Question

If you see 'BM 450' on a map, what does it signify?

• A. The area is approximately 450 meters high.
• B. A permanent reference point with a precise elevation of 450 meters.
• C. A mountain peak measured by a trigonometric survey.
• D. A rugged area where the height is estimated at 450 meters.

Answer 1

Answer:

B. A permanent reference point with a precise elevation of 450 meters.

• 'BM' stands for Bench Mark, which is a precisely known permanent reference. • A Bench Mark is a monumented vertical control point. • Unlike other map heights that might be calculated from a distance (like triangulation) or estimated from photos (like form lines), a Bench Mark is established by "spirit leveling"—a process where a surveyor physically walks the distance from the sea to that spot with a telescope and a graduated rod to confirm the height to the nearest millimeter.