A grading plan shows contour lines with elevations of 98', 100', 102', and 104'. What is the contour interval for this site plan?
Correct Answer
A) 2 feet
The contour interval is the vertical distance between adjacent contour lines. Since the elevations increase by 2 feet between each line (98' to 100' to 102' to 104'), the contour interval is 2 feet.
Why This Is the Correct Answer
The contour interval is the uniform vertical distance between consecutive contour lines on a topographic plan. Looking at the given elevations (98', 100', 102', 104'), each adjacent pair differs by exactly 2 feet: 100-98=2, 102-100=2, and 104-102=2. This consistent 2-foot vertical spacing between contour lines establishes the contour interval as 2 feet, which is standard practice for site grading plans.
Why the Other Options Are Wrong
Option B: 6 feet
6 feet is incorrect because it represents three times the actual interval. This might result from incorrectly calculating the total elevation change (104-98=6) rather than the interval between adjacent contour lines. The contour interval must be the consistent spacing between consecutive lines, not the total range.
Option C: 4 feet
4 feet is incorrect as it represents twice the actual interval. This error could occur from calculating the difference between every other contour line (102-98=4 or 104-100=4) instead of between consecutive adjacent lines. Contour intervals are always measured between immediately adjacent contour lines.
Option D: 1 foot
1 foot is incorrect because the elevation difference between each adjacent contour line is 2 feet, not 1 foot. This answer might result from misreading the elevations or confusing the contour interval with a different measurement standard used on other types of plans.
Memory Technique
Remember 'Adjacent Always' - contour interval is Always the difference between Adjacent contour lines. Count up: 98 to 100 (up 2), 100 to 102 (up 2), 102 to 104 (up 2).
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