Hi, hoping to get answer to this topic. I have this survey plan (see link below) left top corner says "To convert Grid bearings to local astronomic bearings referred to the Meridian through Fd CP (21) add 0 41 19". If some one could explain this to me and how to achieve this. Thank you so much!
How old is the plan, I did not look at the date if it was shown but there is no easy way to do this without doing your own celestial observations, doint the rotation and publishing the date of your observations.
The plan is dated 2021.
I'm noting that the surveyors name is Richard Nixon. That's quite a cross to bear through life.
- If you want the most correct astronomic bearing be sure to also apply the LaPlace correction using the NGS tool DEFLECT18.
"To convert Grid bearings to local astronomic bearings referred to the Meridian through Fd CP (21) add 0°41'19"
A meridian is a line of longitude passing through a point on its way to the geodetic north pole.
A grid bearing at any point will be relative a meridian passing through the origin point of the projection system. If you are on a point which is the same longitude as the origin of the projection system the difference between astronomic bearing and grid bearing will be zero. But as you move away from that grid origin longitude - travelling east or west - the difference between a grid bearing and astronomic bearing steadily grows. Some call this difference the "convergence angle". There are other names.
In theory every point on the survey has a unique convergence angle, each just a tiny bit different from its neighbor. This is a PITA to deal with. In practice, for project level efforts like the subject survey, the surveyor selects a good average convergence angle for the site and uses it for all the points. In this case the surveyor selected the convergence angle at the found control point number 21, which was +0°41'19", to use.
So to convert the grid azimuths shown on the survey to astronomic azimuths, simply add +0°41'19. This simplified approach drives people who are accustomed to dealing with global scale surveys a little squirrely. I've used "geodetic bearing" as a synonym for "astronomic bearing" here, but there is a small difference (the "LaPlace correction") that would become significant if you were surveying all of Canada. It has to do with a plumb line not quite pointing at the center of the earth due to local variations of gravity. You can look that up. For a local site like this one you can ignore it.
145d47'56"+0d41'19"=146d29'15"
Thank you for the input you all. I will do some more research on this.
Uhmmmm this particular survey has made it easy. But to go from grid to astronomical statements are not truly correct. Grid to geodetic is the convergence angle this is a NAD83 so that takes you from grid to geodetic the. Apply t-T to get to astronomical. I know I know it’s minimum difference and I will have to agree the Laplace correction is the best possible solution but may not be necessary on certain projects. But then again if an error or correction can be accounted for why not just do it right the first time. We have way to many other sources of errors we introduce so why not just eliminate this because we can.
. I know I know it’s minimum difference .... why not just do it right the first time.
Because I believe that the OP is not a geodesist or even a surveyor. He is a property owner interested in where to put the fence, not splitting atoms.
The bigger question I have is why reference astronomic north in the first place. I mean, how many, unless you are building an observatory, how many people utilize astronomic north over geodetic north? You need to be doing some serious retracing of the footsteps if you have to break out the solar compass for modern day work.
I do reference the convergence angle on tower design surveys and then layout geodetic north prior to construction but that is a special needs case.
The bigger question I have is why reference astronomic north in the first place.
Years ago 1980's we had to establish astronomic north (using a Polaris shot) for a BofB to layout 5 big radio towers. According to the site engineer, the positions of the towers in a particular pattern, based on astronomic north, would aim the radio transmission in a particular general direction to reduce the jamming or interfering with the radio transmissions of an adjacent radio station.
Edit: Memories of old jobs come back pretty slow these days. The engineer said that the reason for astronomic north based on Polaris (not solar) was a requirement for all tower layouts to try to keep the alignments on the same basis over a large area.
The bigger question I have is why reference astronomic north in the first place.
It may be a statutory requirement.
@norman-oklahoma I know. I was just being a little honorary is all.