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5.3 Spread Distance

Spread distance (SD) is the forward distance a fire spreads in a given amount of time. SD can be calculated from rate of spread (ROS) and projected time (PT).

Spread Distance = Rate of Spread × Projected Time
SD = ROS × PT

Example 1 - What is the spread distance, in feet, for a fire that has a rate of spread of 6 chains per hour for a 3-hour time span? 

Step 1. Use the equation for determining the spread distance.

spread distance

Step 2. The answer needs to be in feet. Set up the cancellation table so all units will cancel, except the
desired unit, feet.

cancellation table

The spread distance is 1,188 feet.

Example 2 - Using the information in Example 1, find the map distance of the fire spread. The map scale is 1:31,680 (inch/inch).

Step 1. Convert inch/inch to feet/inch. Set up the cancellation table so all units will cancel, except the desired unit, feet/inch.

cancellation table

Step 2. Convert the ground spread distance to a map spread distance.

spread distance

The map distance is 0.45 inches.
 

MAP SPREAD WORKSHEET

These values can be summarized in a Map Spread Worksheet.

Map Spread Worksheet (blank version)

Line Input      
0 PP Projection point ______ ______
1 ROS Rate of spread, ch/h ______ ______
2 PT Projection time, h ______ ______
3 SDCM Spread distance, ch (line 1 x line 2) ______ ______
4 SDFT Spread distance, ft (line 3 x 66 ft/ch) ______ ______
5 SCL Map scale ______ ______
6 CF Conversion factor, ft/in (see map scale conversion) ______ ______
  Output      
1 MD Map spread distance, in (line 4 divided by line 6) ______ ______


Map Spread Worksheet (completed)
 

Line Input      
0 PP Projection point A Given
1 ROS Rate of spread, ch/h 6 Given
2 PT Projection time, h 3 From ex. 1
3 SDCM Spread distance, ch (line 1 x line 2) 18 From ex. 1
4 SDFT Spread distance, ft (line 3 x 66 ft/ch) 1188 From ex. 9
5 SCL Map scale 1:31,680 From ex. 2
6 CF Conversion factor, ft/in (see map scale conversion) 2640 From ex. 2
  Output      
1 MD Map spread distance, in (line 4 divided by line 6) .45 from ex. 2


When completing the map spread worksheet, notice that the projection point is line zero. Line 1 begins with the rate of spread.

Fire Area Size Worksheet
 

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

FORWARD RATE OF SPREAD

The shape of a fire is dependent on slope and effective windspeed. The higher the windspeed and slope percent, the longer, more cigar-shaped the burn area.
 

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


 

Safety dictates that firefighters be positioned at the flanks (sides) or back of a fire. A firefighter should never be uphill (at the head) of a fire where the rate of spread is the fastest. See the Standard Fire Orders (F-I-R-E-O-R-D-E-R-S) in the Fireline Handbook, PMS 410-1 or NFES 0065.
 

USING A SCALED RULER FOR SPREAD MEASUREMENTS

When drawing or measuring distances, a scaled ruler is sometimes necessary for direct measurements. A scaled ruler, or engineer's ruler, has 1-inch increments subdivided into 1/10-inch increments, convenient for ground measurements. Ground measurements are usually given in fractions of multiples of 10. Consider a map distance of 3.7 inches, where 1 inch equals 100 feet. In this case, each tenth of an inch will equal 10 feet. Consequently, 3.7 inches equals 370 feet.

image of engineer's ruler

MAP SPREAD WORKSHEET

All data collected and calculated related to spread distance and rate of spread can be put into a map spread worksheet.

Example 3 - Using a ruler scaled in 1/10 inch or 0.1 inch increments, draw a sketch of the burn in Example 2. The effective windspeed is 3 miles/hour. Complete the map spread worksheet. See the steps and illustrations below.

Step 1. Draw a line of 0.5 inches from point A up the slope to point B.
A - B

Step 2. See the fire shapes in the fire shape figure below to determine how wide to make the ellipse.

Step 3. Draw an ellipse around the line drawn.

ellipse around line
 

fire shape wind speed dependent

Fire shape depends on effective windspeeds.

NWCG Latest Announcements

The Next Generation Position Task Book and Incident Position Standards are now available for Safety Officer, Field (SOFF)

Date: July 26, 2024
Contact: Risk Management Committee 

NWCG is excited to announce that the NWCG Incident Position Standards for Safety Officer, Field, PMS 350-81 and NWCG Position Task Book for Safety Officer, Field (SOFF), PMS 311-81 are now available.

The Safety Officer, Field (SOFF) is responsible for monitoring operations on an incident from a risk management perspective to provide for the welfare of incident resources and the public. The new Incident Position Standards and Next Generation Position Task Book are developed through the Incident Performance and Training Modernization (IPTM) effort.

References:

NWCG Safety Officer, Field (SOFF) Position

NWCG Incident Position Standards for Safety Officer, Field, PMS 350-81

NWCG Position Task Book for Safety Officer, Field (SOFF), PMS 311-81

Updated NWCG Standards for Electronic Documentation (eDoc), PMS 277

Date: July 25, 2024
Contact: Incident Planning Subcommittee 

The Incident Planning Subcommittee has updated the NWCG Standards for Electronic Documentation (eDoc), PMS 277.

The NWCG Standards for Electronic Documentation (eDoc) establishes the standards for collection and retention of records on wildland fires. This July 2024 update will provide incident management teams the most current standards required to maintain incident records and submit them to host units at the close of an incident.

References:

NWCG Standards for Electronic Documentation (eDoc), PMS 277

eDoc Box Directory (zip file)

NWCG Off-Highway Vehicle Typing Standard Request for Comment

Date: July 24, 2024
Contact: Mobile Fire Equipment Subcommittee 

The Mobile Fire Equipment Subcommittee has released Equipment Bulletin 24-002 NWCG Off-Highway Vehicle (OHV) Typing Standard - Request for Comment. This bulletin outlines the proposed NWCG OHV typing standard, as well as the business need for establishing the standard. Comments on the proposed standard will be accepted through August 15th using the comment form linked below.

References:

ETC-EB-2024-02: NWCG Off-Highway Vehicle (OHV) Typing Standard - Request for Comment

NWCG Off-Highway Vehicle (OHV) Typing Standard Comment Form

NWCG Standards for Airtanker Operations, PMS 514 - DRAFT

Date: July 18, 2024
Contact: National Interagency Aviation Committee 

A draft version of the new NWCG Standards for Airtanker Operations, PMS 514, is now available. The NWCG Standards for Airtanker Operations establishes the standards for dispatching, utilizing, and coordinating airtankers on wildland fires. As this is the first edition of these standards, the National Interagency Aviation Committee (NIAC) requests review and input into the 2025 final publication.

Please review and provide feedback by September 1st, 2024 for consideration. Feedback can be provided utilizing the NWCG Publication Review Form.

References:

NWCG Standards for Airtanker Operations, PMS 514 - DRAFT

NWCG Publications Review Form