Hi, does anyone know if Grid North is the same as True North in a short distance, say 200 or 300'?
If there is a difference, can anyone tell me how to get mark a True North point from a known position?
Thanks,
Mike
There are items to consider...I assume True means magnetic...How far are you from the Central Meridian of the grid zone? The further E/W from the meridian the more distortion. I would expect higher latitudes to affect your results as well. You can check the Lat/Long of your location for declination to see how much "potential" for variation you could expect. But in 200-300 foot, probably wouldn't see a huge difference.?ÿ
True North is the direction to the earth's rotational pole.?ÿ Grid north is the direction you get on a flat map that has distorted a portion of the earth into a plane. Magnetic north is where a compass points and in most places is considerably different from either true or some map's grid.
The difference between True and Grid north depends on where you are in a zone of State Plane, UTM, or other mapping projection.
The difference can be calculated by many software tools.?ÿ You can go to www.ngs.noaa.gov and look for their tool kit or use Corpscon if you don't already have a surveying software that does it.
Once you find the difference for a point, you can assume it is constant for a small project, but the difference may be significant for very careful work in only a fraction of a mile.?ÿ
I see you are in Texas, so will have less issue with the change than someone at a higher latitude.
Can anyone tell me the difference between mikesurveying12 and a true surveyor?
How do we get from mikesurveying12 to a true surveyor?
Paul in PA
The angle between grid north and geodetic (true) north is not a constant value, it varies from place to place.
As Bill pointed out, 'True North' refers to the rotational pole or 'Geodetic North' and is very close to 'Astronomic North'.?ÿ The differnce between True & Grid is the convergence angle, sometimes called the mapping angle.?ÿ The convergence angle is 0 only along the central meridian of a given projection.?ÿ 'Grid North' plus the convergence angle equals?ÿ 'True North'.?ÿ The convergence angle should be provided on the control sheet, if not then it can be obtained by the methods Bill refferenced.
"True North" is not a defined a scientific term, but usually when people use it, as they often do, they are referring to Geodetic North. Geodetic North differs from Astronomic North by a factor so small the distinction is often ignored. Geodetic North is the direction to the geographic North Pole - which is very nearly, but not exactly, the earth's rotational axis.?ÿ ?ÿ
Magnetic North refers to the magnetic pole and in my area differs from geodetic by 15?ø. And the difference is constantly changing over time. In other parts of the country it is closer to a match. But never suitable for precise surveying.
The direction of grid north relative to geodetic north is dependent on where you are on the projection grid you are using. The difference between the two is called the convergence angle (or mapping angle) , it may be positive or negative, and can be up to +/-2?ø in a state plane projection.?ÿ ?ÿ
SOooooo...grid north and true north are not the same except in very specific places on the projection grid where the convergence angle happens to be zero. The length of the line is not a factor.?ÿ?ÿ
Can anyone tell me the difference between mikesurveying12 and a true surveyor?
How do we get from mikesurveying12 to a true surveyor?
Does he have to be a surveyor to ask questions here, Mr. Crankypants??ÿ
Geodetic North is the direction to the geographic North Pole - which is very nearly, but not exactly, the earth's rotational axis.
For splitting hairs, what is the difference??ÿ I know the rotational axis wanders a bit, so does the earth model used to define a geodetic datum have a defined geographic pole tied to something else?
OK, I'll ask the critical question:
What is the context for this inquiry?
?ÿ
(...am I the only one that despises the term "true north"?)
(...am I the only one that despises the term "true north"?)
I'm not sure that I "despise" it, but it IS somewhat ambiguous, which is NOT a good thing.
If used on a PLAT, it should be explicitly defined (eg. Astronomic, Geodetic, etc.), as well as the source of the value EXPLAINED (Solar, Polaris, GPS, Datum, DEFLECxx, etc.)?ÿ
Loyal
And of course which point/line on the plat it was determined for.
And of course which point/line on the plat it was determined for.
Unless we are talking about a "BLM True Bearing, True Distance" PLAT, in which EACH line (a loxodrome unless DUE North/South) is a function of the unique Lat/Lon of each end.
?ÿLoyal
..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.
