In an airplane the compass reading can be several degrees off the magnetic heading of the airplane, this is known as deviation. It varies around the circle usually being adjusted to zero at magnetic north.
Old Surveyor's toast:
"May all your lines be Agonic!"
I'm guessing that the OP's question was inelegantly stated and what he wants to know is how much the convergence changes moving 200-300' east and west from grid to "true". This is dependent on latitude of course. Where I am near 45d off the top of my head I will guess 4 seconds, less in Texas, doesn't change along the equator, changes rapidly near the north pole.
Doesn't Texas use Lambert??ÿ
If so then there must be large convergence angles, there.?ÿ
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The only time grid north and "true north" are the same is on the central meridian.?ÿ As I recall you can figure it, I think its the Greek symbol called pheta(sp?).
meant to say 2" about 40" in a mile
true north usually refers to a direction parallel to the nearest road allowance ????ÿ
In PLSS States using the Manual, True North is Astronomic North by definition. No ambiguity at all...
In PLSS States using the Manual, True North is Astronomic North by definition. No ambiguity at all...
"The historical practice of using an astronomic meridian versus the current capability to obtain a geodetic value can create a potential ambiguity that must be resolved in certain cases." (emphasis added) See 2009 Manual section 2-27 page 33
I was simply stating the obvious, and pointing out the necessity of supplying "complete" meta-data when making reference to "True North" in the PLSS sense. While the Laplace correction may be trivial in many areas, it is often a NON-trivial consideration in the Mountain West.
Loyal?ÿ
BTW, section 2-3 states:
"The direction of each line is determined with reference to the true meridian as defined by the axis of the earth's rotation."
and;
"The value of the angular difference between the astronomic and geodetic direction, caused by the deflection of the vertical, relates the astronomic meridian to the geodetic meridian, as properly aligned with the axis of the earth's rotation."
That sounds a LOT like geodetic North to me.
Loyal
The ambiguity is not in the definition. The definition is clear and concise in the BLM glossary, which is the authoritative source of what terms in the manual mean.
The problem lies in 2-28 where the writers of the 09 manual utterly failed by not reading thier own glossary. Other than that they almist got it right.
Guess that makes it 'ambiguous', giving the w to Loyal...????
Agreed!!!
Hence my statement about meta-data, and why one should be very clear about which "True North" one is referring to.
An example of the difference from a point just east of Salt Lake City:
Output from DEFLEC12A
Station Name latitude longitude Xi Eta Hor_Lap
dd mm ss.sssss ddd mm ss.sssss arc-sec arc-sec arc-sec
USER LOCATION 40 39 20.00000 111 47 10.00000 -0.71 -39.58 33.99
That would be NON-trivial in my book.
Loyal
Mandrake, you never see a Commie use grid north, it's true north they use, they rarely use grid north and not without good reason. General Jack Ripper, Chief of the Geodetic Survey
We must not allow a state plane projection gap! General Turgidson. Chief of Plane Projections Branch