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

desired unit, feet.

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.__Step 2.__ Convert the ground spread distance to a map 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 | ______ |

### PRACTICE

**Exercise 1.** Fill in the Map Spread Worksheet values for a fire with ROS of 12 chains/hr and PT of 3 hrs. The map scale is 1:21,120.

### 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.

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.

### 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.

Fire shape depends on effective windspeeds.