# How to Calculate North?

I am working for field survey & client requirement is differentiate various north,like magnetic north,true north & grid north.So therefore my question is what is the methodology of calculating north & how can we differentiated various type of north as above mentioned.

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Please explain your question in detail & on which software/instrument you are going to calculate the north ?? – Sunil Feb 1 '13 at 11:06
may be useful for you e-education.psu.edu/natureofgeoinfo/book/export/html/1620 – Sunil Feb 1 '13 at 11:08
I am asking basic geographical question which related to all GIS software. – Mahesh Jadhav Feb 1 '13 at 11:10
Although this is indeed a basic geographical question, the manner of its solution differs radically among GIS software applications, because it requires obtaining information both about the earth's coordinate system and about the projection used in the survey. What @Sunil was looking for was some indication of the computational resources available to you: are you going to code your solution from scratch in C? Would you like to exploit the built-in functions of a GIS? If so, which one? – whuber Feb 1 '13 at 17:19

your question is not clear but anyway let me try if it is useful to you.

1). Differentiate various types of north----

• True north .... i.e Meridian Line(azimuth) (meridian of the place where you are)
• Grid north .... i.e Horizontal and vertical lines making grid (each vertical line pointing towards the grid north of that line)
• Magnetic north ... i don't think i need to explain this...

2). Methodology for calculating various norths

You should know the following terms i.e Azimuth, Grid bearing, magnetic bearing and convergency

• Azimuth of the survey line is the clockwise angle from the true north. Grid bearing of survey line is clockwise angle from grid north and similarly clockwise angle from magnetic north is Magnetic bearing.

• Azimuth can be obtained by astronomical observations(tedious) or can be computed by latitude and longitude (spherical coordinates)

• Grid bearing can be computed by grid coordinates
• Covergency is computed by the arctangent of the product of tan(delta L) and sin(phi), where delta L is L-l (L=longitude of place of observation and l = longitude of grid origin and phi = latitude of place of observation) See wiki/Transverse_Mercator_projection

Grid bearing = True bearing - convergency

So convergency is the difference of angle between True north and Grid north.

Sorry i cant explain with a diagram here but i hope you would be familiar enough with the terms needed to solve your problem...:)

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Conceptual diagram here gis.stackexchange.com/questions/61743/… – Martin F Jan 6 '14 at 11:29
The formula for computing grid convergence depends on the projection. – whuber Jul 29 '15 at 1:18

On the web you can find some Bearing converter calculator like these ones:

http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html (for uk only and reliable)