Hello,
I am a novice surveyor learning the fundamentals behindĀ coordinate systems and have a questions regarding geodetic north.Ā I understand that true north is the geographic north, or the location of longitude convergence.Ā I think of this location as referenced to the physical earth with the prime meridian passing thru the Greenwich observatory.Ā However, with anĀ ellipsoid horizontal datum,Ā is the northĀ associated with the ellipsoid or does it remain associated with the earth?Ā So if I translate to a differentĀ ellipsoid datum, does the northĀ remain fixed on the physical earth or is its orientation impacted by the translation to a different datum ?
Replies
Obviously, geodetic North is defined by the geoid model you are using. NAD'83 was thought to be geocentric but advances in geodetic science has shown that there is a difference between datums that can be significant. I am reading that the difference between NAD83 (CORS96) & WGS84(G1150) can be up to 1 or 2 meters. We now have all sorts of reference frames, ITRS, ITRF, ITRF400, WGS84, & NAD83. NGS is currently getting ready to introduce new reference frames which will replace NAD 83 & NAVD 88. I imagine most of you are aware of that.
As Mr. Hammerstrom might say, you don't have to memorize all this stuff. It appears to be ever changing anyway. I just try to understand the theory & basic principles, keep abreast of the changes & keep my library of reference books up to date. Reading helps a lot as well.
I would also like to compliment Mr. Cavell. His articles are always informative.
The value and importance of all this, of course, is measured by the accuracy & precision with which our work can be retraced & destroyed monuments replaced, thereby better documenting our work.
HI Chuck,
First Kudos to MR . Cavell for pointing this out. True North,there is really no such thing,
if you ask NGS and NOAA, but it is used everywhere you look.Someone will ask or will want to knowĀ which one is real.They are all real and all True, you use the one you need
to use. But if there was one True North, it would be consistent with Geodetic North and run aloneĀ a Meridian that goes through the Geographic North Pole. So where you are
standing,Ā which way is North. It depends on what or how you are using it. Even though
True is being used,it is not true to use it. Did I say that right or true.
Now everywhere you look ,there it is. It is often used to compare with other Norths.
I even catch myself using it ,not to confuse someone even more. They will ask,
What do you mean there is no True North,sure there is I am looking at it.
You will even find it in a Encyclopedia , they will say, see there it is.
Astronomic North is only a few seconds from Geodetic North,no big deal
where others different by Degrees and minutes. As to your question,
which one to use,the one you are using. You can not change the north in your
Data that you are using. The North for it must match it.
You will find it on Plats,Plans,Maps and all kinds of Documents.
I have got to a point to use True in front of all of them.
True (Geodetic) North,to True Astronomic North to True Magnetic North.
All Grids have their on Grid North for that Grid ,a direction of a Grid line,
which is parallel to the central meridian of a Grid projection.
Project,Plan, plant or assume North whatever you use is that .
It is assume. Professionals never use the term True North
in a professional way. But when you say geodetic,Astronomic,magnetic
or Grid and even Plan, Project or Plant then assume. Someone will ask which or where
is the one that is True North. I answer the one you are using.
This is a 9 mb File i can not load it.
Astronomic North - National Geodetic Survey
Ā
I agree,
Ā Ask what north they want?
Mr. Turlington, Whenever possible, avoid the term "True North". ALL the (legitimate) versions of north areĀ true, in that they are accurate.Here are some examples.
- AstronomicĀ North -Ā the direction toward the center of rotation of the celestial sphere in the northernĀ hemisphere..
- Geodetic North -Ā the direction of the meridian line through the point in question. Meridian lines converge as they near the poles.
- Grid North -Ā when a portion of the earth's surface if projected onto a plane oneĀ meridian line is chosen to represent north and the other cardinal lines on the grid are parallel or perpendicular to it.
- Project North -Ā an arbitrary choice for a basis of bearing on a construction plan often chosen for convenience rather than actual northerly direction. Consider a standard design for a fast-food restaurant. Project north may be assigned parallel to the left wall of the building. No matter where the restaurant is constructed, fixing the project north this way allows the plans to be used with almost no changes.
When someone claims to know or want "True North" be sure to get clarification before proceeding so that the true north you give them will be the true north they expect!
JAC
Ok, thanks for that response.Ā Regarding Geodetic North being parallel to direction ofĀ meridian:Ā would I think of Geodetic NorthĀ as referring toĀ meridians on theĀ ellipsoid or referring to meridians on the physical earth?
For this purpose the meridian on the ellipsoid best represent the meridian on the physical earth.
We are used to thinking about plane projections. It turns out that our position (lat, lon) is determined by projecting the physical position on to the ellipsoid.So the geodetic north is along a meridian of the ellipsoid. When the meridian is projected (in reverse) from the ellipsoid to the ground, if you follow it you will be heading north geodetically.
JAC
Oops - oversight on my part.
Totally overlooked Magnetic North.
Its purpose and origin should be easily found.
JAC
true north in relation to magnetic north and grid north is different from each other. magnetic north is where a compass reading is taken base on the magnetic north, you should apply the magnetic declination which vary from time to time. grid north is sometime a reference for a a map generation. Grid north at the central meridian is a straight line toward north or south. the angle of convergence applied when the a certain point of the map is significantly far Ā east or west from for the central meridian.Ā