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.
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...:)
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)
but, usually all oficial national mapping services produce maps with this info.