A site plan shows contour lines at 2-foot intervals. If you need to calculate the slope between two points that are 100 feet apart horizontally and span 4 contour lines, what is the slope percentage?
Correct Answer
A) 8%
The vertical rise is 4 contour lines × 2 feet = 8 feet. The horizontal run is 100 feet. Slope percentage = (rise/run) × 100 = (8/100) × 100 = 8%.
Why This Is the Correct Answer
Option C is correct because slope percentage is calculated as (rise/run) × 100. The vertical rise is determined by multiplying the number of contour lines crossed (4) by the contour interval (2 feet), giving 8 feet of rise. With a horizontal distance of 100 feet, the calculation becomes (8/100) × 100 = 8%. This is a fundamental surveying and grading calculation that contractors must understand for proper site development.
Why the Other Options Are Wrong
Option B: 4%
Option A incorrectly uses only the number of contour lines (4) as the percentage without accounting for the 2-foot interval between lines or properly applying the slope formula.
Option C: 6%
Option D incorrectly calculates the slope by using 10 feet of rise instead of the correct 8 feet, possibly confusing the number of contour intervals with the actual elevation change.
Memory Technique
Remember 'RRH': Rise = contour lines × interval, Run = horizontal distance, then (Rise/Run) × 100 = slope percentage
Reference Hint
Look up site work and grading sections in your construction reference manual, typically found in earthwork or surveying chapters.
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