Carlson Survey 2013 - Geodetic Inverse

Not being a Carlson user I can't answer your direct question.

Here's what I have to say in general though;

What you are looking for is not really a “geodetic inverse” per se (I mean it is and it isn't). Strictly speaking, a “geodetic inverse” would be what you get from the NGS Tool Kit programs “INVERSE or INVERSE3d.” These programs will return MOST of what you need, but you will still have to do some additional (simple) calculations. Also bear in mind that a “geodetic inverse” would normally return a geodesic (on the ellipsoid), NOT a loxodrome (rhumb line) at the mean height of the line.

The Horizontal Laplace may factor in as well (or not...it depends).

I have a program that I use (and regularly tweak), that will convert from BLM “true bearing & distance” to any georeferenced projection grid (and vice versa), BUT it does require a reasonable estimate of the Ellipsoid Height of EACH (and every) CORNER [point].

My recent experience with GPS derived BLM Plats is very encouraging. I can run around half a dozen Sections (or more) and “close” within a centimeter or two, and when I get out in the field, “HIT” all of the BLM Monuments within a couple of centimeters or better. Mileage on OLDER BLM Plats will of course vary a lot!

Loyal

If you are already set up in SPC,

it's COGO
INVERSE
O (for options)

check the boxes you need.

> If you are already set up in SPC,
>
> it's COGO
> INVERSE
> O (for options)
>
> check the boxes you need.

Yup

RFB

I'm curious...what are the "options?"

Loyal

Options

🙂

"True mean bearing" certainly cannot be a geodesic inverse because a geodesic enforces Clairaut's Theorem in that "the rate of change of azimuth along a geodesic is at a constant." It is the shortest distance between two points on the surface of the ellipsoid in this application.

If you average the starting azimuth and the ending azimuth I suppose you can call that the "mean," whatever that is ...

A loxodrome has a constant bearing along the entire line but is not the shortest distance between two points on the surface of the ellipsoid in this application.

Cliff

I agree...

If that isn't what I said, it's what I meant to say (more or less).

🙂

Loyal

Options

Hmmmmmm...

> A loxodrome...

A race for smoked salmon?

LOX

From Yiddish ????? (laks, “salmon”), from Old High German lahs. Cognate to Icelandic lax, German Lachs.

Noun
1.smoked salmon

DROME

From Ancient Greek ?????? (dromos, “a course, race course, road”).

Suffix
1.Used to form words whose original means related to a course, such as a racecourse.

> If you are already set up in SPC,
>
> it's COGO
> INVERSE
> O (for options)
>
> check the boxes you need.

That works!!!

Thanks,

> "True mean bearing" certainly cannot be a geodesic inverse because a geodesic enforces Clairaut's Theorem in that "the rate of change of azimuth along a geodesic is at a constant." It is the shortest distance between two points on the surface of the ellipsoid in this application.
>
> If you average the starting azimuth and the ending azimuth I suppose you can call that the "mean," whatever that is ...
>
> A loxodrome has a constant bearing along the entire line but is not the shortest distance between two points on the surface of the ellipsoid in this application.

Is the mean bearing output by geodetic inverse programs equivalent to the loxodrome bearing?

That would be impossible as an ellipsoidal loxodrome is a straight line only in isometric latitude space which is a Normal Mercator projection.

I've found with geodetic inverses to be VERY close to just using the theta/gamma angle. The only difference is typically a laplace correction, which in my area is like a second or two.

CFU
Coordinate Transformation
select your point
it spits out the theta and GF for that point.

> That would be impossible as an ellipsoidal loxodrome is a straight line only in isometric latitude space which is a Normal Mercator projection.

I think I may have slept through too many of my Geodesy classes...:-D

Now you're sounding like one of my students.:-)

Computing geodetic azimuths can be fairly easy if one uses the global spatial data model (GSDM).

The geodetic azimuth (forward) from "here" to "there" is arctan (delta east over delta north).

The geodetic azimuth (reverse) from "there" to "here" is also arctan (delta east over delta north).

The difference is that delta east and delta north from "here" to "there" are slightly different than delta east and delta north from "there" to "here" (because meridians are not parallel).

As I understand it, true mean bearing is the average of the two. Although it may be very good, true mean bearing is an approximation.

For computational details see http://www.globalcogo.com/3DGPSAZ.pdf

If you have any questions, email me at the address found on my website.