its about engineering survey... now im study at Poli, so i'll never upload more about form 6 subject.

1. Types of north.
There are three North’s commonly in use Greet British.
Grid North the direction of a grid line which parallel to the central meridian on the National Grid.
True North is the direction of a meridian of longitude which converges the North Poles.
Each day the Earth rotates about its axis once. The ends of the axes are the True North and South poles.
True North on a map is the direction of a line of longitude which converges on the North Pole.
Magnetic North is the direction indicated by a magnetic compass. Magnetic North move slowly with a
variable rate and currently is west of Grid North in Great Britain.
Ordnance Survey map showing Grid North, Magnetic North and True North variations for
Purbeck UK, 1977
Google Maps North
Google Maps use a variant of the Mercator projection for its map images. The Mercator map was
designed as an aid to navigators since straight lines on the Mercator projection are loxodromes or
rhumb lines - representing lines of constant compass bearing - perfect for 'true' direction.
True North on Google Maps is not shown, but for a normal Mercator projection, grid north and true
north will coincide and it will follow any vertical line (or meridian) to the top of the map.

2. DIFFERENCES
 The horizontal angular difference between True North and Magnetic North is called MAGNETIC
VARIATION or DECLINATION.
 The horizontal angular difference between Grid North and Magnetic North is called GRID MAGNETIC
ANGLE.
 It is this angle which needs to be applied when converting between magnetic and grid bearings.
 Difference between the three North’s are given on our 1:50 000 scale OS Land ranger and OS
Explorer 1:250 000 maps.
CALCULATING MAGNETIC
Variation.
Example -
REFRERENCE
1. Different types of north - Google Compass
 googlecompass.com/TypesofNorth.htm
2.
-
.

3. FORE BEARING.
The line is measured in the forward direction example along the progress of
survey is known as FORE BEARING. Fore bearing = BACK BEARING +-180
Fore bearing columns
Measured the absolute compass angle to the next station.
Fore Bearing and Back Bearing are average to calculate the coordinate of the
next station.
A magnetic deviation from north can be entered in the horizontal angle reference
dialog.
It is also possible to change absolute angles to turning angle to minimize the
influence of local magnetic attractors.
Calculation of corrected bearing and internal angle.
Theoretical difference between fore bearing and back bearing should be 180˚.
Local Attraction.
 If the difference between magnetic. Fore bearing and back bearing of the
line is not exactly 180˚. It may be due to presence of local attraction at one
of the both station. If this difference is exactly 180˚ then both stations are
free from local attraction.
Practice problem.
In the below table observed bearings are given,
Line observed
F.B
CORRECTION
B.B
CORRECTED
F.B
AB
B.B
,B=-
BC
-
CD
-
DA
-
147˚00