Patrol Grid Trace and Safety Reporting
A training area is a rectangle 600 m east-west by 400 m north-south. Grid references are given in metres from the south-west corner, easting (east-west) first, so the north-east corner is (600, 400). HQ is at (100, 100); the radio mast is at (450, 320); the bridge is at (520, 80). A section leaves HQ and marches: 200 m east, 150 m north, 250 m west, 100 m south, where it halts. Visibility is poor and the section commander reports position only as a grid reference. Two safety rules apply: no patrol may come within 100 m of the bridge, and any patrol more than 400 m from the radio mast must send a position report by radio.
1. Give the section's final grid reference and its straight-line distance from HQ, and compare that with the distance actually marched.
Read the worked answer
Track the easting and northing separately.
- Start: (100, 100)
- 200 m east: (300, 100)
- 150 m north: (300, 250)
- 250 m west: (50, 250)
- 100 m south: (50, 150)
Displacement from HQ: easting 50 - 100 = -50 m; northing 150 - 100 = +50 m. Distance = sqrt(50^2 + 50^2) = sqrt(5,000) = 70.7 m (roughly 71 m) to the south-west of HQ.
Distance marched: 200 + 150 + 250 + 100 = 700 m.
The section has walked 700 m to finish 71 m from where it started - the legs largely cancel. This is why reporting a route is not the same as reporting a position.
2. Does the section breach the bridge exclusion, and is it obliged to send a radio report? Show the calculations.
Read the worked answer
Bridge check. Bridge at (520, 80); section at (50, 150).
- dE = 520 - 50 = 470 m; dN = 80 - 150 = -70 m
- Distance = sqrt(470^2 + 70^2) = sqrt(220,900 + 4,900) = sqrt(225,800) = 475 m
475 m is far outside the 100 m limit, so no breach. Any single axis being large is enough to keep the section clear; here the easting difference alone is 470 m.
Radio check. Mast at (450, 320); section at (50, 150).
- dE = 450 - 50 = 400 m; dN = 320 - 150 = 170 m
- Distance = sqrt(160,000 + 28,900) = sqrt(188,900) = 434.6 m
434.6 m exceeds 400 m, so a position report is required - but only just, by about 35 m. The commander should report and expect the safety staff to treat the section as marginal, not comfortably inside the limit.
3. The section is ordered from its halt to move to a point exactly 300 m due north of HQ. Give the new grid reference, the shortest distance of that move, and any boundary issue.
Read the worked answer
New position: 300 m due north of HQ (100, 100) means the same easting, northing 100 + 300 = 400, so (100, 400).
Shortest distance from the halt at (50, 150):
- dE = 100 - 50 = 50 m; dN = 400 - 150 = 250 m
- Distance = sqrt(50^2 + 250^2) = sqrt(2,500 + 62,500) = sqrt(65,000) = 255 m
Boundary issue: northing 400 is exactly the northern edge of the training area, so the position lies on the boundary rather than inside it. A section standing on the boundary line needs to be told which side it is meant to be on, and the safety trace should record it as a boundary position, since any small navigation error puts the section outside the area.