When sampling a flat surface, how far must each sampling location be from the edges?

• A. At least 6 inches
• B. At least 1 foot
• C. At least 2 feet
• D. No minimum distance

Answer: (B)

When sampling a flat surface, you want to avoid edge effects that can distort the results. Placing each sampling point at least one foot from the edges puts the sample in the interior where material properties are more representative of the whole area. Sampling too close to an edge can pick up material that's been disturbed by handling, edge compaction, or segregation, which biases the result. Six inches is too close to reliably avoid those disturbances, two feet is more than necessary for standard practice and can be impractical, and having no minimum distance invites edge influence. So, the recommended distance is at least one foot from the edges.