yourYour 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
Differentiate various types of north:
- True north .... Direction of meridians (lines of equal longitude) at your survey site. On a globe, these meet at the poles. On a map, it depends on the map projection.
- Grid north .... Direction of vertical lines (of equal eastings) making up the map grid.
- Magnetic north ... Direction of local magnetic field.
Methodology for calculating various norths:
You should know the following terms i.e Azimuth: azimuth, Gridgrid 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 of a 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)
Azimuth can be obtained by astronomical observations (tedious) or can be computed by latitude and longitude (spherical coordinates and spherical trigonometry).Grid bearing can be computed by grid coordinates
Grid bearing can be computed by grid coordinates (simple trigonometry).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
Covergency is the difference in angle between True north and Grid north. It will depend on which map projection is used. For example, on a transverse Mercator projection, it is computed by the arctangent of the product of tangent of delta lamda and sine of phi, where delta lamda is the difference between longitudes of place of observation and of grid origin, and phi is 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 beare familiar enough with the terms needed to solve your problem...:)
