Congress mandated that "All lines shall be referenced to True Bearings". That was a useful fiction. It was useful, in a world, that did not have high accuracy survey instruments. It was useful, in that it made the people of this nation, feel like things were "Oriented to True".
Problem is.... As you move back and forth, East and West, TRUE NORTH is a converging series of lines.
I had a good surveyor about 10 yrs ago, ask me "Why when I move my GPS base, and back shoot my last base station, the BEARING of the line always has a little error in it. Sometimes 2, 3 or 4 minutes."
I replied, "Convergence". True north, is ONLY true, at one geographical location. (unless, of course you move NORTH or SOUTH on the meridian line) thus, we have a "Fiction" for all lines, that are of any E-W dimension. A 1 mile long E-W line, in Arkansas, when measured off of true north, going East to West, will have a bearing, that differs, from a line Measured from West to East. Unless, you use a system, that uses the MIDDLE of the line... and that also has it's own problems... it merely shifts the difference to a slightly different location.
I just got several files from the above mentioned surveyor. They were a bit difficult to work with. Built from several GPS base setups. That were NOT due north, or south of each other.
It is something very unfortunate, that TDS, had a "One Point Localize", that set the thing to run off TRUE bearings, at the BASE Station, and Ground combined scale, at the base station. Then, the user, did another job, a mile away, and ASSUMED that they were on the same bearings. BECAUSE they were called the same thing. "True North". IF you want to work on that coord system, you MUST find your old base station, at least pretty close.
When you MIX a radial basis of bearing, with rectangular coordinates, they are only on TRUE at one meridian line. One Line of Longitude.
And, now, you have a survey, of any size, EW, it and it is referenced to TRUE BEARINGS, then it is strictly on TRUE, at a particular line of longitude. So, if you have 4 surveys, in the same vicinity, all done with individual base stations, and they all set on TRUE NORTH, then they are not on the same bearings. They all have 4 differing bearings. They are all labled "True North", but they are all referenced to their own Radial Line.
I have used several sun shot programs, that will yield "GRID BEARINGS" After you tell it what projection, it will allow you reference the GRID. Rectangular bearings. Grid is strictly a knowledge of how far E or W of the C/M you are (central meridian) for that projection.
Look at the attached file. The converging lines, are "True North".
I recommend that surveyors NOT mix a radial bearing system, with a Rectangular coordinate system. Unless they pick a common line of Longitude, to reference to.
I have been tempted to use this for my Basis of Bearings:
Basis of Bearings: True North, as taken at the central meridian, of the State of Arkansas.
Carry on...
N
Nate,
According to the 2006 Arkansas Standards of Practice, Section 4.1.A.9, Publication of Results:
[INDENT=2]North arrow with basis of direction (bearings). A statement shall be made adjacent
to the north arrow to explain how direction was obtained, and shall include
document (book/page) references if based on deed or survey record bearings.
When the basis of direction is derived from the Arkansas Coordinate System 1983
(geodetic or grid system), the convergence angle and the scale factor shall be
shown with a notation specifying the location where the calculations were made.[/INDENT]
[INDENT=2] [/INDENT]
I've taken the liberty of providing you an example (see attached PDF).
DDSM [hl]å© April 7, 2013[/hl]
I dunno Nate, it's worked pretty well for the PLSS for the last couple hundred of years...B-)
Here's a recent BLM Plat from Utah:
I have navigated to most every one of these corners using GPS, and have yet to miss one by more than a centimeter (maybe 2).
I was using several RTK Base Stations, and NAD83 LDP Coordinates computed from the Plat's "true bearing & distances," converted to the LDP. Of course the "grid inverses" (Bearings) do NOT match the Plat, but then they are not supposed to!
:angel:
Loyal
Loyal, you know alot more than I do.
That is the FIRST time I've ever seen a BLM plat, with accuracies to to the sixth of a minute. And, it is also the first time I have ever seen one that CONSISTENTLY showed the chains to 3 places.
I just now HAVE a tool, to manage, those pesky "true North Bearings". I can do data entry, with the JAVAD LS, in "True North Mode" to do this, you have to have a lat lon to kick it off. Then, you key in the bearings and distances, as GEODETIC. And, ground distances. It's pretty cool. I have never known a good way to enter a geodetic bearing, as though it were a grid bearing. Especially when convergency around here is around 55" per mile, and the bearings on the BLM plats are rounded to the minute.
So much to learn.
N
You make some good points, Nate. Most of the time people thinking they are on True North bearings are on True North at one Longitude. They are actually on a Grid, turning angles from a reference direction instead of measuring directions... well, directly.
It would take some real effort to produce a geodetic bearing for each call in the old days, I think. Also you would have a poorer closure error because our process of calculating latitudes and departures for closure is based on Cartesian geometry (not Spherical). Small areas, you won't really see the difference, but larger areas certainly do.
I've taken to providing a basis of bearings that reads something like this:
Bearings related to a local grid having an origin of North Latitude xxå¡xx'xx" and West Longitude xxå¡xx'xx". Convergence angle at POB: xxå¡xx'xx".
Now if I'm using State Plane, I don't do that, but most of my work these days is related to a Low Distortion Projection (Hail, Loyal!).
I believe in the it's all relative rule...only Google can connect the world...every BOUNDRY survey is localized to itself...no matter how large or small...A section need only conform to the adjoining sections, a 40...the same...1 acre....the same...only pipelines or power lines or highways need to connect with infinity!!!! Grid is a dangerous thing when it comes to farmer Joe's farm....so I guess I agree about true north being useful fiction....but I love that story, and I guess I will keep telling it down here in Mississippi...fancy rebar caps and all.
Loyal, post: 364788, member: 228 wrote: I dunno Nate, it's worked pretty well for the PLSS for the last couple hundred of years...B-)
I am not necessarily saying it does not work. I am saying that MIXING geodetic polar coordinates, with rectangular, is not often understood, and taught. I am saying, that NO surveyor that I know, publishes that kind of thing. ONLY the BLM does. And, often, the surveyors do not understand what the BLM is actually publishing.
And, I am saying, that we should understand, that WHEN we MIX geodetic polar coordinates, (True North) with rectangular, we should be aware of what we are doing, and it's limitations. TRUE north, (pretty close to Geodetic north) is a radial line. And, as such, it is a potential error source, when you don't manage your convergences.
N
Nate The Surveyor, post: 364857, member: 291 wrote: I am not necessarily saying it does not work. I am saying that MIXING geodetic polar coordinates, with rectangular, is not often understood, and taught. I am saying, that NO surveyor that I know, publishes that kind of thing. ONLY the BLM does. And, often, the surveyors do not understand what the BLM is actually publishing.
And, I am saying, that we should understand, that WHEN we MIX geodetic polar coordinates, (True North) with rectangular, we should be aware of what we are doing, and it's limitations. TRUE north, (pretty close to Geodetic north) is a radial line. And, as such, it is a potential error source, when you don't manage your convergences.N
Natester,
You are correct in that too many "surveyors" don't really understand the basic terminology (and geometry) involved in this discussion. There are any number of [root] causes for this situation, from poor education (mentoring), to the [over] reliance on canned software, and computers/box-boxes in general.
If you (by chance) tried to put the BLM Plat that I posted into a computer (CAD), or data collector in order to generate "search Coordinates," you no doubt noticed that the Plat LACKS a critical piece of information....NO Heights!
By definition, "True Bearings" and "True Distances" in the context of the PLSS "datum," are a 3 dimensional proposition.
It could be said that EACH LINE (Section Corner to å? Corner OR å? Corner to Section Corner), is for all practical purposes, a Mini-datum. The "Central Meridian" is unique to the mean Longitude of the ends of the (curved) Line, and the ("true" horizontal) distance is a function of the mean height of THAT line.
The area covered by the Plat that I posted, is pretty "flat" in Sections 3, 4, 9, & 10 (about 4960 to 5020 NGVD29), but Sections 1, 2, 11 & 12 go from about 5020 to 5490 NAVD29. As surveys go round my neck of the woods, this is "pancake land," and a single height [estimate] could be used for most any practical purpose. However, a similar sized plat located only a couple of miles away, could have 5000 feet of relief, in which case the height issue would be nontrivial.
Anywho...I feel your pain, and totally agree with you about the widespread misuse, abuse, and general misunderstanding of these basic principals. Too many folks are totally dependent on whatever software/hardware that they are using, and afraid (or disinterested) in looking behind the curtain, to see what the wizard is REALLY doing (or NOT doing).
One size NEVER (really) fits ALL...
The same can be said about the "standard operating procedures" that I routinely see from many of the Engineering/Survey Firms around my area. Fugarwe "calibrations" and such, work fine (for them) on postage stamp sized residential projects down in the valley, but blowup in their face once a project involves larger areas, and/or thousands of feet of relief in the mountains.
😉
Loyal
Loyal, post: 364975, member: 228 wrote: Natester,
You are correct in that too many "surveyors" don't really understand the basic terminology (and geometry) involved in this discussion. There are any number of [root] causes for this situation, from poor education (mentoring), to the [over] reliance on canned software, and computers/box-boxes in general.
If you (by chance) tried to put the BLM Plat that I posted into a computer (CAD), or data collector in order to generate "search Coordinates," you no doubt noticed that the Plat LACKS a critical piece of information....NO Heights!
By definition, "True Bearings" and "True Distances" in the context of the PLSS "datum," are a 3 dimensional proposition.
It could be said that EACH LINE (Section Corner to å? Corner OR å? Corner to Section Corner), is for all practical purposes, a Mini-datum. The "Central Meridian" is unique to the mean Longitude of the ends of the (curved) Line, and the ("true" horizontal) distance is a function of the mean height of THAT line.
The area covered by the Plat that I posted, is pretty "flat" in Sections 3, 4, 9, & 10 (about 4960 to 5020 NGVD29), but Sections 1, 2, 11 & 12 go from about 5020 to 5490 NAVD29. As surveys go round my neck of the woods, this is "pancake land," and a single height [estimate] could be used for most any practical purpose. However, a similar sized plat located only a couple of miles away, could have 5000 feet of relief, in which case the height issue would be nontrivial.
Anywho...I feel your pain, and totally agree with you about the widespread misuse, abuse, and general misunderstanding of these basic principals. Too many folks are totally dependent on whatever software/hardware that they are using, and afraid (or disinterested) in looking behind the curtain, to see what the wizard is REALLY doing (or NOT doing).
One size NEVER (really) fits ALL...
The same can be said about the "standard operating procedures" that I routinely see from many of the Engineering/Survey Firms around my area. Fugarwe "calibrations" and such, work fine (for them) on postage stamp sized residential projects down in the valley, but blowup in their face once a project involves larger areas, and/or thousands of feet of relief in the mountains.
😉
Loyal
You forgot to mention "modifrickinfried" somewhere in your post.
😉
Dave Karoly, post: 364984, member: 94 wrote: You forgot to mention "modifrickinfried" somewhere in your post.
😉
Ya mean "modifrickinFIED" right?
Well..."fried" works too I suppose.
I just noticed that I wrote "box-boxes," when I meant to write "black-boxes" (although most of them are yellow).
B-)
Loyal
Loyal, I am one of them... Used to be anyway. I used to get so frustrated with BLM plats that did not close.... Did not know why.
There are 2 methods of using "True North", that I am aware of.
1.) Bearings are from the Point of Origin. So, a Bearing of N 89å¡50' W, (with 55 seconds of convergence) would read S 89å¡49' E due to rounding, if it were written from the west, going east.
2.) Bearings are taken from AVERAGE of the two, (bearing is taken in the middle of the line) and the distance is also an average of the elevations. So, it is really a "Fully Ground Distance" in most senses of the word.... Less and except private undulation! Could be a big mountain in the middle, or a valley... go figure!
Anyway, I'm taking it that most BLM plats are published using # 2 above.
How do you enter these into a "Rectangular coordinate" software? BLM has some fancy stuff, for goobering this just right. I remember discussing this with the BLM surveyor. He was using a HP 48 calculator, to double check all the math... to make sure that after all the asinine rounding, and setting all lines to be off of "True North" that the section still "Closed" within min standards.....
I think that was the day that "Set me off".
🙂
Nate
Nate The Surveyor, post: 365004, member: 291 wrote: How do you enter these into a "Rectangular coordinate" software?
I wrote my own software (BASIC) years ago. Enter an LDP/SPC/UTM coordinate & Ellipsoid Height (or Lat/Lon/eH) at any given Corner, and then just input the BLM (PLAT) Bearings and Distances.
Whole bunch of whizzbang stuff happens "behind the curtain," and the program outputs Coordinates (and Grid Bearing/Distance) to both the screen, and several ASCII Text files (.csv & stuff).
Closures around a Section are pretty much perfect on modern plats (subject of course to the BLM rounding to 6 arc-seconds and 0.066 ft on the plat). Older Plats (19th Century-ish) are not really worth the effort, and I usually just input the Plat data as if it were rectilinear in nature.
There is an "inverse" function as well, that returns "True Bearing & Distance" from LDP/SPC/UTM coordinates and heights.
Although useful (and quite "accurate") the program is essentially "laboratory software," that's not particularly user friendly (even to me sometimes). In fact, I probably use the inverse function more than the direct function...
The inverse function also writes a polyline into a .dxf file that follows the "arc" (@ 33 foot vectors). I use this polyline when subdividing Sections, or doing single/double proportion calculations.
As you alluded to above, there ARE topographic scenarios that require additional "modeling" from time to time.
Loyal
Nate, Trimble handles true north pretty well, you do have to know how to massage the data entry...........
Since the BLM started using GPS those corners are tight. You will find a 1/4 on an east-west line right at 2" south of midpoint (the curve at my lat.)
Given enough distance you will find a curve along state, base, standard lines.
Just remember to go the direction of the bearing, don't reverse it B-)
OK, now the question comes to mind. What method does the BLM software use?
No. 1 above,
Or No. 2?
Or something else?
This was ONE of the handy things, that JAVAD does... you can directly enter GEODETIC bearings. (But, I don't know it's methods!)
Dang, I'm deeper in surveying than I mean to get, (But I like it!)
🙂
Nate
Nate The Surveyor, post: 365010, member: 291 wrote: OK, now the question comes to mind. What method does the BLM software use?
No. 1 above,
Or No. 2?Or something else?
This was ONE of the handy things, that JAVAD does... you can directly enter GEODETIC bearings. (But, I don't know it's methods!)
Dang, I'm deeper in surveying than I mean to get, (But I like it!)
🙂
Nate
The BLM has a program they use or did use, I don't remember the acryomn, but it would calculate in true north and I THINK convert to a format useful in computer language.
But remember, they have lots of time and aren't doing data intensive projects like large subdivisions or highway layouts...........
And it's not just east-west lines that get curved.:whistle:
Nate,
As I read [interpret] the 2009 Manual (Chapter II Measurement):
Basis of Distance:
2-2. "The distance reported is the horizontal measurement at the mean ground elevation for the line above sea level."
Basis of Direction:
2-3. "The direction of each line of the public land survey is determined with reference to the true meridian as defined by the axis of the earth's rotation."
Chapter II goes into MUCH greater detail than earlier Manuals in regard to just WHAT THAT MEANS... and I highly recommend a careful reading of the entire chapter.
Based on recent retracements of "modern" BLM Surveys, I would say that 'they' (the BLM) are using a program that does what the Manual requires...which is in effect, your No.2.
AND, (as Mighty alluded to above), ALL of the PLSS Lines have "some" curvature, unless they are perfectly "North-South." As a practical matter, we only "worry" about the East-West Lines, but not every PLSS Line is semi-cardinal. When you start playing with LONG meander/reservation/etc. Lines that are at "odd" bearings, you need to pay attention to the curvature (sometimes).
Loyal
Am I safe in assuming because I am using a solar transit (Smith solar attachment) and invar chain, this topic is of no concern to me?
Mike Marks, post: 365125, member: 1108 wrote: Am I safe in assuming because I am using a solar transit (Smith solar attachment) and invar chain, this topic is of no concern to me?
It Depends...on how you use it.
😀
O dang. I'm in trouble.
Confession time.
I have used BLM plats wrong for a long time. Without realizing what convergences were. And, that each line was on it's own little system. (Really now, who draws plats, with MULTIPLE DATUMS, and does not warn the user!)
Well, the BLM does. User is not supposed to be un educated! (ha!)
So, long and short, Loyal is right.
And, I am working to get up to speed on this.
One complaint I do have though, is that There is NOBODY doing this for all the 30+ years of Continuing Education.
I recognize the need for me to "get fluent" with this. And, I see alot of others "abusing" the system, ie, just like me.
N
Nate The Surveyor, post: 365136, member: 291 wrote: O dang. I'm in trouble.
Confession time.
I have used BLM plats wrong for a long time...One complaint I do have though, is that There is NOBODY doing this for all the 30+ years of Continuing Education.I recognize the need for me to "get fluent" with this. And, I see alot of others "abusing" the system, ie, just like me.
N
Nate,
Here is your chance to SHARE with your fellow Arkansas surveyors. Publish a paper or Teach a class at an ASPS short course. Quit complaining and TEACH!
DDSM:beer: