Geodesy help please

Geezer,

Why not just reference the grid bearing. It seems to me that if you rotated the Geodetic azimuth by the convergence angle your bearings are actually based on the grid.

John

Don't Worry, Every Geodetic Line Is Curved

Many more than not in a ess shaped horizontal curve. They are curved horizontally and vertically.

Assuming your observable area on the ellipsoid is actually a sphere does simplify the math somewhat.

What is with the *, the Character Map always works °, alt 0176 works in most text programs but kicks me out of the entry screen on this Board.

Paul in PA

Don't Worry, Every Geodetic Line Is Curved

I use alt + 248 °

Thanks, ALT 248 ° Works Here

Thanks Doug, now the trick is to rember it.

Paul in PA

Don't Worry, Every Geodetic Line Is Curved

Darn. Why cant it be alt 360 for degrees 😉

ALT 1/360 Would Be Better

But too logical.

Paul in PA

:good:

From what I remember about geodesy from the college class (so I'm not claiming to be a geodesist) the correct term would be forward bearing if you're referencing the geodetic bearing from the first 10" spike (point A) to the second 10" spike (point B). It's a back bearing from point B to point A. It truly is a perfectly straight line between points A and B, but only with respect to traveling along the surface of the ellipsoid. If you project that line onto a grid then it appears in the S shape you see in geodesy books, but it's not as if when driving in a car between the points you'd have to turn the steering wheel at all to travel directly between them, it's just the result of the projection process that gives it the curved appearance. The convergence of the meridians is the only reason there is even a bearing difference between the forward and back bearings, and that difference only manifests itself if there is an longitudinal difference between the points. If points A and B were perfectly north-south then the forward and back bearings would be identical, which obviously isn't the case here since there is over a minute of difference between the forward and back bearings.
The BLM describes using a mean geodetic bearing on cadastral survey lines. What that physically means is that if you could hold a 'geodetic compass' that referenced the geodetic north pole and were standing at point A (or point B and just flip the quadrants), you would set your compass bearing to the mean geodetic bearing and start walking a straight line. Since the meridians converge as soon as you start accruing longitudinal distance from your starting point, the geodetic compass needle would start to slowly spin and your bearing wouldn't be the same anymore unless you started to turn to counteract the spinning of the needle which is searching for geodetic north. You would have to keep pivoting slightly the entire distance to maintain that constant mean geodetic bearing reading, but as long as you did that you would end up exactly at the other point. So this mean geodetic bearing 'line' really is a curved line instead of a straight line unlike when using forward and back bearings. If you were standing at point A, you would set your geodetic compass to the forward bearing to point B, then head out on a perfectly straight line to point B and ignore the compass needle. As you travelled along this straight line, the bearing of the geodetic compass would slowly spin from the forward bearing at point A and eventually stop spinning at the back bearing once you reached point B. So this really is a straight line, not a curved line like when using mean geodetic bearing. What you are describing by your method sounds like a modified grid projection, which I like to use all the time, which means mean geodetic bearing (a truly curved line) wouldn't really be the appropriate term. I would scale all points by the combined scale factor for whatever projection you were using at a central point to take the distances from grid to ground, then rotate about a central point to get bearings to be on an essentially 'pseudo' geodetic bearing basis. If you start inversing between points you should see all ground distances be identical on your modified grid compared to your GPS processing software except for rounding here and there, but the bearings you'll see seconds of difference up to a minute or perhaps more. The more longitudinal distance your project has, the more discrepancy you'll see between your modified grid bearings and the actual geodetic bearings you're trying to replicate. This is just the result of using two different reference surfaces (grid and ellipsoid) and you can't really correct this discrepancy. A small enough project means you can call the modified grid bearings geodetic and that's essentially true since the difference is so small, but bigger projects that just isn't a close enough approximation to call them geodetic bearings and not have discrepancies start popping up. By what you describe, they really are grid bearings (though some might be very close to actual geodetic) and that's what you should make sure you state.

Paul

° °

First degree mark is alt248.
Second is alt0176

I didn't get kicked out. Makes me wonder what I am using different than what you are using.

If you are surveying in Custer, So. Dak., I am sure you have run onto some of my posts?

I resurveyed the township that Custer is in.

Keith

The "geodetic bearing" would be a curved-line bearing. I wouldn't use the word "true" because that might mean different things to different people. Might it be better to use a state-plane bearing base and call out the rotational value at the different points? What are you publishing? Are you showing coordinates?

I am not sure I can answer as well as a lot of poeple here. The term "true" is a little bothersome to me, is all.

Mark,

That was the word that was really getting to me. I don't publish any coords. I just have a map showing PLSS corners and a few intermediate or near by property corners.

I just want what I say to the "right" and reproducable.

thanx,
Geezer

Howdy,

The NGS Geodetic Glossary has this. Note the second to last sentence.

http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml

true - (adjective) (1) Actual, existing, or observed, as contrasted to ideal or theoretical.
(2) Corresponding to what would be predicted by a theory accounting for all systematic effects.
(3) Average for all possible cases.
(4) A term applied to distinguish among similar concepts, such as true anomaly from mean anomaly and eccentric anomaly.
In surveying and navigation, true is used to distinguish astronomical quantities from corresponding magnetic ones. The terms "true bearing", "true meridian", "true north", etc. occur frequently in reports of land surveys, distinguishing them from magnetic quantities. In descriptions of boundaries, the use of true has legal significance and, except in rare instances, refers to values based directly on astronomical observations. For terms containing true as a modifier, see under word modified, such as Equator, true; equinox, true; anomaly, true; value, true; and place, true.

Note that an astronomic and geodetic azimuth differ by the quantity reported by the DEFLEC program as Horizontal Laplace.

Cheers,

DMM

Paul

Alt167 also works for me. º

Why not just report the grid bearing between them and quit killing yourself?

You mean the roughly 700 pages of resurvey notes and 24 plats that go along with T3S R4E? You bet I've seen your posts out there! Seems like I reference those weekly sometimes and have sure handed out copies of various pages repeatedly. Yesterday I had somebody in retracing that survey. 1968-1972 or so if I remember right?