Kindly have a look at this thread

http://www.pdmsworld.com/forum/members-chat-area/7707-why-only-north-direction.html

I had replied on spur of the moment, but is that a correct answer?


I remember, there were few threads about this on our forum. But I am having second thoughts and want some

Warm Regards

Your posting is very good, it should make people think. Below is an answer to a related question I got sometime back:

North, Geographic, Plant, etc.
The question:
What is the difference between geographic north, plant north and isometric north? What are the criteria to apply to these directions during plot plan development and piping study?

My answer:
You have asked a very good question. There are a number of “North’s” and every designer, engineer, project manager and client needs to know and understand what these are and what they means to the project.
So let’s look at the list of common North’s that are found on process plant projects along with a description of what it is and why it is used.
• True North: Every place on earth has a True North and it is pretty much the same no manner where you are. It is located at 90 degrees north of the equator, it is the North Pole. It does not move.
• Geographic North: Same as True North. The term True North is normally the preferred terminology for this.
• Magnetic North: Magnetic North is not normally used on the average process plant project. The magnetic North Pole is always moving. In 2005 the earth’s Magnetic North was located at 82.7 degrees North Latitude and 114.4 degrees West Longitude. This places it far off the north coast of Canada. Because it is always moving it is not reliable as t fixed reference point.
• Plant North: For a project on dry land this is the most important North Arrow used in the process plant engineering, design and construction business. It is rare to find a piece of property that has the major axis of the property aligned with True North. Because of the way all of our ancestors settled the countries and lands of the world then built roads everything is out of alignment with the North Pole. When a project is first started, the property (of some shape) is displayed on a property map that will normally include a North Arrow showing True North. The property may be at any angle (angle of declination) off True North. It is very difficult to do any kind of design if everything is at an odd angle. To simplify the design process a new North is created for the project and this is called the Plant North. It is normal to include the Plant North and the True North on the project Plot Plan along with the angle of declination. Other documents such as structural foundations and piping plans only need the Plant North. as a reference point.
• Platform North: For an offshore drilling platform or processing facility platform this is the most important North Arrow. The Platform North will be assigned for each specific platform and may or may not relate to actual north.
• Isometric North: There is not (or should not be) anything called an “Isometric North.” Isometrics must have a North Arrow on them but it is the Plant North, not a different north.

This presents an interesting scenario which has occured in the project I am currently involved in. On plot plans we usually establish a local plant grid which is aligned with plant North. This is a local rectangular grid and is tied to a UTM real world co-ordinate value and the angle to True North. The grid is a flat 2D tool used to establish column lines, buildings etc. However the world is not flat and the lines of longitude curve toward the poles so the closer you get to North or South pole you have to allow for that factor. These lines are actually curved not flat. This produces a disconnect between local plant co-ordinates and UTM. When a site is large say more than 1 mile long, this factor should be calculated as it must align with the pipelines in and out of the plant which would be designed using UTM co-ordinates.
We had a difference of 1 metre over a 3km long plant site to align with differing co-ordinate systems.
There are different methods and ways of handling this and for small distances it is not a factor but over long distances it comes into play.