Hi
First I'd like to clarify that I am not a surveyor but a GIS specialist working on updating parcel dataset per subdivisions. I work in NJ using ArcGIS Pro.
On one subdivision plat, I see that the parcel boundary along Cumberland Avenue has a bearing of N31-31-41W. In ArcGIS Pro, it says the bearing is N41-07-11W when use a satellite map with State Plane projection. The only reason I can think of is the difference between true north and magnetic north, but in many other plats there are no such big difference between bearing shown on plat and bearing measured from map.?ÿ
As you can tell, I am new to the survey world, so any thought on this can be helpful.
Thanks!
Bearings are sometimes a way to encode the interior angles, and there is no rhyme or reason if you try to make it fit any known datum.
On one subdivision plat, I see that the parcel boundary along Cumberland Avenue has a bearing of N31-31-41W. In ArcGIS Pro, it says the bearing is N41-07-11W when use a satellite map with State Plane projection.
First off, ArcGIS Pro is a great program, but it's not an authority with respect to legal boundary lines.
What's the plat basis of bearing? Plats aren't necessarily referenced to global or national datums. Could be "true" north (there's no such thing but it used to mean astronomic), geodetic north, magnetic north, grid north, or assumed north. Or even grid north for a custom projection. Doesn't really matter as long as the plat is consistent within itself.
in many other plats there are no such big difference between bearing shown on plat and bearing measured from map.?ÿ
As you can tell, I am new to the survey world, so any thought on this can be helpful.
Being new to the survey world, I think you're going to find that there are LOT of plats that don't match up with GIS lines. Or with each other for that matter. Doesn't mean they are wrong or invalid. I like consistency as much as the next guy, but numbers are just numbers without something physical in the real world to reference them to.
The bearing is what the creator of the plat has decided it is.?ÿ As mentioned already, it can be a wide variety of things.?ÿ The critical part is that you can slap one subdivision up against another and get rather close symmetry.?ÿ The bearings may have been called something different, though.?ÿ Sort of like doing a jigsaw puzzle, but rotating it one way or the other, yet the pieces fit together rather well.
Historically, it has been difficult to establish true north with any degree of accuracy. And a plat is tied into older parcel lines whose location is known, so bearings based on true north aren't particularly useful in retracing platted lines. The only thing that's important is to use the same basis of bearings throughout the plat. You can expect that will have been done.
I've been surveying here in Minnesota for 40 years and have never seen a plat 20 or more years old with geographic (true north) bearings as opposed to assumed bearings. The assumed north is generally within a few degrees of geographic north, but if it is not it doesn't do any harm.
Nowadays, some county surveyors require new plats to use bearings on the County or State grid. They say that's to make things easier for the GIS people, which is reasonable enough. It does away with the puzzle-piece fitting that Holy Cow describes.
You didn't mention the age of the plat. If it's new, and if local regulations require it, there might be a note explaining the basis of bearings. Otherwise, probably not.
The basis of bearings might have come from an older underlying plat, survey, or deed, whose bearings were in turn based on a magnetic compass reading taken 100 or 200 years ago. There is most likely some historic reason for it. But it's not a problem.
@rplumb314?ÿ you think horizontal is hard to correlate. Try correlating NGS benchmarks, FEMA FIRM Maps and OPUS Elevations.
The north arrow references a book and page so I would look at that for an explanation of the basis.?ÿ It might be magnetic of some older date.
I was just dealing with this for a road signalization project that runs through 4 filings of 3 subdivisions, and in the end, I threw up my hands and beseeched the Surveyor to help make a decision for the BOB of the drawing because I had 6 to choose from all between the same monuments, and yeah.?ÿ I lost sleep over that one....not fun.
I??d like to see state plane grid on all surveys.?ÿ
or
a reference to spc.?ÿ
or
geodetic. With location where it is geodetic.?ÿ
since more information is now known, we should see more of the above.?ÿ
n
Oregon tried that a couple decades ago.?ÿ Subdivisions were required to tie to geodetic control located within a certain distance.?ÿ Problem being they only required a tie to one geodetic mon.?ÿ I utilize geodetic referenced grid coordinates on about 99% of my projects.
Thank you all for the replies. I am getting better understanding now and less worried that I might have made some mistakes in the GIS program.
Being new to the survey world, I think you're going to find that there are LOT of plats that don't match up with GIS lines. Or with each other for that matter.
Indeed, but most differences that I saw were within one or two degrees. I've only worked with 2020s subdivisions so I haven't seen an older plat.
?ÿ
The basis of bearings might have come from an older underlying plat, survey, or deed, whose bearings were in turn based on a magnetic compass reading taken 100 or 200 years ago.
Is this why the north arrow says BOOK G96 PAGE 482? I saw another plat with north arrow saying NJSPCS (NAD83), and I did find that one easier to work with.
Thanks again!
Sort of like doing a jigsaw puzzle, but rotating it one way or the other, yet the pieces fit together rather well.
Yes.
So one might takeaway from this that bearings are nothing more than a guideline when trying to place various plats relative to each other. The GIS department shouldn't depict overlaps or gaps just because that is how the bearings of the associated plats fit relative to each other.
@lurker?ÿ
the big problem that GIS has is that everyone puts everything into polygons and they think that that's the way the words are supposed to be done and don't realize that each line from each point or corner or monument is actually where it comes from in those can change based on the geodetic references they're trapped in their own hell of making everything close as a perfect box circle octagon hexagon etc
Most plats done before GNSS, and a good many after still using total stations, were measured as angles, but there is a tradition of giving bearings even though bearings were not measured, only a starting bearing assumed.
