Benchmark

Key Takeaways

• A benchmark is a permanent physical mark of known, published elevation that surveyors use as the starting point for vertical measurements.
• Benchmarks answer how high. The corners and monuments of the government survey answer where. Both are physical marks, but they control different dimensions.
• An elevation only means something relative to a datum, the agreed reference surface every height in the system is measured from.
• In Washington the Department of Natural Resources is the official state survey agency and maintains records of reliable survey monuments and bench marks statewide (RCW 58.24.040).

What It Means

A benchmark is a permanent marked point whose height above a reference surface has already been determined and published. The familiar form is a brass or bronze disk, stamped with an identifier and set into a concrete post, a bridge abutment, or a building foundation, placed there because it is unlikely to move. A surveyor who needs the elevation of a point on a parcel starts at the nearest benchmark and measures the difference in height between the two, instead of trying to establish an absolute height from nothing.

Elevation is meaningless without a starting surface, and that agreed surface is the Datum. A benchmark carries a published height above that datum, so every elevation worked out from it lands in the same system as every other survey nearby. That is what makes the numbers comparable across jobs and across decades: a base flood elevation, a finished floor height, a building height limit measured from average grade, and a sewer line invert all trace back through some benchmark to the same zero. Lose the mark and the survey loses its vertical reference, which is why these marks are treated as public infrastructure worth preserving.

How It Works in Washington

Washington treats survey marks as a public record to be preserved rather than as private hardware. The Survey Recording Act, chapter 58.09 RCW, states its purpose as "to provide a method for preserving evidence of land surveys by establishing standards and procedures for monumenting and for recording a public record of the surveys," and RCW 58.09.040 makes filing mandatory: within ninety days after establishing, reestablishing, or restoring a boundary corner or a general land office corner, a land surveyor must file a record of survey with the county auditor. Those recorded surveys are the evidence later work is tied to, and they are what a Legal Description is checked against when a boundary is questioned.

Bench marks are named directly in RCW 58.24.040, which designates the Department of Natural Resources as the official state survey agency and directs it to "assemble and maintain records of all reliable survey monuments and bench marks within the state." Note the split of duties: a Survey of your parcel is recorded county by county with the auditor, while the statewide inventory of control marks sits with the department. The same statute lets a section corner or other land boundary mark be temporarily removed for construction only if a registered professional engineer or land surveyor references it to the Washington coordinate system first and replaces it, or sets a suitable reference monument, within a reasonable time afterward.

Example

Beckett is buying a riverfront lot in Skagit County for $315,000, and his lender wants an elevation certificate because the parcel sits beside a mapped flood zone. His surveyor, Ines, starts at a benchmark set in the concrete headwall of a county bridge about 400 feet away, published at 42.60 feet above the datum.

Ines runs levels from that disk to the proposed building pad and measures a rise of 5.85 feet, which puts the pad at 48.45 feet. The base flood elevation for the parcel is 47.00 feet, so the finished floor as designed sits 1.45 feet above it. The lot is buildable without raising the pad and the flood insurance quote comes back lower than Beckett feared. Notice what carried that result. The metes and bounds fixed where the lot is. A brass disk on a bridge, and the published number stamped in the county's records against it, fixed how high it is.

Common Mistakes and Exam Traps

• A benchmark controls elevation, not location. Baselines, principal meridians, township lines, and section corners control location.
• A benchmark is not the datum. The datum is the reference surface; the benchmark is a physical mark whose height above that surface is known and published.
• A benchmark is not a corner monument. Monuments mark horizontal positions such as a section corner or a property corner, and a benchmark usually sits nowhere near a boundary line.
• Washington keeps two separate sets of records: records of survey are filed with the county auditor under chapter 58.09 RCW, while the statewide inventory of monuments and bench marks is held by the Department of Natural Resources under RCW 58.24.040.