Calculating Convergence based on Google Earth Lat/Long

Near San Francisco I can locate a fence corner with GPS & OPUS on current datum, and its generally within 10' of the position shown on Google Earth.

A convergence angle calc based on a google earth position should be close enough for your purposes. Running a test on google earth positions of the front lot line of my suburban house lot and the back lot line (110') yields a difference in convergence angles of under 2 seconds. Google earth positions can be worse than that, but are usually much better. Your error would probably be under 1 second. Good enough for ya? Using the lat/long from your smart phone would likely be just a little better in your case by eliminating the problem of identifying your position on the ground in GE in a rural area with few landmarks.

> A convergence angle calc based on a google earth position should be close enough for your purposes. Running a test on google earth positions of the front lot line of my suburban house lot and the back lot line (110') yields a difference in convergence angles of under 2 seconds. Google earth positions can be worse than that, but are usually much better. Your error would probably be under 1 second. Good enough for ya? Using the lat/long from your smart phone would likely be just a little better in your case by eliminating the problem of identifying your position on the ground in GE in a rural area with few landmarks.

Greetings Norman. Hadn't even thought of confirming the coordinates with my iPhone. Duh. I checked it where I am now (indoors), and it's less than 30" from where Google Earth says I am (in Lat/Long)...That translates to about 20" in convergence. I think if I start with the Google Earth number (I can see where I'll be to within a few feet), record the location with my phone multiple times for the hour or so I'm on station, and mean the works, I'll be good to go. Thanks.

> Greetings Norman. Hadn't even thought of confirming the coordinates with my iPhone. Duh. I checked it where I am now (indoors), and it's less than 30" from where Google Earth says I am (in Lat/Long)..
I think you might get a much better position from your iphone if you step outside.

> I think you might get a much better position from your iphone if you step outside.

Ya, I get that, but...uh...no thanks. Perhaps another time. +o(

When computing a plane azimuth from an astronomic be sure to apply the Laplace correction as well as the convergence angle. There is, of course, a tool on the NGS site to calculate the Horizontal Laplace. Go to: http://www.ngs.noaa.gov/cgi-bin/GEOID_STUFF/deflec12A_prompt.prl

I know it's only 4", but the NGS language makes me crazy!

> When computing a plane azimuth from an astronomic be sure to apply the Laplace correction...
I know to do that, but every time I do, it absolutely boggles my mind to absorb the language from the Deflec12 Readme (Kent posted this last fall), and I understood it no more now, than then:

[pre]
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.
[/pre]

WTF? Not just "in many textbooks", but in the Deflec12a results itself.

I'm west of the Central Meridian, therefore Convergence is negative.
I know:
Geodetic AZ = Astronomic AZ + LaPlace
and,
Approx Grid AZ= Geodetic AZ - Convergence.

Therefore:
Approx Grid AZ = (Astronomic AZ + LaPlace) - Convergence.

I know that if Convergence is negative, subtracting a negative is a positive (and is therefore added).

But what about LaPlace? If it's negative (it is), and is ADDED to Astro AZ, Then Geodetic AZ is less than Astro AZ, correct?

If the Astro AZ to the Back Sight is, say, N80E, then:
The Geodetic AZ is N79-59-56.1E, and
The Approx. Grid AZ is: N80-03-21.2E