That zone isn't for everyone.

A turnstile or a card reader control who goes through a door. What happens afterwards, inside, nothing controls: whoever got in can go anywhere.

And on many sites there is no door to put in. A loading area, building work inside a running plant, the perimeter of a machine: places where you can't put up a partition and fit a reader.

So it gets solved with a sign and with people knowing. Which works until someone walks in who doesn't know.

You mark the area over the camera image with the mouse, and say what has to be true to be inside: wearing certain equipment, being accompanied, being there at certain hours, or simply not being there at all.

Changing the zone means drawing it again. That matters more than it sounds on a site or in a plant that gets reorganised every few weeks: the control moves with the work, not the other way round.

And you can have several zones over the same camera, each with its own rule.

In a steelworks, the safety radius around the casting ladle changes with the stage. Two zones are drawn over the same camera: the normal working one and the extended one, which switches on while the pour is under way.

Nothing new has to be installed every time the process changes. You change the drawing.

First you look at the camera that is already there. What it sees, from what height and at what angle. A zone drawn on a very oblique view distorts: what is three metres in the image can be eight on the ground.

Then you draw the area over the image and write the rule in one sentence. From there it is left running for a few days without alerting anyone, just recording. That is what tells you whether the zone is well placed: if two hundred alerts come out in three days, the zone is wrong, not the people.

And finally you decide who it alerts and how: a screen in the control room, a message to the manager's mobile, a beacon in the zone itself. The rule is the same; what changes is who finds out.

Building work inside a running plant: for six weeks there is a crane working in a yard people normally walk through. You can't put up fixed fencing because the yard is still used for other things.

The crane zone is drawn as a prohibited area during lifting hours, and as free passage the rest of the time. When the work finishes, the drawing is deleted. Nothing had to be installed or removed.

'How many zones can I have over one camera?' As many as fit in the field of view and can be told apart. In practice, three or four per camera is reasonable: more usually means you need another camera, not another zone.

'Can I change them myself?' Yes, and that's the point. A zone is drawn with the mouse over the image; you don't have to call anyone or stop anything. On a building site, or in a plant that gets reorganised every few weeks, that is the difference between it being used and not.

'And if someone moves the camera?' The zone stops being where it was, and that is a real problem. That's why the camera is fixed down when it's set up, and checked: if yours is the kind of installation people touch, better to know beforehand.

'How long does it take to alert?' It depends on the rule. Presence in a prohibited zone is immediate; a ten-minute stay alerts after ten minutes, which is what was asked for.

Each alert leaves the camera, the time, the zone and the video of the moment. What it doesn't leave is who it was: the system tells that there is a person, not which one.

Those records serve two different purposes. The immediate one is reacting. The other, the one almost nobody uses, is the pattern: which zones get entered most, at what hour and on which shift. That is what makes you change a schedule or move a door, which fixes the problem at the root instead of reporting it every day.

It doesn't replace physical access control where the rules require one. It's a layer on top, not a replacement for the turnstile or for the light curtain on a press.

And a zone only exists inside the field of view. What happens outside the image nobody sees: covering a large area means several cameras, and that has to be said at the start.