In Washington State the surveyor is required to state the basis of bearing being used. I am looking for; as simple as possible statement, when North (geodetic) has been established by GPS method.
What Is North ?
You must state various parts of how you established North including which North.
Did you try to do it by a single observation, multiple points with a single GPS unit, multiple points with multiple receivers, RTK, Static, Fast Static, Stop and go, using OPUS-S or OPUS-RS?
Then define your coordinate system.
Tell us what you do do and we will help you put it in words.
Paul in PA
What Is North ?
OPUS-RS at two points on the internal the traverse. OPUS-RS on the 1/16th controlling corners. Checked by calculating previous surveys that tied both the exterior controlling corners and interior corners tied with traverse.
What Is North ?
So you have a previous survey with a record bearing? Why not get on that record bearing?
What Is North ?
You probably are using GEODETIC north, and if you are, that would be true at the base.
Thus, you could have a BOB like this:
Basis of Bearings, Geodetic north, based on GPS, and taken at _____Lat _____Lon
Or, you can do what I do, and that is to calc the convergency angle at the base, (Theta) then, you are STATE PLANE GRID, and you would have:
BOB Arkansas State Plane Grid bearings, via GPS.
This is nice, because essentially all your surveys are on the same BOB, even if your base is moved around.
N
PS, Deflection of the vertical does not introduce errors OF SUBSTANCE for my area.
N
What Is North ?
I do have a number of previous surveys within the quarter, none of which actually tied all four corners on the same survey. As luck would have it they all calculated at least one of the controlling corners by accepting just bad measurements of yet others. As it turns out my basis does match one prior survey that used a solar observation on one of the controlling legs (section corner to 1/4 corner). However the county I will record in requires tie a between the basis used and State. I thought just using geodetic basis at a lat/long near to center of the survey would be best.
What Is North ?
Sounds like you are on a grid.
GPS is really irrelevant to the basis of bearings.
If you want to say something about GPS I suppose you could.
But it would be better to state that you are using grid bearings-it probably is a tranverse mercator (maybe lambert) projection that the GPS software created.
Just state that the basis of bearings are grid based on a tranverse mercator projection with an origin lat of____ and origin long of____NAD83. Distances are surface.
That is probably what the GPS software is doing.
But without seeing what's going on I can't be sure.
From A Recent Project
DRAWING AND LINEWORK ROTATED TO REFLECT THE NEW JERSEY STATE PLANE COORDINATE SYSTEM NAD83 (US SURVEY FEET), ELEVATIONS ARE BASED ON NAVD88, OBTAINED BY STATIC GPS OBSERVATIONS ON AUGUST 1, 2012 PER OPUS SOLUTIONS REFERENCED TO "NJWC" CORS, "PAMS" CORS & "NJSC" CORS. COMBINED SCALE FACTOR = 0.99989812, POINT 393 HELD FIXED.
Paul in PA
What Is North ?
Unless you are using geodetic coordinates (lat & Long) then your bearings are most likely in grid. Grid angles are consistent throughout conformal projections such as all of the systems normally used in state and local coordinate systems. Geodetic north converges and thus is different from one place to the next. This makes them a little hard to map with.
As for the basis of bearings on a record of survey, I always try to pick to monuments of record to as a reference line and then state which horizontal datum, epoch and coordinate system the bearing is based on. When I do a tower survey that requires a geodetic bearing, I usually state the convergence angle at the center of the tower. Just remember that that is the only place it is good for.
John
We publish State Plane Coordinates on our surveys and so our metadata is particular to our purposes. Lately I've been using custom projections. My latest incarnation of bearing metadata:
"Bearings related to a local grid having an origin of N Lat: and W Long: To convert reported bearings to bearings related to Grid North for the Texas Coordinate System of 1983, North Central Zone, rotate reported bearings clockwise by the mapping angle of "
From A Recent Project
what is the purpose of listing the particular CORS used and the date of the survey? I will get the same azimuth whether I use any particular set of CORS or even don't use any CORS (i.e. a baseline not connected to control). The rotation between ITRF and NAD83 is much less than 1".
Just saying NJ SPC zone is sufficient.
The fact is that there is practically no azimuth bias between different flavors of NAD83 (typically 1" or less), and even between NAD27 and NAD83 it is usually less than a couple of seconds, which is probably less than the accuracy of the particular survey. Unless, of course, the central meridian is changed from NAD27 to NAD83.
From A Recent Project
I will add the CORS, HARN, other control info (including epoch) if coordinates are given. Then it allows a surveyor to "get on" your system at a later date. In this case there should be site control with Lat, Lng, Hgt, N, E, El somewhere included in the survey information. Also need the Geoid model info.
If coordinates are not given then I don't, simply stating that it's based on the name your state coordinate system, NAD83, xxxx zone is enough for the bearings.
A Monument Reference Is Required.
In the past it would have been the NGS reference point that was occupied. Since we did not occupy the CORS stations, the CORS name is the reference provided.
If it were an OPUS-RS solution that was used, I would only reference the 3 closest. On some OPUS-RS projects because of observation times and CORS data quality you may not get the same 9, usually in that case you match 8 of 9, but I have seen only 7/9.
Paul in PA
From A Recent Project
> what is the purpose of listing the particular CORS used and the date of the survey? I will get the same azimuth whether I use any particular set of CORS or even don't use any CORS (i.e. a baseline not connected to control). The rotation between ITRF and NAD83 is much less than 1".
Yes, citing CORS sites doesn't give you anything. The information that you really want is the uncertainty in bearings reported.
I would suggest reading this document, specifically section 5.
Very good suggestions, in my opinion.
Page 69 of 78 for a quick guide to documentation.
A Monument Reference Is Required.
I would also reference the geoid and epoch used. Thus is purely info for future genrations especially fro vertical reference here. I don't want someone guessing what went on decades from now.
Sort of like adding the declination condions on an old plat from yesteryear.
Great document & information. Thanks Kevin
