True North

[MisterJingles]

Hi there

 

I received a hard copy of a simple site plan (See below)

 

I need to recreate this in Autocad and obviously need to show which direction North is. I'm finding this tricky, I have worked on co-ordinated drawings before but never had to create from scratch.

I have the co-ordinates for points A - F but cannot place the points until I set up my north.

 

Can anyone show me how to go about setting up True North before I go about setting out the 6 boundary points?

 

Thanks

 

[Organic]

If that is all you have just take north as straight up the page as shown on the north point there, without any additional control or info you won't be able to get it any more accurate.

[Tiger]

Coordinates are coordinates - if you have them, you don't need to know North beforehand. Or am I missing something?

[MisterJingles]

Dink my concern is that when I do that, and then place my boundary points they are orientated completely differently than they show on the image above.

See below.

 

[MisterJingles]

Tiger my concern is that when I plot out the boundary points the site doesn't resemble the one in the hard copy. It almost seems a mirror image of it.

I assume its my North orientation which is the problem?

[eldon]

When surveys look like mirror images, check which order you plotted the coordinates. You might have Easting and Northing OR Northing and Easting. Try plotting in the reverse order.

[Tiger]

Ah, now I see the trouble!

 

I would go with Eldons suggestion, we have to have our tounges in the right mouth that at the office when we send out coordinates because the X in our CAD-drawings are the Y in the real life - but not in every town that we work in.

[eldon]

Another thing to look for, you are in the Southern Hemisphere, are the Northings actually Southings (minus Northings)

 

Because you can mirror your second image and then rotate it 180 degrees, before it looks like your first image.

[eldon]

Or even your coordinates could be Westings and Southings

[Tiger]

Coordinates give me headache

[MisterJingles]

Yeah I have a headache too Tiger, but I also have a drawing which now works. Thanks to you guys for the help.

I did have to swap my X and Y around. and also had to drop the -.

 

Man I love coming here.

[Organic]

Projections and all the different systems data can be on confuses me (getting the right system is easy enough although then making sure you are in the right zone/town etc makes it a bit trickier), I would rather just leave it up to the surveyor to work out the correct spatial position in the world and give me the data I can work on confidently without having potentially screwed up the coordinate system right from the start. I'm not sure what 'W N' stands for (it may be local to your country, i.e. all the projections and systems I know are localized to my country).

[Tiger]

I ran into a fun thing when I started a new project with a new client.

 

Since a few years back our major client switched from a local city-coordinate system to the main Swedish coordinate system (Swedish main grid is based on the 15-degree meridian (or median, never know which one is which. I know latitutes are lazy and lie down, that's about it)) but with a twist - they based it on the 18-degree which is in fact closer to our town.

 

This client uses the the official 15-degree grid - but that is not always true I learnt. There is a 15-degree coordinate system. Then there is the 15-degree TM coordinate system which is the real main coordinate system of Sweden. And those two (15 and 15TM) are not the same ofcourse...

 

Let's just say I transmogrified enough files that day to last me a year!

[BIGAL]

True North or magnetic north which one they are different ?

[SLW210]

True North or magnetic north which one they are different ?

 

In some places they are the same. There is also Grid North. True North is also divided into Geodetic true north and Astronomical true north. Geodetic true north also differs very slightly from Astronomical true north (typically by a few arc seconds) because the local gravity may not point at the exact rotational axis of the earth.

 

Drawings should use Geodetic true north as this is TRUE NORTH at all times. Some drawings will have more than one indicated, but N is for TRUE NORTH.

[ctdlc888]

yes, just understand the x and the y in right order.

i.e., for review x is the offset perpendicular distance from the vertical(0)

and y is the offset perpendicular distance from the horizontal(0)

in autocad.in some countries it's the opposite though.

[ScribbleJ]

in some countries it's the opposite though.

 

I'm curious about that. In what countries is this a true statement?

X and Y values are mathematical expressions which is a universal language.

[Organic]

I'm curious about that. In what countries is this a true statement?

X and Y values are mathematical expressions which is a universal language.

 

Maybe the poster was referring to how in some countries we have

 

Eastings Northings (correct way in my opinion)

 

while others use

 

Northings Eastings

[ScribbleJ]

Maybe the poster was referring to how in some countries we have

 

Eastings Northings (correct way in my opinion)

 

while others use

 

Northings Eastings

 

 

It is a relative then. In the U.S. it has always been Northing and Eastings (which is correct in my opinion). I have never seen any text stating Eastings then Northings. However I have seen Westing first but this generally referred to global coordinates such as latitudes and longitudes. Of all the survey maps I have ever created the surveyor has expressly stated the labeling of points to be North first and East afterwards. As I said it is relative, so in each country it is different.

[Dana W]

True North or magnetic north which one they are different ?

 

True North is the convergence point, the northern geographic center, for all the lines of longitude drawn on the globe. It is merely a geographic measurement point.

 

Magnetic North is the actual point at which your compass needle points as it is drawn into alignment with the magnetic field lines generated by the Earth's iron core.

 

Since the iron core moves and sloshes a bit, it is not always round, neither is the Earth for that matter. The Earth, and the magnetic field are greatly influenced by gravitational forces from the other nearby bodies in the solar system.

 

Magnetic north continually moves around at a speed somewhere between 27 and 34 miles a year. It normally stays a few hundred miles to the South of the geographic North Pole but it is always moving.

 

The same goes for the South Pole, but not to as large of a variance.

 

If you draw a line from the North magnetic pole, to the South magnetic pole, the line will not pass through the geographic center of the Earth.

 

True North and magnetic North only line up with the geographic North and South poles, if you happen to be on a line that intersects all three points, with your compass in your hand. This alignment will only last long enough for you to perceive it, than it moves on. You can get the compass to point to both North Poles by being on a line that intersects both poles, but you need to keep moving.

 

There is no point anywhere on earth that you can truly state that magnetic North and true North are in the same direction from where you are standing, ever. By the time you go "Yaaahooo, I found it", it will be a few centemeters away and moving further on its merry journey.

 

All this is why the system of longitude and latitude was invented in the first place, and why inertial navigation and the Global Positioning System were designed to improve upon that.

 

In Columbus' time, you could repeat a long distance voyage, say a couple thousand miles using your compass, but you would only be able to get to the same spot again within a margin of error measured in tens of miles at best.

 

Luckily, the Earth is sooooo Huuuuuge, that our little Boy Scout compass will keep us from getting lost in the woods with no worries. Uh, that is, unless you go camping way up north of the Arctic circle.

 

That gives you some idea why those north pole explorers were such awsome dudes, huh? Or delusional. Or both.