What is a benchmark in surveying?

A benchmark is a fixed, permanent marker whose height (elevation) above a reference level, such as sea level, has been carefully measured. Surveyors use it as a starting point to measure the height of other things.

What one looks like

• A small metal disk set in concrete or rock
• A rod driven into the ground
• A mark cut into a bridge, building foundation, or similar sturdy structure

What it's used for

• Setting building heights and drainage grades
• Working out how a property sits relative to flood levels
• Making sure different surveys use the same height reference

Benchmark vs. boundary marker

These are easy to confuse. A benchmark tells you how high a spot is. It generally does not tell you where a property line is. Property corners are usually marked by separate boundary monuments, such as iron pins or concrete markers. A benchmark may happen to sit near your land, but that doesn't make it a property corner. Only a licensed surveyor, looking at your deed, plat, and the evidence on the ground, can say what a particular marker means for your boundaries.

Things to know

• Elevations are tied to a named reference system (a "datum"). Older benchmarks may use a different datum than newer ones, so the numbers may not match.
• Benchmarks can be moved, damaged, or buried over time. A surveyor typically checks one before relying on it.
• Government agencies keep records of many benchmarks, but availability varies by location.

If you've found a marker on your property and aren't sure what it is, avoid moving or digging it out, since that can destroy useful evidence.

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