How Far Back, How Much Perimeter, and Who Held It CORDON
A four hundred metre cordon sounds like a line you could hold with a handful of people. It is two and a half kilometres of perimeter and twenty-five posts before anybody is relieved. That is knowable in the first minute.
What is at the middle of it
These are published planning distances, and they are not a decision. They exist so that a first responder has a defensible number to work to in the first two minutes, before anybody qualified arrives. The person who owns the scene sets the cordon; where ordnance is involved that is the ordnance officer, on the ground, looking at the thing. Nothing on this page has seen it.
What that costs in perimeter and people
The three-reliefs figure is the honest one. The number everybody quotes on the radio is the single-shift number, and a cordon held past a shift change needs the multiple.
Eight places to send people
A centre and a radius are not an order anybody can act on. Eight positions are. These are computed on the WGS84 ellipsoid, not on a sphere, because a sphere is a fifth of a per cent out and a cordon point is somewhere a person has to stand.
Who is on which corner
The page that answers “was the north-east junction held at eleven minutes past”, three weeks later, without anybody having to remember a radio call.
The cordon plan
Questions people ask about CORDON