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7.1 Slope & Scale

Slope (Degrees)

Slope (Percent)

10º

17.6 %

20º

36.4 %

30º

57.7 %

40º

83.9 %

45º

100 %

50º

119.2 %

60º

173.2 %

70º

274.7 %

80º

567.1 %

90º


Standard LANDFIRE slope themes are represented in units of degrees (°). Many locally produced landscapes over the year’s stored slopes in percent (%). It is much easier to estimate slope in %, estimating the elevation change and the horizontal distance and calculating the ratio. BehavePlus, and BEHAVE tools before that, default to slope input in %.

To convert from slope in degrees (°) to slope in percent (%), a scientific calculator is needed.

  • Enter the slope in degrees
  • Press the Tangent button
  • Multiply the result by 100 to get slope in %



7.1.1 Calculating slope from contour map measurements

See Firefighter Math (http://training.nwcg.gov/courses/ffm/) for additional explanation, examples, and exercises.

The process for directly calculating slope with measurements from a contour map: 

1. Determine the contour interval. This is the elevation change between adjacent contour lines.

        Example: 40 ft.

2.  Determine the map scale and conversion factor. The map scale must be found in terms of the number of feet that each inch on the map represents (ft/in).
 
    a. Map scales are usually given as the number of inches per mile, such as 2 in/mi, or as a representative fraction such as 1:31,680.

    b. Use the spacing of section lines to determine the map scale. Normally section line spacing is 1 mile; be careful of foreshortened sections; look around on         the map and find square sections with equal spacing. Measure the distance with a ruler graduated in inches and tenths of inches. Divide 5,280 by the map         distance between section lines.

Example: Map dist. between section lines = 2.64 in
Map scale = 5,280/2.64 = 2,000 ft/in


Scale

Representative

fraction

Map (in/mi)

Map (in/ch)

Feet per map inch

1 : 253,440

253.44

0.25

0.0031

21120

1 : 126,720

126.72

0.50

0.0063

10560

1 : 63,360

63.36

1

0.0125

5280

1 : 62,500

62.5

1.01

0.0127

5188

1 : 31,680

31.68

2

0.025

2640

1 : 24,000

24

2.64

0.033

2000

1 : 21,120

21.12

3

0.0375

1760

1 : 15,840

15.84

4

0.05

1320

1 : 7,920

7.92

8

0.1

660



3. Determine rise in elevation by counting contour intervals and convert to feet.

                        Example: 11 contour intervals
                        11 x 40 ft/contour interval = 440 ft.


4. Measure the horizontal distance with a ruler graduated in inches and tenths of inches, and convert to feet with the map scale from step 2.

                        Example: Map dist. of slope length = 1.2 in
                        1.2 in x 2,640 ft/in = 3,168 ft.



5.       Divide the rise in elevation from step 3 by the horizontal distance from step 4.











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