After asking the question here:
https://surveyorconnect.com/index.php?mode=thread&id=309307
I spent two days going back reading every thread I could find explaining what the LaPlace correction is and how to use it. Enjoyable reading, but when the tripod hits the dirt, I just wanted to know whether to add or subtract the thing from the Astronomic Azimuth.
The incomprehensible instructions in the DEFLEC12A README say:
>The computed Laplace correction (Hor.Laplace) should be ADDED to a
>clockwise astronomic azimuth, to obtain a "near-geodetic" Laplace azimuth.
> Note: the deflection correction is usually negligible, yielding a geodetic azimuth.
>Note: in many textbooks, the Laplace correction is shown with the opposite sign and is subtracted from astronomic azimuth.
The formula is:
Approx Grid AZ = (Astronomic AZ + LaPlace) - Convergence.
I decided to get it from one whose knowledge would be indisputable, so emailed Dan Martin. Here's what he says:
>In these cases, you always add and subtract algebraically....
>...So yes, you are correct that you want to ADD a NEGATIVE number.
>Dan
>Daniel J. Martin
>Vermont Geodetic Advisor
>NOAA/NOS/National Geodetic Survey
Bottom line: In the case of LaPlace, whether it's published positive or negative, you add the published number.
And in the case of Convergence, whether it's published positive or negative, you subtract the published number.
Careful with your convergence angle. The convention is that you add algebraically (add if positive, subtract if negative)- the same as the LaPlace correction. if you are going from grid to geodetic. If you go from geodetic to grid you must reverse the sign.
Laplace is where you arrive when you take a drive in Le Car
> Careful with your convergence angle. The convention is that you add algebraically (add if positive, subtract if negative)- the same as the LaPlace correction. if you are going from grid to geodetic. If you go from geodetic to grid you must reverse the sign.
You're technically correct. If Convergence is published positive, it becomes a negative; if it's negative, it becomes a positive, but it's this thinking that can cause confusion.
I therefore prefer to just use the formulas, and the data as published.
Grid = Geodetic - Convergence (positive or negative)
Geodetic = Grid + Convergence (positive or negative)
Geodetic AZ = Astronomic AZ + LaPlace (positive or negative)
If you're "going the other way", Convergence just moves to the other side of the formula.
Is predictive text doing all the capitalization?
Funny!
Unless I am east of the central meridian in a transverse Mercator projection...
I've operated successfully for 42 years not using it and not understanding what it is! I'm a Land Surveyor, not a Geodisist...
> Unless I am east of the central meridian in a transverse Mercator projection...
Not correct. East of the central meridian, convergence is published as a positive number. Subtract it and you're good to go.
South of the equator though, is another story. You do reverse the formula then. I've never looked at published numbers though, so they could already take that into account.:-)
:good:
I have shared the frustration with trying to understand the words on this and other things that can be much clearer as formulas.
Depending on whether you are going grid to geodetic or vice versa. 😉