Line Crossing Detection: How It Works on Site Cameras

Siteguard Single Cam view of a rural gravel driveway leading to a farm gate, with a blue line crossing rule drawn across the track — A line crossing rule across a farm driveway. Nothing alerts until a person or vehicle crosses the blue line.

A camera that alerts on every movement soon gets ignored. Wind in the shade cloth, headlights, a possum on the hoarding: by the third night, nobody opens the notifications. A line crossing detection camera fixes that by watching one thing, a virtual line drawn across the image, and raising an event only when a person or vehicle crosses it.

Quick answer

A line crossing detection camera triggers an alert only when a person or vehicle crosses a virtual line drawn on the camera image, such as a fence line or gateway, in the direction you set. Because it ignores movement either side of the line, it throws far fewer false alarms than plain motion detection on a site at night.

What is a line crossing detection camera?

Line crossing is a video analytics rule. You draw a line on the camera's view, along the inside of a fence, across a gateway or along the edge of a laydown yard, and the camera treats it as a virtual tripwire. Nothing happens until someone goes over it.

Two settings matter. The first is direction: A to B, B to A, or both, so a back-fence line flags someone coming in, not a worker heading out to the skip bin. The second is object type. Siteguard's units use person and vehicle detection rather than plain motion, so a kangaroo or a plastic bag drifting across the line isn't treated as an intruder.

How line crossing works on a site camera tower

Behind every alert is a short sequence:

Track: follow each moving object.
Classify: sort it as a person, a vehicle or something to ignore.
Check the line: register when it passes from one side to the other.
Check direction and hours: count it only in the armed direction, during armed hours.
Act: send the event with its linked response.

That last step is set in Siteguard Foreman, the browser-based dashboard where you manage detection zones and configure event linkage: siren, strobe, relay outputs, push alerts, or all of them. The full chain to a police call is laid out in what happens when monitored cameras detect an intruder.

Choose the camera before you draw the line

Single Cam for one gate, Twin Cam for a long fence, Pan & Zoom 360° Cam for open ground. Compare what each tower covers.

Compare the towers

Line crossing vs motion detection: where each one wins

Motion detection asks whether anything in the picture changed. Line crossing asks whether a person or vehicle went over one line. On an open construction site with a road beside the fence, trees overhead and wildlife after dark, it decides whether your alerts are worth reading.

Construction site compound with site sheds and temporary fencing beside a road and a park with a rail fence, at dusk — A road, a park and a site fence in one view. A line inside the fence ignores the first two.

Situation	Motion detection	Line crossing
Fence beside a busy road or footpath	Triggers on passing cars, walkers and headlights	Ignores the road and fires only when someone crosses inside the fence
Site gate after hours	Triggers on anything moving near the gate	Fires on entry through the gate, in the direction you choose
Open ground with access from every side	Picks up movement anywhere in view	A single line is easy to walk around, so an area zone fits better

For a plant compound or container yard that nobody should enter after hours, use an area rule instead, as covered in how intrusion detection zones are set up on a construction site. Most sites run both: lines on the boundary, zones around the assets.

Where to draw virtual tripwires on fences and gates

A line in the wrong spot either misses people or fires all night. Work around the site boundary in this order:

Back and side fences: draw the line a metre or two inside the fence, not on it, so people on the footpath stay outside it. Arm it for inward crossings only.
Gates: run the line across the driveway just inside the gate and set it to inbound. Add number plate recognition for a time-stamped record of every vehicle.
Laydown edges: a line here catches someone who got over the fence where no camera was pointed.

Site camera view along a dirt track and wire fence line on a rural property, with steel livestock gates on the left — Aimed along the fence, the camera sees people cross the frame side to side.

Angle matters too. Analytics track people best when they move across the frame rather than straight at the lens, so aim the camera along the fence and keep the line well inside the image, not hard against the edge. On Siteguard's solar camera towers, the Single Cam gives a 4K 8MP view across 95° for one gate, the Twin Cam covers 180° along a perimeter, and the Pan & Zoom 360° Cam can zoom up to 1km on the far boundary.

A tripwire alert still needs someone to answer it

Siteguard's Australian-based control room verifies every alarm, issues audio warnings and the siren, and escalates to authorities when required, weekends and public holidays included.

See control room monitoring

How line crossing cuts false alarms, and what still causes them

Line crossing removes most nuisance alerts. What's left usually comes from setup, not the camera:

Armed hours that don't match the site. A line armed at 5pm when trades work until 6:30 flags your own crew. Update it for night works and the Christmas shutdown.
Both directions armed on a fence where only entry matters.
Lines left behind after the temporary fencing moves. Redraw them when the boundary shifts.

The last filter is a person. With 24/7 control room monitoring, every alarm is checked by an operator before action is taken, which stops false alarm fatigue. Fewer false triggers save power too: on a solar tower, the siren and strobe draw from the same battery as the cameras. How the analytics tell a person from a shadow is explained in how AI detection works on a CCTV tower.

Key takeaways

Line crossing fires only when a person or vehicle crosses a virtual line, in the direction you set.
It beats motion detection on fences beside roads, at gates and where wind or wildlife move in frame.
Draw lines just inside the fence, aim cameras along the boundary and keep lines off the frame edge.
Operator checks and accurate armed hours remove most of the rest.

Frequently asked questions

What is line crossing detection on a security camera?

It's a virtual line drawn on the camera image that triggers an event only when a person or vehicle crosses it, in the direction you choose, such as along a fence or across a gate.

Is line crossing better than motion detection?

For fence lines and gates, yes. Motion detection reacts to anything that changes in the picture. Line crossing ignores movement either side of the line. For an enclosed area, an intrusion zone often suits better.

Does line crossing detection work at night?

Yes, as long as the camera can see the person clearly enough to track them. Siteguard's Pan & Zoom 360° Cam is built for low light, and Ghost Cams use low-light and thermal imaging to capture footage in near-total darkness.

What happens when a line crossing alarm goes off?

The event triggers the responses linked to it in Siteguard Foreman, such as a siren, strobe and push alert. On a monitored site, an operator checks the footage, issues audio warnings and escalates to police when required.