How Wildfire Perimeter Maps Are Made — and Why They're Often a Day Old

The outline drawn around a wildfire on a map looks precise, but it's a snapshot of where the fire edge was at a particular moment — and that moment is often many hours ago. Knowing how perimeters are made tells you how much to trust one when a fire is near you.

Where perimeter lines come from

Infrared flights at night

For large fires in the United States, the most detailed perimeters usually come from aircraft carrying infrared scanners, flown at night through the U.S. Forest Service's National Infrared Operations program and by state agencies. Night is deliberate: the ground has cooled, so the heat of the fire stands out clearly, even through smoke. An infrared interpreter turns the imagery into a perimeter, often marking intense heat and scattered heat inside it, and delivers it before the morning briefing so crews can plan the day.

Mapping on the ground and from the air

Field observers walk or drive sections of the edge with GPS units, and helicopters fly the line. Drones are increasingly used too, especially at night and in smoke when crewed aircraft can't fly. These updates can be more current than the overnight flight, but they often cover only part of the fire.

Small fires

Most small fires never get a mapped perimeter at all. The incident is just a point with an acreage figure, which is why many fires on a map appear as dots or circles rather than shapes.

How often perimeters actually change

We looked at how often perimeter shapes changed in FireRadar's own feed for US fires that grew past 1,000 acres between January and October 2026. Of 808 such fires, 248 received two or more distinct perimeters. Across those, the median time between perimeter updates was 25 hours, and half of all updates arrived between 23 and 48 hours after the previous one.

That rhythm reflects the overnight mapping cycle: one good perimeter per day, published the next morning. It also means that during a fast, wind-driven run, the line you're looking at may be well behind the actual fire edge.

Things perimeters don't show

• Unburned islands. The area inside a perimeter isn't all burned. Fires skip patches of green, wet ground and even whole neighbourhoods, so the acreage inside the line is a maximum, not a damage count.
• Spot fires ahead of the line. Embers can start new fires well outside the mapped edge, especially in strong wind.
• Which edges are secure. A perimeter doesn't tell you which parts are contained. That's what the containment percentage is for, and some incident maps mark controlled line separately.
• What happens next. A perimeter is historical. Wind direction and terrain decide where the fire goes from here.

Filling the gap with satellite hotspots

Satellite detections fill the hours between perimeter updates. Polar-orbiting VIIRS sensors pass over most places a few times a day, and each detection carries its own time. If you see fresh hotspots outside last night's perimeter, the fire has very likely moved since it was mapped. Our guide to how satellites detect wildfires explains what a hotspot is and its limits.

A practical way to read the two together:

1. Note the time of the latest perimeter.
2. Look for hotspots detected after that time.
3. Turn on the wind layer to see which edge the wind is pushing.
4. If new activity is on the side facing you, act on official evacuation levels — and don't wait for the next perimeter to confirm it.

On FireRadar's map, perimeters, VIIRS and MODIS hotspots and the wind layer sit on the same map, and you can scrub through a fire's growth history to see how each new perimeter compares with the last.

Methodology: US incidents in FireRadar's database discovered between January 1 and October 31, 2026, whose latest reported size was at least 1,000 acres. A perimeter update is counted when the boundary geometry changes; time between updates is measured from when each new shape first appeared in our feed, which is checked roughly hourly.