..I assume True means magnetic...
As of February of 2019 there was NO place in Texas where magnetic = true (assuming the definition of "true" used here refers astronomic or geodetic)
See https://www.ngdc.noaa.gov/geomag/WMM/data/WMM2015/WMM2015v2_D_MERC.pdf
I could accept that question from a non surveyor, as I have a good idea where to start. But such a question from a surveyor with out full context, gives me no place from which to start.
Cranky Pants Paul in PA
Not the right guy to define the definition, between the two, but in dealing with earth’s “wobble”, the earths axis is defined as being the axis position on January 1, 1980.
https://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml
The NGS Glossary provides the following:
?ÿ
Its not a good term, but it means either geodetic or astronomical north, and for a land surveyor the difference doesnt matter unless you are in far north Greenland or the Artic Ocean, so it is harmless.
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
?ÿ
?ÿ
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...????
