I have been working on a project since the early 90's (I have been employed by 6 different companies since then) whose original control was established by GPS, and the coordinates based on NAD 27 State Plane Local Zone (except they had to truncate the coordinates to fit in the data collectors and maybe even the computers of the time). To this day, I can take cad drawings and or legal descriptions written during and since that time, translate them to NAD 83 coordinates, and see little to no rotation needed to hit called for monuments. There are still subdivisions being built today with the engineering based on those NAD 27 truncated coordinates.:-X
I've found the same to be generally true. Different models or datum's might yield different coordinates, but the bearings seem highly reproducible.
> "True North, by GPS observation".
>
> My problem with this, is that it leaves the question hanging... does he even know what his Basis of Bearings are?
>
> IF he sets his base up again, a mile east of it, to do another survey, and he uses the "Same" BOB, on the second survey, will he understand why the bearings on the 2 jobs are NOT the same?
>
> Why not just use STATE PLANE GRID bearings, on both of them?
>
>
> N
They will be the same, Nate, they will both be true north. But, I know what you mean, the angular variation between a north south line and an east west line will be different.
But now tell me, why is it important that to parcels a mile apart have related directional systems?
Stephen
> What is "True North"... really...
True north is that point on the earth's surface that intersects the axis of rotation.
It's easy to find by the way, just sight Polaris when at apogee or perigee.
Stephen
We surveyed on the surface, not some imaginary line that nobody can see. There were lots of checks against direct solar observations on certain projects and we would vary maybe 5" in bearing. It seems that the software in the controller allowed setting to whatever you desired to use, astronomic or grid. Anyway, we didn't seem to have any problem and we did our work up on the ground where everything is. You people using grid bearings and ground distances are trying to mix apples and oranges.
Surveying along the surface relates to how we report distances. It is fairly insignificant to how we report bearings. Unless you are computing each bearing by geodetic inverse your bearings are grid. They may be a local grid but they are still a grid. The further you get east or west from your bearing source the greater the convergence angle will be.
Next year we'll be surveying the tract between those two tracts a mile apart. Won't it be nice when we can piece it together using the surveys on either side like a tightly fitting puzzle?
c. The reference meridian used to conduct the survey shall be stated on the survey plat. A definitive north arrow shall be shown on the plat. All surveys will be referenced to a true meridian by accepted methods with the following exceptions: (a) those used in existing subdivisions; (b) those shown on city or town plats; or, (c) those shown on a previous survey when the current survey is a division of said previous survey and enough monumentation is available to establish the original orientation. If Global Positioning System equipment is used to obtain the reference meridian, it shall be stated on the plat whether the bearings are grid or geodetic. If any published horizontal control stations are occupied during the survey, they shall be listed on the plat and the horizontal datum used shall be listed on the plat. If a meridian established by the compass is used, the compass must be properly declinated and adjusted to a True Meridian. Regardless of the meridian used, the survey must be referenced to a well defined line, group of monuments, reference points, etc. of a normally assumed permanent nature so the orientation of the survey can be re-established. This reference line and its relation to the meridian used must be clearly shown on the survey plat.
> c. The reference meridian used to conduct the survey shall be stated on the survey plat. A definitive north arrow shall be shown on the plat. All surveys will be referenced to a true meridian by accepted methods with the following exceptions: (a) those used in existing subdivisions; (b) those shown on city or town plats; or, (c) those shown on a previous survey when the current survey is a division of said previous survey and enough monumentation is available to establish the original orientation. If Global Positioning System equipment is used to obtain the reference meridian, it shall be stated on the plat whether the bearings are grid or geodetic. If any published horizontal control stations are occupied during the survey, they shall be listed on the plat and the horizontal datum used shall be listed on the plat. If a meridian established by the compass is used, the compass must be properly declinated and adjusted to a True Meridian. Regardless of the meridian used, the survey must be referenced to a well defined line, group of monuments, reference points, etc. of a normally assumed permanent nature so the orientation of the survey can be re-established. This reference line and its relation to the meridian used must be clearly shown on the survey plat.
Gee, isn't that nice.
But isn't it embarrassing that such explanations/requirements are necessary? Wouldn't it be better if something so basic to a supposed "profession" was actually understood by the members of that "profession", at least to the extent that it isn't necessary to have such a hand-holding statute?
Truly embarrassing.
BTW, what exactly is a "normally assumed permanent nature"???
My favorite is still: Bearings are assumed but relative
I figure the east-west lines are uncles and aunts of the north-south lines which are cousins.
And I'm with you, I don't want basis of bearings statements legislated.
It isn't all that difficult to construct a simple statement that explains how the bearings were derived. Maybe someday we will all work geodetically; basically going forward to the past.
I see bearing statements that go on and on even giving coordinates for the origin point of an LDP when there aren't any coordinates given on the entire survey. Kind of a waste of time. I see legislated basis of bearings rules getting way out of hand. And if there are monuments on the ground and bearings between them then it's not all that difficult to get onto what was done before.
I would rather leave it as it is and then you can tell if the surveyor knows what he's doing by what his statement says-often times it says nothing.
Well, Stephen, I got started in this, one day, when I did a survey, with a theodolite, with a mechanical problem. I ran around 160 acres, and found 3 ft of error. I indexed the gun out, and found an erratic problem. Like the circle was off center or something. I went out, and set up about 6 times, (with another theodolite) and did sun shots. I computed STATE PLANE GRID BRGS from these sun shots, and then figured a correction to the survey. Potentially, there is up to 1/2 a foot of error per 1/4 mile on this survey, but I'd bet it is less than that. I rotated out the 3 ft error. (There was also a rotational error, on the closing bearing shot, but I don't remember what it was)
Then, I discovered that GRID BEARINGS can be used, to keep all my surveys on the same BOB. (Yes, there is something like a few seconds of error, doing an inverse from one side of the state to the other, but it is insignificant, in the area of 30 miles or so.)
Adjusting to TRUE north, means that you are adjusting to lines that converge.
State Plane Grid Bearings mean:
Take the middle of the state (Here in Arkansas anyway) and point at TRUE, or GEODETIC north. Then, work all surveys off parallel lines from this central meridian.
I could put this note on my plat:
Basis of Bearings, Geodetic North, based upon the central meridian of Arkansas. (Since we are way to the west of CM, the convergence angle here is usually around 0°55' or so. Convergence turns out to be about 55" per mile of EW change.)
I have always used TRUE north and Geodetic north as about the same, as the difference between these is not great.
Have I added to the confusion?
🙂
Nate
Convergence turns out to be about 55" per mile of EW change.)
surely not that much.
> ... so the orientation of the survey can be re-established....
This statement implies that, under these standards, it is acceptable practice to retrace a boundary by jumping on a basis of bearings and running out the bearings and distances from a map or deed.
Ugh.
> And I'm with you, I don't want basis of bearings statements legislated.
>
I agree. If someone goes to the effort to prepare a survey with a reproducible bearing basis, they'll proudly note that on the survey. If it's an assumed bearing or a record bearing from who-knows-what, it won't change how I reconstruct their survey. I'll have to find two points, relate to those two points and find the remaining points. It's of no value to me to see "held record bearing" or "assumed bearings".
> It isn't all that difficult to construct a simple statement that explains how the bearings were derived. Maybe someday we will all work geodetically; basically going forward to the past.
>
amen
> I see bearing statements that go on and on even giving coordinates for the origin point of an LDP when there aren't any coordinates given on the entire survey. Kind of a waste of time. I see legislated basis of bearings rules getting way out of hand. And if there are monuments on the ground and bearings between them then it's not all that difficult to get onto what was done before.
>
Grid coordinates of the LDP origin would be useless, but Geodetic coordinates of the LDP origin will allow a surveyor to calculate convergence, which would be of great value, particularly on large with an East-West extent of more than a few thousand feet.
> I would rather leave it as it is and then you can tell if the surveyor knows what he's doing by what his statement says-often times it says nothing.
yep
Grid coordinates of the LDP origin would be useless, but Geodetic coordinates of the LDP origin will allow a surveyor to calculate convergence,
Yeah, I didn't say that very well, For an LPD giving the lat long of the origin point is very important, but giving out the X Y is nice, but, when there aren't any coordinates provide for the survey it's mostly pointless (I suppose you can call the other surveyor and get his coordinates....if you want to go to the trouble). You can assign any XY to the origin point and end up with the same bearings at the monuments the other guy did.
It is for Arkansas. The way that ours is set up, N zone, and S zone.
N
:good:
Uh, maybe we are talking different things. I see about 50" of convergence per mile, but that's at 45degrees latitude. I'm thinking at lower latitudes the convergence would be less.
If you are on a "state plane" bearing base, you are on grid bearing base parallel to "true" north being at that particular system's defined longitude. If you took a stellar observation, you might still be on a "grid" bearing base, parallel to north at the point where you took the observation.
I wonder if there might be simple terms to explain just where north is true on your 'plane' survey. Most of us, I think, use plane geometry for our work as opposed to spherical geometry.
I don't know...
Also to another point made in this thread. I agree that we shouldn't have to have minutiae legislated in statutes on things like how a basis of bearing statement should be written.
I think the correct terms are upper or lower culmination.
