. If it's a steep slope, the reduced horizontal distance from one end could be different than the reduced horizontal distance from the other end, and a person would need to make sure and mean the two distances to more closely replicated the horizontal distance from the mean elevation. Even then, it isn't precisely the distance you might get if you measured 80 chains or less at a time along the geodetic surface.

I think it might be a "trick" question. Maybe Mr.Ince can give us his answer.

We could discuss ellipsoid height verses orthometric height, deflections of the vertical, horizontal Laplace corrections, etc. etc., until the cows come home...BUT for most practical purposes, these are trivial considerations when you are only talking about a couple of square miles (or even MOST townships).

HOWEVER, the basic 3-dimensional geometry can becomes NON-trivial pretty fast.

More relative relief = larger horizontal corrections.

Higher Latitudes = more convergence between N-S lines and MORE curvature in the E-W lines.

The BIGGER the project (the more Sections involved), the more likely that this “stuff” will become a non-trivial issue.

Modern BLM Surveys (at least the ones that I am currently looking at) return True Bearings to the nearest tenth of an arc-minute (6 arc-seconds), and distances to .001 chains (0.066 feet). By making the proper corrections for height and convergency, these plats “close” at or BELOW this threshold (~2 centimeters) when transformed into a County LDP, and recent field observations using RTK return spatial agreement with these BLM Plats (and notes) at or below this same threshold across entire townships. These surveys contain anywhere from a few hundred to a couple of thousand feet of vertical relief, so even the horizontal corrections are non-trivial in many cases.

The (preliminary/draft) plats that I am working with do NOT return any height data, so I am using USGS Topographic maps to ascertain that component. Generally speaking, this works quite well, and once we get GPS observations on each corner, more “accurate” corrections are easy computed.

Now whether or not my computational methodology is EXACTLY the same as the BLM software is currently unknown (to me anyway), but we are getting the SAME answers at the centimeter or so level, and that isn't too bad for RTK!

I can basically enter the BLM's NAD83 Lat/Lon on a given corner, and then “run” around every section that is of interest to me by simply entering the True Bearings & average distance BLM returns, and the program burps out County LDP Coordinates that we routinely verify at the above stated threshold (once I have aligned the initial BLM Lat/Lon with our Network adjusted Lat/Lon at that corner).

Adamsurveyor's above comments are spot on, and these variations can (in some rare cases) factor into [say] a single proportion between two Section Corners of radically different height, where ALL (or most) of the vertical difference is manifest in only East or West (or North or South) HALF mile.

All of this stuff is reasonably easy to calculate, but I don't think that Autodesk (for example) supports these 'esoteric' considerations very well (if at all).

Loyal

PLSS question> Loyal

I have given much thought to the difference in our environments, me at near sea level and you up in the near stratosphere. I am glad to not have to deal with the severe changes in elevation!!!!

But this statement …” the Public Lands System Datum is based upon measurements at actual average ground elevation along the line.” means that horizontal measurements are made, at the ground level, and therefore scale, ortho heights, etc are not taken into consideration, doesn't it? I assumed that the distances shown on the plat are reflective of the field notes, and we all know that slope distances don't really count without a vertical angle to reduce to a horizontal...

PLSS question> Foggy

Welllll...a "chain" (66.00 feet these days) is a chain, is a chain...

A 66.00' HORIZONTAL distance measured at 10,000' above "sea level" is 66.00 feet just like it is AT "sea level," (in the local sense), but is NOT THE SAME in a geodetic sense. The BLM returns horizontal distances in the "LOCAL SENSE" not in the geodetic sense, SO when we start playing with EDMIs, and GPS (georeferenced coordinate systems in particular), we ALSO NEED to project things (distances) into the "local sense." Which may or may not make sense (sorry).

There are several ways to do this, some more rigorous than others, BUT all of them rely on a certain assumptions and therefore contain some uncertainty (EXCEPT of course ACTUALLY pulling that chain up the side of the mountain with your trusty inclinometer).

Because EACH line (say half mile between PLSS corners) is based on the [more or less] AVERAGE elevation of THAT line, and the TRUE bearing is of course subject to the mean Longitude of THAT line (and mean Latitude on an East West line), corrections must be made to transform THAT data into a rectilinear Cartesian system (like SPC/UTM/LDP). ALL of this “stuff” can be computed in a georeferenced coordinate system (SPC/UTM/LDP) to the millimeter (at least in theory).

Total Stations (with FEW exceptions) return (compute) the “horizontal distance” at the “elevation” of the axis of the instrument. So the same line measured “down” the hill, returns a LARGER [horizontal] distance than the SAME slope distance measured ”UP” the hill (simple geometry). This DIFFERENCE is usually rather small, but IT DEPENDS on the difference in relative height between the two stations (corners).

Loyal

Edit...I probably misstated the "longitude" thing above (didn't really SAY what I meant), but I have to get some coordinate estimates ready for the field crew before they hit the hill in a few minutes. I'll try and get back to this later.

Modern BLM Surveys (at least the ones that I am currently looking at) return True Bearings to the nearest tenth of an arc-minute (6 arc-seconds), and distances to .001 chains (0.066 feet).

That's new to me. The 2000 era resurveys I'm looking at are still nearest minute and .01 chains. Are these smaller tract surveys? The only ones I'm dealing with are township retracements.

Mighty

These surveys cover about a dozen sections (in one case), and all of them cover at least 2-4 sections. These are BRAND NEW (2008-2011) dependent Resurveys, and are NOT yet approved or available online.

I have been very favorably impressed with these surveys, and can only ASSUME they represent a general trend within the BLM.

Loyal

PLSS question> Loyal

> I have given much thought to the difference in our environments, me at near sea level and you up in the near stratosphere. I am glad to not have to deal with the severe changes in elevation!!!!

Not to imply that I am tag-teaming with Loyal (as he is ten times smarter than I am). But I agree with his response that it is not the elevation above sea level we are really referring to. The old surveys we are retracing did not reduce their section line distances to sea level. They are still a mean 'horizontal' distance between points at the general elevation of the line. Since they are a series of "horizontal" distances as they wrap around the "geoid" they are closer to a kind of series of tangents. They also don't really make that much difference in a mile, and you are right that in the general sense, running a piece of property we can for the most part treat our measurements as though the world is flat, and the possible reductions to geodetic north for each line or to geodetic distances are pretty meaningless.

Being the average modern-day surveyor, we need to keep the possible changes in mind if we are measuring great geodetic distances, dealing with multiple townships, or large-route surveying.

Loyal

This is the latest one I know of. A couple more should come out this winter. This is a portion of a larger retracement of 54-79, 6th. Done in 2007.

Since the BLM started using GPS the surveys have gotten quite a bit more accurate. I know one old-timer that really resisted until he got a unit. Then he was telling me how great it was because he could run right down the line with the GPS and see what was going on so quickly. Which is something I really appreciate.

From talking to some of the BLM guys, I would be careful about data from the BLM that doesn't from the actual plat. Such as GCBD data. From what I'm hearing the Lat., Long data from the survey is not getting into that database. I've heard two varations of how this data is getting put into the GCDB and both I found a bit disturbing. Anyway, I'm using the plat itself to get my numbers for now.

Mighty

Here's a piece of one of the plats that I was talking about (assuming I do this right). This one represents Field Work in 2006, and was approved in 2009. It might be on the Utah BLM's website (I haven't checked).

Loyal

Loyal

Very interesting, .1 minutes, .01 chains.
That would be just about right on for the newer equiptment that the BLM is using.

Loyal

The concept of 0.1 minutes in conjunction with .001 chains probably came from me back when I was developing plat drafting stuff for BLM in the early 1980's.

This was based on the old idea that precision of reporting should be somewhat commensurate with the actual accuracy of measurement. So we determined bearing by solar observation. Good to better than a minute, but certainly not to a second. Similarly distance measurement at the time had evolved to EDM and one should expect the distances between points connected by edm traverse to be better than a link, but really not to 1/100th of a foot.

So I built annotation otions into our HP and then AutoCad CAPD systems for 0.1 mintues of bearing and .001 chains for distance that persist to this day.

However the degree of precision reported on returns is a manner of policy of the particular office that approves the surveys. Some offices feel that staying with a link an a minute is appropriate. If there are subdivisions perhaps there is some allowance for fractional (decimal) links.

On the other extrene OR SO decided for some reason to go to seconds and .01 feet equivalents on their records. So there are a variety of opinions within BLM.

jlw

Jerry

Thanks for the clarification.

Like I indicated above (somewhere), I have found these .1 arc-minute and .001 chain returns to be VERY realistic when compared to our own independent observations on the physical monuments on the ground.

Loyal

Bob Dahl's Presentation from ACSM, San Diego, PLSS Datum and the Local Surveyor

91 Pages of info ought to provide your answer.

Follow the footsteps

Get a 66-foot chain and a compass. Break chain as required in steep areas. Use a tension gage and plumb bobs. That is the ONLY precise way to determine the correct distance.

Follow the footsteps

Don't follow the compass, determine the declination where the GLO did, it is noted on the plats at that point, use the adjusted compass bearing at that point to establish the line and follow that line with a transit. Those old timers knew better than to follow a compass and expect to leave a straight line behind them, we should be observant enough to have figured that out, even with the teachings of those who teach and do not do or only done enough to be dangerous, working against us. When the manual tells you to sight the corner if it can be seen and follow that line to that preset corner, while measuring along that line so you can come back and set the midpoint, should be a huge clue, that should not be ignored but is, about how the GLO ran their lines. The Colonial States seem to have followed the compass directly for a most of their work, but I am not talking about that early work or the work built upon its leavings.
jud