Wildfire data field guide
Fire Perimeter vs Burned Area: what is the difference?
A fire perimeter is the outer boundary. The area inside that boundary can include unburned islands, so perimeter-enclosed acreage and truly burned surface are not always identical.
Core distinction
Boundary geometry vs what actually burned.
NWCG defines a fire perimeter as the entire outer edge or boundary of a fire. Operational acreage is commonly calculated from that mapped polygon, but the polygon can contain unburned or unburnable islands.
The outer line or polygon boundary mapped for the incident at a specific time.
The portion of the landscape actually affected by fire; depending on the dataset, this may exclude unburned patches inside the perimeter.
Islands
Green can exist inside the black.
NWCG training defines an island as an unburned area inside the fire perimeter. That means a polygon-derived acreage can include land that did not burn.
Why it matters
Perimeter acreage is not a perfect ecological burn footprint.
USGS research has documented substantial unburned area within mapped wildfire perimeters. For ecological effects, carbon accounting or severity analysis, assuming every acre inside a perimeter burned uniformly can overstate the affected surface.
- Incident acreage is useful for operational scale.
- Perimeter polygons are essential for spatial chronology.
- Post-fire burned-area or severity products answer a different question.
- Unburned islands can be important refugia and seed sources.
Worked example
One boundary, two valid area figures.
Suppose a mapped outer boundary encloses 1,000 hectares. A separate classification identifies 150 hectares inside it as unburned and 50 hectares as unburnable. For this teaching example only, assume these categories do not overlap, the remaining area is classified as burned, and there are no missing observations.
Classified burned area = 1,000 − 150 − 50 = 800 hectares.
Burned share of the enclosed area = 800 ÷ 1,000 × 100 = 80%.
The boundary-enclosed area remains 1,000 hectares. The classified burned area is 800 hectares. Neither number should silently replace the other: they describe different quantities. Label both the measurement and its source when comparing reports.
Why the denominator changes the percentage
The excluded 200 hectares represent 20% of the enclosed area: 200 ÷ 1,000 × 100. However, using 1,000 hectares as if it were the classified burned area would give a value 25% above 800 hectares: (1,000 − 800) ÷ 800 × 100. The same 200-hectare difference produces two percentages because the reference quantity changes.
Do not turn this example into a correction factor
These invented values demonstrate arithmetic, not a typical unburned fraction. The USGS-listed study of unburned areas within wildfire perimeters examined three parks and found variation across fires and study areas. Its results do not establish a universal percentage to subtract from another incident.
If some pixels are unclassified, cloud-obscured or otherwise missing, the subtraction above is no longer sufficient. Keep that unknown area separate. The burn-severity map guide includes a worked example showing how missing data affects a reported percentage.
A comparison record you can reproduce
- Identity: record the incident identifier, dataset and version.
- Time: retain the observation time and time zone for each layer, as well as the download date.
- Area definition: state whether the value describes an outer boundary, a polygon with excluded holes or selected mapped classes.
- Method: record the units and area calculation method; do not compare hectares directly with acres.
- Coverage: list excluded categories and missing observations, and state the denominator used for each percentage.
Before interpreting a difference as fire growth, check whether the two records describe the same area definition and compatible observation times. For measurements made from an image or printed map, also check the map scale and screenshot-resizing limitations.
Black Box use
Store perimeter, acreage and burn effects separately.
Sources