I have been asked to display some traverse work that I completed in SPC (2112) to bearings referenced to Geodetic North.
I have read some tidbits on SPC at the link below and am having trouble getting Grid Azimuth and Projected Grid Azimuth related to Geodetic Azimuth through my thick skull.
http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf
Page 18 dangled a carrot, but I'm not quite getting it.
I only need to get an idea for the correction to get me to the nearest degree, but it would be helpful if I could get to within 30 minutes of angle to a Geodetic North reference.
In other words, I don't need to be exact, just closer than raw SPC bearings for illustrative purposes.
An example of my OPUS convergence angle is "-1.25291287" for one of my adjustment reports in this project area.
Thanks
The simple equations are:
Grid Azimuth = Geodetic Azimuth - Convergence
Geodetic Azimuth = Grid Azimuth + Convergence
Note:
Equations assume that the arc to chord (t-T) correction is negligible (which it is on short lines).
Be aware that the Geodetic Azimuth with be different on each end of the line. Take the mean of both solutions and that will yield the mean geodetic bearing.
Learn By Doing
Download an SPC utility for your State and Zone.
Starting in the middle enter Lat and Lon, note SPC and the divergence.
Enter Lat and Lon for a point to the North etc.
Go to the limits of your state zone and redo.
You will soon get a feel for the proper corrections.
Then near the limits of your state zone enter the SPC for a point you had entered Lat and Lon for previously.
Then enter SPC for a point due N-S or E-W on the SPC grid, note Lat and Lon and convergence.
Paul in PA
> The simple equations are:
>
> Grid Azimuth = Geodetic Azimuth - Convergence
>
> Geodetic Azimuth = Grid Azimuth + Convergence
>
> Note:
> Equations assume that the arc to chord (t-T) correction is negligible (which it is on short lines).
>
> Be aware that the Geodetic Azimuth with be different on each end of the line. Take the mean of both solutions and that will yield the mean geodetic bearing.
Thank you for your input and help on my query.
At what distance would that become a factor on pancake terrain?
Some information I did not include in the original post was that my traverse was based on two separate pairs of adjusted OPUS-RS points.
Basically I was running North and South, from two pairs of positions on the opposite side of a lake.
The BS on the South side was 1/2 mile between two OPUS-RS points and I traversed 1/4 mile to the South shore of the lake, where I left my target prior to closing in from the North.
The BS on the North side was about 1/4 mile between the OPUS points and I traversed about 1/4 mile South to the North shore (40' of elev over 800 feet) where I shot across to check in.
My check-in from the North was .11', half in the Easting and half in the Northing.
I was surprised and very happy to see that small of a misclosure using his method.
geodetic v plane azimuths
FWIW,
The difference between a grid and geodetic azimuth are due to the fact that a grid azimuth is computed on the basis of a projected coordinate system. We take a spherical Earth and generate a rectangular system that allows easy computations using plane trigonometry.
In the design of a plane coordinate system like the State Plane System a Central Meridian (CM) is specified. At this longitude a grid azimuth and geodetic azimuth are equal. Everywhere else a correction angle (convergence angle) must be computed.
As a state plane coordinate system is by definition a plane, height differences must be dealt with for distance reductions. Azimuths do not change as a function of height differences.
You might want to look at some internet sources: Peter Dana or Charles Ghilani pages on state plane coordinates. I also created some materials at http://geodesyattamucc.pbworks.com/w/file/13931132/Class23_SPCS_FancyPlaneSurveying.ppt --- or --- http://geodesyattamucc.pbworks.com/w/file/63366627/Lec5_2013_SPCS_lecture.pdf In the latter presentation see slides 37 and 39.
As stated in other responses, geodetic azimuths are computed on the ellipse not a plane making the difference between line (A to B) and line (B to A) NOT 180 degrees except on the CM. While astronomical azimuths are no longer frequently used they differ from geodetic azimuths; transforming one to the other requires knowledge of the laplace correction.
HTH,
DMM
