Site Contour lines max and min based on PPI example in PPD

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Dwalklive13

July 13, 2026 01:32

If your contour lines are 1’ apart and you have 1% min and 20% max how do you determine the run for each. PPI has 5’ as a Min and 100’ as a maximum. I don’t understand using the rise/and run formula ?

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  kkgalicinao July 24, 2026 20:33

  Since the contour interval is 1 ft, the rise is always 1 ft.

  Use the slope formula: Slope (%) = (Rise ÷ Run) × 100

  Solve for run: Run = Rise ÷ (Slope ÷ 100)

  • 1% slope: 1 rise ÷ 0.01 run = 100 ft run
  • 20% slope: 1 rise ÷ 0.20 run = 5 ft run

  A helpful shortcut for 1-ft contours is to remember:

  • 1% = 100 ft
  • 2% = 50 ft
  • 5% = 20 ft
  • 10% = 10 ft
  • 20% = 5 ft

  As the slope gets steeper, the run gets shorter, which is why contour lines appear closer together.

  Hope this helps!

  Kiara | Black Spectacles | Community

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