A back azimuth is a projection of the azimuth from the origin to the opposite side of the azimuth circle. There are 360 degrees in the azimuth circle, so the opposite direction would be 180 degrees (half of 360 degrees) from the azimuth.

A back azimuth is calculated by adding 180° to the azimuth when the azimuth is less than 180°, or by subtracting 180° from the azimuth if it is more than 180°. For example, if an azimuth is 320°, the back azimuth would be 320° - 180° = 140°. If the azimuth is 30°, the back azimuth would be 180° + 30° = 210°.

An azimuth is the direction measured in degrees clockwise from north on an azimuth circle. An azimuth circle consists of 360 degrees. Ninety degrees corresponds to east, 180 degrees is south, 270 degrees is west, and 360 degrees and 0 degrees mark north.

Map Scales and Equivalents

Map Scale Conversion Factors
 

A contour line is a line drawn on a topographic map to indicate ground elevation or depression. A contour interval is the vertical distance or difference in elevation between contour lines. Index contours are bold or thicker lines that appear at every fifth contour line.

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

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.

The map scale is printed in the map legend. It is given as a ratio of inches on the map corresponding to inches, feet, or miles on the ground. For example, a map scale indicating a ratio of 1:24,000 (in/in), means that for every 1 inch on the map, 24,000 inches have been covered on the ground. Ground distances on maps are usually given in feet or miles. 
 

Map Scale Conversion Factors