A fire perimeter is the entire outer edge or boundary of a fire, usually drawn on a map as a line or polygon around the area the fire has reached. Each mapped perimeter is a snapshot from a particular time and method, so on an active fire the real edge may already be somewhere else.
How perimeters are mapped
Crews and specialists map perimeters in the field by several methods. A U.S. Geological Survey report on interagency perimeter data lists infrared (IR) flights and Global Positioning System (GPS) units carried on foot, in vehicles or in aircraft.
An infrared interpreter uses imagery from each night's mission to identify the heat perimeter and areas of intense, scattered and isolated heat. The interpreter delivers the products, including map layers and PDF maps, to the incident and gives a morning briefing as the incident requires.
Where public perimeters come from
The Wildland Fire Interagency Geospatial Services (WFIGS) group, hosted by the National Interagency Fire Center (NIFC), publishes the best available perimeters for wildland fires in the United States. Its service refreshes every 5 minutes and pulls from multiple sources, using rules to pick the most current or most authoritative shape.
WFIGS states plainly that perimeters are not available for every incident. For the full set of reported fires, WFIGS points to a companion incident locations service.
Reading a series of perimeters
A progression map shows the areas a fire affected over time. National Wildfire Coordinating Group mapping guidance shows each time period shifting from cool to warm colors, so the newest growth stands out.
After large fires, the Monitoring Trends in Burn Severity program maps burned extent and severity, with Landsat as its primary data source. It covers fires of 1,000 acres or more in the western United States and 500 acres or more in the eastern United States.
What a perimeter does not tell you
The line marks the outer edge, not whether everything inside burned or how hot. NIFC says its perimeter data are not legal documents and carry no warranty of accuracy or completeness, so treat the line as the best estimate at the time it was mapped.