What is True North?

[Comach]

The True North that we all use on our layout drawings - exactly what North is it?

There seems to be a number of variations - Astronomic North, Geodetic North and Magnetic North.

Most surveyors drawings that I have recently seen refer to Astronomic North - which is described in some literature as "True North" although in others it defines Geodetic North as being "True North" - apparently a correction value (called Laplace) is applied to the Astronomic North to define Geodetic North.

This is terribly confusing and I wondered if anyone can cast a light on this subject and clarify the definition of True North as we would normally use on our cad drawings and that used by our site construction teams?

[Brian Myers]

I agree with the Google definition:

http://www.google.com/search?q=define%3Atrue+north&btnG=Search&hl=en&lr=&rls=GGLG%2CGGLG%3A2006-14%2CGGLG%3Aen

A good article is here:
http://www.pobonline.com/CDA/Archives/c194817cac0f6010VgnVCM100000f932a8c0____

Ultimately North must be defined by points on the globe and nothing else. The reason is that over VERY LONG periods of time even the stars in the sky (hundreds of thousands of years in some cases) will seem to have moved when view from the Earth's surface. Since sailing by the stars is rather new in such astronomical terms that's never been an issue. The "magnetic north" actually wobbles slightly so to be honest it's never a 100% perfectly accurate measure, especially once you get close to that point. Plus (and here's the mind blower) every "X" number of years (which is again a VERY, VERY Long time) the magnetic Poles flip!!! Yep, South and North have flipped several times in the history of the Earth.

Sooo... you really need to pick a point on the map and say "this is North" and use it as a reference.

[Comach]

Ultimately North must be defined by points on the globe and nothing else.
Thank you Brian,

I would agree with this statement and as mentioned on your google link, this would define True North as being a true geographic north of the globe - makes sense!.

I understand from National Geographic that we are long overdue for a flip in the magnetic poles!

Getting back to the issue in hand - the more tangible terrestrial geographic North is perhaps the logical choice - so why do land surveyors define the bearings from an Astronomic North?

[Brian Myers]

so why do land surveyors define the bearings from an Astronomic North?

Tradition. Did you know that the former American President Abraham Lincoln used to do survey work as a young man? This was right before he started to study law and get into politics about 150 years ago. In other words, its a very old profession and while things may have changed a lot over the years in how surveys were done, its still handled in terminology in much the same way. Astronomic North is pretty close to North and its likely that no matter where that old surveyor would go, regardless of the map and what standardizations would have (or would not have) taken place, that North was still going to be North by the stars.

I compair it to Architects designing wood-frame contruction. Here in the states we use boards that we call 2x4, 2x8, etc because of their size. This was based on the fact that these used to be the actual sizes of these boards. Today with the new milling processes we still call a 2x4 that even if the dimension is now 1-1/2 x 3-1/2 and a 2x8 is actually 1-1/2 x 7-1/4. Its simply the tradtion of the terminology.

[Comach]

Astronomic North is pretty close to North
I guess it is the pretty close bit that concerns me. This will probably still require some further research to completely satisfy my curiosity with this one.

Brian - Thanks for taking the time to reply on this issue your input was very informative.

[Brian Myers]

I guess it is the pretty close bit that concerns me. This will probably still require some further research to completely satisfy my curiosity with this one.

Brian - Thanks for taking the time to reply on this issue your input was very informative.

May I ask, what concerns you? It would seem that regardless of which one they used they would still be plotting the boundary the same, just in perhaps slightly different terminology. Is this question leading to "What is the EXACT difference between the two" so you know how to adjust for that? That's an interesting question...

[robert.1.hall72202]

Upside down or right side up?

Is there a standard here?

Or,

Do companies tend to use one term more than the other to the extent that
it "just" gets adopted?

[Comach]

Is this question leading to "What is the EXACT difference between the two" so you know how to adjust for that? That's an interesting question...
Plainly speaking that is a good assessment - it bothers me that there is a marginal difference and over a 300 meter site that can relate to a substantial measurable difference - if indeed it is a difference of measurable magnitude - I agree therefore we should at least determine what the actual difference is..

Having followed up with some additional research and informal corridor discussions with colleagues it is apparent that the general understanding from the design perspective is that True North on the cad drawings would relate to the actual earthly orientation.

So if the design team think on earthly terms and the surveyors think on astronomical terms - (theres got to be a potential dig at surveyors there!) - then it is possible for a margin of error to exist between design intent and actual build!

Robert - you are probably right that this is something that "just"gets adopted, an assumed unwritten perception of something that has always been there!

[blads]

FWIW I've always taken a more simplistict approach to the definition of True North. It being - The geographic north of the globe and represented on the Lat/Long grid by the lines of longitude or meridians.

Wheras Magnetic North is the direction that a magnetic compass will point. Can be 10°-15° different than True North.

[Augi Doggie]

FWIW I've always taken a more simplistict approach to the definition of True North. It being - The geographic north of the globe and represented on the Lat/Long grid by the lines of longitude or meridians.

Wheras Magnetic North is the direction that a magnetic compass will point. Can be 10°-15° different than True North.
Not only is Magnetic North different from True North by varying amounts depending on where you are located on the globe, but it also changes slowly.

In my area, I believe it shifts about 0 degrees 7' towards the East each year.

[jaberwok]

Not only is Magnetic North different from True North by varying amounts depending on where you are located on the globe, but it also changes slowly.

In my area, I believe it shifts about 0 degrees 7' towards the East each year.

And every now and then it reverses -



.... The last reversal happened about 780,000 years ago, over a period of several thousand years. Now Gauthier Hulot from the Institute of Earth Sciences in Paris and his colleagues think they have spotted early signs of another reversal....

Oh dear.

[Brian Myers]

And every now and then it reverses -
Oh dear.

That's true, but life on Earth has seen many of these and its not been effected by it. That being said, with the GPS units of today/tomorrow the compass will soon be obsolete in any practical application, so no harm done. :roll:

[Augi Doggie]

That's true, but life on Earth has seen many of these and its not been effected by it. That being said, with the GPS units of today/tomorrow the compass will soon be obsolete in any practical application, so no harm done. :roll:
That's true, but the idea that most scientists don't know what causes a magnetic reversal, and don't know how it will affect us, does make one a little anxious.

[H.Hunter]

That's true, but the idea that most scientists don't know what causes a magnetic reversal, and don't know how it will affect us, does make one a little anxious.Everything will be constructed upside down. That would freak people out if all the sudden their cars were going the wrong way.

Since "True North" is a varying condition then, like the ground through erosion, doesn't it fall under one of those contractor verification categories for existing site conditions? Now what woul dbe bad is if the poles flipped in the middle of a project. Although I assume you would then just go by "True South."

[daniel.dennin]

The normal purpose of defining which North is being used (True, Magnetic, or both) allows for the information to be measured and converted for everyone involved. This is much the same as using standard drill sizes for a certain tap size; everyone can look at the standard and know which tool to use for a certain specification.
While both forms of North shift, the change is measurable, which allows for calculations to be done to accurately locate the same point at a later time.

One additional advantage is that by showing declination, the difference between Magnetic and True (aka Map) North, the military can continue to make 2nd Lieutenants look like idiots during map exercises.

[Augi Doggie]

Everything will be constructed upside down. That would freak people out if all the sudden their cars were going the wrong way.

Since "True North" is a varying condition then, like the ground through erosion, doesn't it fall under one of those contractor verification categories for existing site conditions? Now what woul dbe bad is if the poles flipped in the middle of a project. Although I assume you would then just go by "True South."
I have to make a little correction for you here. True North does not change, Magnetic North does.

[H.Hunter]

I have to make a little correction for you here. True North does not change, Magnetic North does.But what is True North? ;)

[Augi Doggie]

But what is True North? ;)
I've found this post to be very handy in the past.

[jaberwok]

I've found this post to be very handy in the past.

But we're in the present.

;-)

[Comach]

I would like to thank everyone for their contributions. The discussions are very informative.

[cadtag]

Grid North -
as a civil nerd, North is generally determined by the state plane coordinate system we are working with. In FL at least it's very commonly used and well documented. Although a number of surveyors seem to be fixated on using an arbitrary origin of 5000,5000 and can't be bothered to convert their info to state plan until we complain vocally... I believe all the US states have their own state plane coodinate systems, with varying number of zones, and in some locales the local authroitues have defined county coordinate systems. FL for example uses three zones to cover North, East, and West

[Augi Doggie]

Grid North -
as a civil nerd, North is generally determined by the state plane coordinate system we are working with. In FL at least it's very commonly used and well documented. Although a number of surveyors seem to be fixated on using an arbitrary origin of 5000,5000 and can't be bothered to convert their info to state plan until we complain vocally... I believe all the US states have their own state plane coodinate systems, with varying number of zones, and in some locales the local authroitues have defined county coordinate systems. FL for example uses three zones to cover North, East, and West
It's good to see a fellow Floridian in here. I come across surveyors using arbitrary points of origin all the time too. A lot of them are coming around, especially considering the increasing technology in GPS is making it much easier for them to use state plane.

The state plane coordinate system is becoming much more widely accepted in the US. It's based on the North American Datum (NAD) 1983. It's much more accurate than using Latitude and Longitude coordinates. The name is a little deceptive, it is a country wide program that's based on monuments located worldwide. This NOAA document (http://www.ngs.noaa.gov/PUBS_LIB/NOSNGS-50.pdf)describes it in way too much detail.

The FAA is now requiring all Airport Layout Plans to be drawn in State plane coordinates so they can be added to the database more easily. I've noticed a lot of municipalities are starting to do the same.

[Opie]

Grid North -
as a civil nerd, North is generally determined by the state plane coordinate system we are working with. In FL at least it's very commonly used and well documented. Although a number of surveyors seem to be fixated on using an arbitrary origin of 5000,5000 and can't be bothered to convert their info to state plan until we complain vocally...
Unless the work is being done for a government entity or is required by a governmental entity, the expense to convert to a state plane coordinate cannot always be justified.

[Opie]

It's good to see a fellow Floridian in here. I come across surveyors using arbitrary points of origin all the time too. A lot of them are coming around, especially considering the increasing technology in GPS is making it much easier for them to use state plane.
Not all surveyors want to lug a GPS unit out to a job site to get to the state plane coordinates either. The hand held GPS units will get you close, a few feet I believe, but that isn't the most accurate information either.

[Augi Doggie]

Not all surveyors want to lug a GPS unit out to a job site to get to the state plane coordinates either. The hand held GPS units will get you close, a few feet I believe, but that isn't the most accurate information either.
The technology that has come out, in as recently as the past year, is really impressive. If you have a base unit near the job site it's extreamly accurate.

There is at least one company here in FL (I won't name, I don't work for them) that has GPS units in their grading equipment, that will look at their location as compared to the design and indicate if they are in the right location/depth.

The important thing here is of course the base unit not the handheld mobile units. Surveyors will have no interest in lugging something that big around for a small job. It's just not cost effective.

[Doodlemusmaximus]

We use a similar system over here as a national grid reference. This being based upon a global north, though from my understanding recently there is some debate going on with useing this system and perhaps adopting a different version. And I just got used to this one.

[cadtag]

Unless the work is being done for a government entity or is required by a governmental entity, the expense to convert to a state plane coordinate cannot always be justified.

What is the expense, or rather what causes the expense of setting up the survey data in state plane instead of an arbitrary grid? My last hand-on surveying was in school, with transits and steel chains/tapes, so in that environment, the arbitrary scheme made some sense. AFAIK back then none of the benchmarks were located in the grid system for horizontal control, just vertical.

Most of the time the sites I deal with are large enough and the jobs last long enough that a consistent coordinate system is critical, and even when they are small - it can be very helpful to have state plane coordinates. We use LABINS extensively for much of our site reference material and planning level data, and that info is all on state plane. Always being on state plane can save jobs and budgets big-time!..

case in point was a beach renourishment job. the Surveyor ran topo at 5 locations post hurricane, county paid contractor for new sand hauled in, surveyor re-ran the same five sites so county could check sand volume on-site vs volume allegedly delivered. All five sites came in the first time with arbitrary coordinates, so instead of a single LDT project, I had to create 5. The post-renourishment survey also used a separate 5000,5000 coordinate for each of the five sites, but did not choose the same points as the previous survey, and did not have the exact same direction for north. Each site had to be rotated and moved a unique distance and angle-,, and that didn't happen until after the EIT who was doing the volume calcs had spent a couple of hours trying to reconcile mismatched data,

Is it really that much cheaper to not bother with real locations and directions, and only use 'assumed' north and 'assumed' coordinates?

[Opie]

What is the expense, or rather what causes the expense of setting up the survey data in state plane instead of an arbitrary grid? My last hand-on surveying was in school, with transits and steel chains/tapes, so in that environment, the arbitrary scheme made some sense. AFAIK back then none of the benchmarks were located in the grid system for horizontal control, just vertical. Back then, everything was pretty much by hand anyway. Very few, at least in these parts, used computer based drafting.


Most of the time the sites I deal with are large enough and the jobs last long enough that a consistent coordinate system is critical, and even when they are small - it can be very helpful to have state plane coordinates. We use LABINS extensively for much of our site reference material and planning level data, and that info is all on state plane. Always being on state plane can save jobs and budgets big-time!..
The size of the site will be one factor in the decision to provide state plane coordinates.


case in point was a beach renourishment job. the Surveyor ran topo at 5 locations post hurricane, county paid contractor for new sand hauled in, surveyor re-ran the same five sites so county could check sand volume on-site vs volume allegedly delivered. All five sites came in the first time with arbitrary coordinates, so instead of a single LDT project, I had to create 5. The post-renourishment survey also used a separate 5000,5000 coordinate for each of the five sites, but did not choose the same points as the previous survey, and did not have the exact same direction for north. Each site had to be rotated and moved a unique distance and angle-,, and that didn't happen until after the EIT who was doing the volume calcs had spent a couple of hours trying to reconcile mismatched data,
The lack of planning on the surveyor's part in not using the same points or provided a means to use the same coordinates caused the additional expense. The survey crews that I have dealt with, know if they need to maintain that control. And if they don't maintain that control, it just adds to the cost of the project and that is not good business.


Is it really that much cheaper to not bother with real locations and directions, and only use 'assumed' north and 'assumed' coordinates?
It's cheaper here to have a person in the office to work with it instead of sending a survey crew out at a higher rate. It's also easier on the crews since they are the ones out in the extreme environments.

Hope that makes sense.

[cadtag]

But, what causes the expense of putting a survey on state plane? If the surveyor is working from a known horizontal control, would not the state have that already located in state plane? If that's not the case, what does a survey need to accomplish to properly place their point shots in a state plane system?

>>The lack of planning on the surveyor's part in not using the same points or provided a means to use the same coordinates caused the additional expense. The survey crews that I have dealt with, know if they need to maintain that control. And if they don't maintain that control, it just adds to the cost of the project and that is not good business.

I'll concede that, bearing in mind that a tropical storm came through between runs and relocated the landscape a bit.... However, if all the points were shot on state plane for each pass, any variance would have been more readily apparent.

[Opie]

But, what causes the expense of putting a survey on state plane? If the surveyor is working from a known horizontal control, would not the state have that already located in state plane? If that's not the case, what does a survey need to accomplish to properly place their point shots in a state plane system?

>>The lack of planning on the surveyor's part in not using the same points or provided a means to use the same coordinates caused the additional expense. The survey crews that I have dealt with, know if they need to maintain that control. And if they don't maintain that control, it just adds to the cost of the project and that is not good business.

I'll concede that, bearing in mind that a tropical storm came through between runs and relocated the landscape a bit.... However, if all the points were shot on state plane for each pass, any variance would have been more readily apparent.
It is far easier to setup control on an arbitrary coordinate system, than it is to setup on known control points and then traverse to the site. There is no additional time needed to use an arbitrary coordinate system. To bring a known coordinate system to the site just takes time, and money. It is not a quick task to run the control to the site over long distances.

I have never had to deal with control being removed due to natural disasters. The surveyor should have still had some way of knowing where their control was located. There should still be some features that remained that would have allowed them to setup on the same coordinates. ~shrug~

[jesser.81782]

The state plane coordinate system is becoming much more widely accepted in the US. It's based on the North American Datum (NAD) 1983. It's much more accurate than using Latitude and Longitude coordinates. The name is a little deceptive, it is a country wide program that's based on monuments located worldwide. This NOAA document (http://www.ngs.noaa.gov/PUBS_LIB/NOSNGS-50.pdf)describes it in way too much detail.
I'm not sure what you mean by your comment on state plane being more accurate than lat/long? Coordinates can be mathematically transferred between LL and SPCS making them equally accurate. IMHO, LL with ellipsoid height is probably the most accurate since changes in elevation do not affect it like the SPCS.

[Augi Doggie]

I'm not sure what you mean by your comment on state plane being more accurate than lat/long? Coordinates can be mathematically transferred between LL and SPCS making them equally accurate. IMHO, LL with ellipsoid height is probably the most accurate since changes in elevation do not affect it like the SPCS.
It's strictly a tolerance issue, UTM and Lat/Long are accurate to a varience of plus or minus 1 unit per 2500 units. State plane is accurate to 1 unit per 10,000 units.

Of course, this would probably be to small to effect most projects anyway. Also it's only fair to point out that not even all of the government agancies use the SPCS. Most of my projects for the FAA are converted from State plane to Lat/Long. We use Corpscon, it's a great program and it's free.

[thair]

Not sure if someone already supplied the link but I do this all the time when I have to enter hydrographic data into a modeling program which asks for the magnetic declination from true north to Magnetic north etc. All you need to know is where in the world you are (lat long)
http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl

Hope it helps.
Trey