|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).
Map spread worksheet
These values can be summarized in a Map Spread Worksheet.
Map Spread Worksheet (completed)
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
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.
1. Do I have the information I need to fill out a map spread worksheet?
The correct answer is a, yes. If you know the rate of spread, projection time, and map scale, you can fill out a map spread worksheet.
2. What is the rate of spread?
The rate of spread is 12 chains/hr.
3. What is the projection time?
The projection time is 3 hours.
4. What is the spread distance in chains?
The correct answer is d, 36 chains. SD = ROS × PT = 12 × 3 = 36.
5. What is the spread distance in feet?
36 chains × 66 ft/chain = 2376 ft.
6. What is the map scale (in/in)?
The correct answer is a, 1:21,120.
7. What is the conversion factor to ft/in?
The correct answer is c, 1760 ft/in.
8. What is the map spread distance?
The correct answer is 1.35 in. A ground distance of 2376 ft translates to 1.35 in on this map.
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.
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.