Skip to main content

5.4 Perimeter and Area Estimation Charts

The perimeter and area of a fire can be determined by using estimation charts as shown below. These charts require knowledge of the spread distance in chains and the effective windspeed in miles per hour. A fire area/size worksheet is provided to be filled out with the data collected.

EFFECTIVE WINDSPEED

Effective windspeed is the midflame windspeed, corrected for the effects of slope on fire spread. Changes in slope affect how wind influences a fire and the overall fire spread. For slopes above 20%, each additional 20% increase in slope translates to an additional 1 mph increase in the effective equivalent “upslope” component of the wind. For slopes between 20% and 40%, for example, a 1-mph slope-equivalent upslope component would be added to the flame-level wind. Between 40% and 60%, a 2-mph slope-equivalent upslope component would be added, and above 60%, flame-level winds would increase by 3 mph.
 

screengrab of animation of flame heights increasing as slope increases
Click to view animation of slope effects on fire

Example 1 - A fire starts at point A. Given the following values, determine the fire perimeter. 
rate of spread = 5 ch/h, projection time = 3 h, effective windspeed = 10 mi/h

Step 1. Use the equation for determining the spread distance (see Section 5.3).
ROS × PT = 5 ch/h × 3 h = 15 chains spread distance.

Step 2.> Look at the perimeter estimation chart. Find a spread distance of 15 chains on the vertical axis. Move your gaze across horizontally.

Step 3. Find an effective windspeed of 10 miles/hour on the horizontal axis, which is between 9 miles/hour and 11 miles/hour. Follow the column down vertically.

Step 4. Where the two lines intersect, read the number. In this case there are two numbers.
34 chains and 33 chains

Step 5. Take an average of the two values.
34.0 chains + 33.0 chains / 2 = 33.5 chains

The perimeter of the fire is 33.5 chains.

Example 2 - Determine the area of the fire in Example 1 and complete the fire area/size worksheet. The spread distance is the same, 15 chains.

Step 1. Reference the area estimation chart for a spread distance of 15 chains. Follow the row across horizontally.

Step 2. Look for an effective windspeed of 10 miles/hour, which is between 9 miles/hour and 11 miles/hour. Follow the column down vertically and read the numbers from the two windspeeds.
4.9 acres and 5.7 acres

Step 3. Take an average of the two values.
4.9 acres + 5.7 acres / 2 = 5.3 acres

The area of the fire is 5.3 acres.

Perimeter estimation for point source fires.
Table showing Perimeter Estimation

Area estimation for point source fires.
point source fires

Fire Area/Size Worksheet

Line Input    
0 PP Projection point A
1 ROS Rate of spread, ch/h 5
2 EWS Effective windspeed, mi/h 10
3 PT Projection time, h 3
4 SDF Spread distance, ch 15
  Output    
  PER Perimeter, ch 33.5
  AC Area, ac 5.3

NWCG Latest Announcements

IBC Memo 24-01: OF 297 Signature Order and 2024 Fire Season Use IBC Memo 24-02: Summary of Changes for SIIBM, PMS 902

Date: July 15, 2024
Contact: Incident Business Committee 

The NWCG Incident Business Committee (IBC) has recently released two memorandums. The first provides direction on the use of the Emergency Equipment Shift Ticket, OF 297 for the 2024 fire season. Due to delays in hard copy printing, both the 2024 revision and the older version of the Emergency Equipment Shift Ticket, OF 297 are acceptable for use during the 2024 fire season.

The second memorandum documents the updates of the newly revised NWCG Standards for Interagency Incident Business Management (SIIBM), PMS 902. The summary of changes attached to the memo covers the major updates and process changes from the 2022 version.

References:

IBC Memorandum 24-01: OF 297 Signature Order and 2024 Fire Season Use

IBC Memorandum 24-02: Summary of Changes for the NWCG Standards of Interagency Incident Business Management, PMS 902

IBC Memorandum 24-02 Attachment: Summary of Changes

NWCG Standards for Rapid Extraction Module Support, PMS 552

Date: July 10, 2024
Contact: Incident Medical Unit Subcommittee 

A new publication from the Incident Medical Unit Subcommittee is now available. The NWCG Standards for Rapid Extraction Module Support, PMS 552 will be used as a guide and as an opportunity to begin to build out Rapid Extraction Module Support (REMS) modules for the remainder of Fire Year 2024. These standards will be fully implemented as the minimum standard starting in January 2025.

NWCG Standards for Rapid Extraction Module Support outlines the roles, duties, qualifications, and equipment pertinent to REMS. A REMS team, strategically stationed at wildland fires, plays a pivotal role in prioritizing swift access and medical treatment to injured or ill firefighters for safe and efficient egress off the fireline. This ensures their rapid transport to definitive medical care in cases of emergency during firefighting operations, highlighting the invaluable contribution of the REMS team to firefighter safety and well-being.

References:

NWCG Standards for Rapid Extraction Module Support, PMS 552

National Wildland Firefighter Day

Date: July 2, 2024
Contact: National Interagency Fire Center 

The National Wildfire Coordinating Group (NWCG) would like to recognize July 2, 2024, as National Wildland Firefighter Day (NWFFD). Established in 2022, NWFFD honors the dedication of wildland firefighters and support personnel. This day falls within the 2024 Week of Remembrance (June 30 - July 6), providing an opportunity to renew our commitment to wildland firefighter safety while remembering those who have fallen in the line of duty.

NWCG continues to provide leadership to enable interoperable wildland fire operations among federal, state, local, Tribal, and territorial partners. NWCG standards and training establish common practices to contribute to safe, effective, and coordinated national interagency wildland fire operations.

References:

NWCG.gov

Week of Remembrance

National Wildland Firefighter Day

2024 Week of Remembrance

Date: June 27, 2024
Contact: 6 Minutes for Safety Subcommittee 

As we approach the 2024 Week of Remembrance (WOR), June 30 to July 6, we dedicate this time to thoughtfully reviewing and recognizing the events of the 2018 Mendocino Complex. As such, this year’s theme of “Learning From the Mendocino Complex” embodies a longstanding hallmark of WOR, honoring through learning.

Throughout the week, our energy will be directed toward fostering generative conversations in briefing rooms and at tailgates.

References:

6 Minutes for Safety - 2024 Week of Remembrance

Wildland Fire Lessons Learned Center