I know that the PLSS datum is based on true bearings and meridians converge. Conceptually I understand, but I'm a little lost when it comes to applying it on the ground.
For instance, in calculating the center of section, I would typically intersect opposite 1/4 corners in AutoCad. Simple enough, but isn't this ignoring curvature? The C 1/4 is just an example, but wouldn't the same hold true for any aliquot subdivision, proportioning etc.?
Any example of proportioning based on coordinates seems to ignore this principle, even in the manual.
Maybe I'm overthinking it...
For 1/2 mile I ignore curvature. For anything longer I proportion using geographic coordinates.
I hear you. At the section level, the framework is perhaps densified enough to get away with ignoring it. I live in NW Washington where convergence amounts to about 1' per mile. It adds up pretty quick...
Like Jim, I disregard curvature within sections. Given that I would be retracing a local surveyor who first subdivided the section, there are footsteps to follow.
The BLM Course (Advanced Cadastral Survey) I took back in 1993 (which predated the CFeds Program) taught that they used a "red book" that gives you the change in bearing as you traversed east to west. They corrected the bearing backsight with the appropriate correction bearing as they backsighted and moved forward This ran the coordinates with a correction as they moved along. This methodology keeps them on track for traversing to the next corner bearing-wise, but the coordinates would never close if they tried to loop back around.
The fact that x/y or N/E coordinates are on a grid, you can't escape that you are practicing plane surveying on a curved surface. I suspect, however, that you can apply the same "red book" principle to your autoCAD coordinates. It would be better if you could work in spherical coordinates and intersect the latitude with a longitude.
THe red book is simply a red book of tables that tells you how much change in bearing you experience with respect to distance in the east-west direction based on what latitude your at.
I agree, that 1/2 mile is relatively minimal in bearing change in the contiguous United States (maybe a tenth different than if you ran a straight plane-line). But that wasn't your question. I suspect you would have to take it a bit more seriously in Alaska.
(I hope I worded that comprehensively, sometimes I get a little convoluted when I type as I think)
I guess that is what it comes down to. The orthogonal system most if us work in only jives with the PLSS datum at a givin point. The question is at what point does one have to deal with that difference?
For me I think it would be easier to learn how to proportion geodedic coordinates than trying to work in a grid system where true north is a consistantly moving target.
I've calulated latitudinal lines using the redbook tables before and for me it is a cumbersome process and one that I've never fully grasped. All of the examples I've seen usually have an emphasis on standard parallels or long distances, but I'm surprised on how much of a difference it does make even in a mile.
Again I live close to the canadian border.
At the 49th Parallel...
1 Mile East-West (to set the å? corner, including of course a Center å?)
The å? Corner would be ~0.19 feet South of a "straight line" between the opposing Corners. This would mean that "Grid Bearings" would indicate a ~0å¡00'30" deflection @ the å? Corner.
That would be nontrivial in my book, but your mileage may vary.
Loyal
Jim_H, post: 366212, member: 11536 wrote: I know that the PLSS datum is based on true bearings and meridians converge. Conceptually I understand, but I'm a little lost when it comes to applying it on the ground.
For instance, in calculating the center of section, I would typically intersect opposite 1/4 corners in AutoCad. Simple enough, but isn't this ignoring curvature? The C 1/4 is just an example, but wouldn't the same hold true for any aliquot subdivision, proportioning etc.?
Any example of proportioning based on coordinates seems to ignore this principle, even in the manual.
Maybe I'm overthinking it...
There is much discussion amongst us regarding the GLO/BLM meaning of "intersecting opposite quarter corners." Many maintain that ALL latitudinal lines in the PLSS system are curves, many that maintain that within a Section they are straight, and many that just plain can't decide. It is my understanding that the BLM Cadastral Measurement Management (CMM) software uses curved lines. If you search this forum you will find much discussion on this topic.
The BLM does not proportion using coordinates - they proportion using distance and cardinal offsets.
It would be cool to get ahold of one of those "red books" for my library.
The "Red Book" was not always Red...
I have a "red" EIGHT EDITION 1956 (BLM), and a "black" THIRD EDITION 1923 (GLO).
The 1923 Third Edition is virtually brand new (found it at a used book store years ago).
The 1956 Eighth Edition is falling apart (I carried it around between about 1968 and sometime in the 1980s).
They both have a place of honor on my "Manual" shelf, along with copies of the various GLO/BLM Manuals, Corner Restoration Circulars, and other related "old" books.
Loyal
Jim_H, post: 366212, member: 11536 wrote: I know that the PLSS datum is based on true bearings and meridians converge. Conceptually I understand, but I'm a little lost when it comes to applying it on the ground.
For instance, in calculating the center of section, I would typically intersect opposite 1/4 corners in AutoCad. Simple enough, but isn't this ignoring curvature? The C 1/4 is just an example, but wouldn't the same hold true for any aliquot subdivision, proportioning etc.?
Any example of proportioning based on coordinates seems to ignore this principle, even in the manual.
Maybe I'm overthinking it...
Maybe under thinking it. The vast majority of lines in the PLSS are not intended to be true parallels or meridians. Most are supposed to be parallel with and perpendicular to some other line, or straight lines connecting previously established monuments. Hence the need for both single and double proportioning of lost corners depending on which lines or arcs are involved. But as someone mentioned, how it was actually done in any given area is more important than the theory or instructions in most instances.
In fact, the "Blue Book" (Manual of Survey Instructions) was not always Blue.
My 1930, 1947, 1973, and 2009 are Blue, but my 1881, 1894, and 1902 are Black, and the Advance Sheets (1924 Edition) are Green (paper cover).
Loyal
Jim_H, post: 366224, member: 11536 wrote: I guess that is what it comes down to. The orthogonal system most if us work in only jives with the PLSS datum at a givin point. The question is at what point does one have to deal with that difference?
For me I think it would be easier to learn how to proportion geodedic coordinates than trying to work in a grid system where true north is a consistantly moving target.
I've calulated latitudinal lines using the redbook tables before and for me it is a cumbersome process and one that I've never fully grasped. All of the examples I've seen usually have an emphasis on standard parallels or long distances, but I'm surprised on how much of a difference it does make even in a mile.
Again I live close to the canadian border.
t sounds like you already know about the "red book" practice making my earlier post kind of moot.
Anyway, I work more in retracing surveys, and I am, for the most part, going to accept monuments I find instead of setting something a little bit away because I used the latitudinal arc. I am more interested in finding evidence of boundary acceptance, and original monuments whether I am using a plane survey coordinate base or figure things on an arc. So many boundaries have ripened, and the surveyors who set the corners may well have used other means. You need to establish the means and intent of the earlier surveys more so than the "right" way to survey. (unless, of course you are doing some original surveying yourself).
I performed a survey for the BLM several years ago establishing government lots for a 'withdrawal' and I was instructed to put the sixteenth corners on the arc. As I recall it amounted to a little over a tenth of a foot per quarter mile.
Ric Moore, post: 366244, member: 731 wrote: It would be cool to get ahold of one of those "red books" for my library.
It's funny, right after I took the course I went in to a used book store and looked through what survey material I could find and found that "red book".
The instructions are clear. The township is laid first. And center sections are originally set on straight line last connecting the standard corners. So for center section, it's not a matter of ignoring latitudinal arc, but instructed to run 'straight line connecting' the standard corners. Reestablishing a lost corner have rules too. Lost or obliterated corners:
http://www.blm.gov/style/medialib/blm/ak/aktest/cadastral.Par.86851.File.dat/lost_oblit.pdf
INTERIOR CORNERS
10. A lost interior corner of four sections will be restored by double proportionate measurement.
When working with sections the instructions are complicated, but there is hierarchy and priority. Range lines, township lines, then sections, and the west and north sections suffer the deficiency.
Working on a project in section land, read the instructions.
1-30. The basic provisions require that the public lands ÛÏshall be divided by north and south lines run accord- ing to the true meridian, and by others crossing them at right angles, so as to form townships six miles square;Û that ÛÏthe townships shall be subdivided into sections, containing as nearly as may be, six hundred and forty acres each;Û and that ÛÏthe excess or de ciency shall be specially noted, and added to or deducted from the western and northern ranges of sections or half-sections in such townships, according as the error may be in run- ning the lines from east to west, or from south to north.Û The system of rectangular surveys ts the basic require- ments to the curved surface of the earth.
In this rectangular plan, the township boundaries are intended to be due north and south or due east and west.
The boundaries running north and south are termed ÛÏrange lines.Û The boundaries running east and west are termed ÛÏtownship lines.Û In this general plan, all the lines are rhumb lines and cross each meridian at a con- stant angle.
The range lines are great circles of the earth that, if extended, would converge and intersect at the North Pole. This convergency becomes apparent in the mea- surement of the township lines. The convergency is taken up at intervals by the running of standard paral- lels, on which the measurements are again made full. On the standard parallels, rst termed ÛÏcorrection lines,Û there are offsets in the range lines and two sets of corners, standard corners for the lines to the north and closing corners for lines to the south. The usual inter- val between the standard parallels is 24 miles, but there were many exceptions in the older surveys.
To make the sections represent ÛÏsquare milesÛ as nearly as may be, the meridional lines are run from south to north and parallel to the east boundary of the township for a distance of 5 miles from the south boundary. These are run and monumented as true lines. The remaining section lines are all run by random and true between the established section corners. This produces the rectangu- lar sections, 25 of which contain 640 acres each, within allowable limits. The sections along the north and west boundaries are subdivided on a plan for certain lottings to absorb the convergency and the excess or de ciency in the measurements. These sections provide a maxi- mum number of aliquot parts (160-, 80-, and 40-acre units) or regular subdivisions of a section, the remainder being shown as lots for which the contents are computed according to the eld measurements.
.... Rules of Survey. The public lands shall be divided by north and south lines run according to the true meridian, and by others crossing them at right angles, so as to form townships of six miles square, unless where the line of an Indian reservation, or of tracts of land sur- veyed or patented prior to May 18, 1796, or the course of navigable rivers, may render this im- practicable; and in that case this rule must be departed from no further than such particular circumstances require.
Third. That quarter-quarter-section corners not established in the process of the original survey shall be placed on the line connecting the section and quarter-section corners, and mid- way between them, except on the last half mile of section lines closing on the north and west boundaries of the township, or on other lines between fractional or irregular sections.
Fourth. That the center lines of a regular section are to be straight, running from the quarter-section corner on one boundary of the section to the corresponding corner on the op- posite section line.
http://www.cfeds.org/docs/sml/ManualOfSurveyingInstructions2009_060414.pdf
Larry Scott, post: 366305, member: 8766 wrote: instructed to run 'straight line connecting' the standard corners
At the time those instructions were first given, the distinction between latitudinal arc and optically straight line was meaningless because a) the tolerances were far wider than the difference and b) the line was run with a compass which if everything was perfect would cause them to follow chords to the latitudinal arc. Depending on sighting conditions, that could be anywhere from one to many chords.
And the meridians and parallels were directed to be used for township and range lines. Not for the interior sections. And in retracing, the instructions for lost corners apply, and proportionate distances prevail.
You silly people. Everyone knows the Earth is flat. All this curve stuff is just like the movie Capricorn One.
It goes to the original question: is the C section on a latitudinal curve? And the answer is that per the instructions is no.
And the 25 standard section lines aren't either. The township boundary is meridians and latitudes. The 25 standard sections are run parallel the the east and south lines of the township, by distance from the east and south lines of the township. Starting in the SE. The N 1/2 and W 1/2 of the 11 sections on the north and west perimeter absorb the excess or deficiency.
(And, this isn't about performing an original survey.)
So, the C section was not on a latitudinal arc. And standard section lines aren't either. And the BLM has published texts, and frequently holds workshops on the subject because it's not always obvious.
