All surveys connected to the NG should have their measured distances reduced to the horizontal, and then to MSL; and should then be multiplied by the local scale factor to reduce them to grid distance Consider Figure 8.24 in which stations A, B and C are connected into the NG via a link traverse from OS NG stations W, X and Y, Z:
(1) The measured distance D1 to D4 would be treated as above.
(2) The observed angles should in theory be corrected by appropriate (t−T) corrections.
These would generally be negligible but could be quickly checked using:
E = easting of the mid-point of the line
R = an approximate value for the radius of the ellipsoid for the area
(3) There is no correction for grid convergence as the survey has commenced from a grid bearing and has connected into another.
(4) Grid convergence and (t − T) would need to be applied to the bearing of, say, line BC if its bearing had been found using a gyro-theodolite and was therefore relative to true north (TN). This procedure is sometimes adopted on long traverses to control the propagation of angular error.
When the control survey and design coordinates are on the NG, the setting out by bearing and distance will require the grid distance, as computed from the design coordinates, to be corrected to its equivalent distance on the ground. Thus grid distance must be changed to its MSL value and then divided by the local scale factor to give horizontal ground distance.
The setting-out angle, as computed from the design (grid) coordinates, will require no correction.