California Surveyors Forums

I've entered measured Lat / Long data of two stations into the Inverse program from the NGS website.

The program gives me a FAZ result of 103d 00' 29.75" but a BAZ of 283d 01' 19.18". This doesn't seem to add up.

When I enter the lat / long into me data logger, and do an inverse, the FAZ is 103d 27' 49" and BAZ is 283d27'49"

Which set is correct ? I've never seen these conflicting results before.
All data entries have been checked and verified ?

"Bad" GPS ???

Your D/C is planar and the NGS is spherical?

NGS is giving you geodetic azimuth, which will be different from depending upon where you are on the line. That's because our common concept of azimuth is on a plane, and when placed on the surface of the earth (or the spheroid model) can't hold a constant relationship to North.

The only time that your geodetic azimuth will be exactly 180 degrees different fwd vs back, is when you are on a North-South line with an azimuth of 000-00-00.0.

Your data collector program is working on a plane, not on the spheroid, so it keeps the calcs simple.

not a flat earth?

http://www.ngs.noaa.gov/CORS-Proxy/Glos ... ossary.xml

azimuth, geodetic - The angle at a point A between the tangent to the meridian at A and the tangent to the geodesic from A to the point B whose geodetic azimuth is wanted.
Until 1985 the U.S. National Geodetic Survey had considered a geodetic azimuth to be positive clockwise starting from south. See azimuth . The azimuth is called the "forward azimuth" for the line AB. The angle at B between the tangents to the meridian and to the geodesic is called the "back azimuth" or "reverse azimuth"for the line AB. Because of the convergence of the meridians, the forward and back azimuths of a line do not differ by exactly 180o, except where the two end points have the same geodetic longitude or where the geodetic latitudes of both points are 0o.

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Ian: you said it best but the "?" threw me.
more is better, less is more better

Sorry, Peter, I was trying to convey some of my sarcasm in the post.

I suppose that I could have popped in the French "nez pas?".

Yep, I soon realized that the earth in most peoples world is round.

The problem with being "sharp as a tack" is that you may be also just as "flat headed."

Who is Nez Pas ? and is he related to Fox Pas ?