03Survey marks, benchmarks and the nails in the curb

The short version: The round metal disk set in a curb or bridge, the nail in the asphalt and the capped rod at a property corner are all survey marks: fixed points that surveyors measure from. US government disks, mostly from the National Geodetic Survey and its predecessor the Coast and Geodetic Survey, record a precise position or elevation, and each one in the NGS database has a public datasheet you can look up. In the US, willfully defacing, changing or removing a government survey mark is a federal offense. In Britain the equivalent is the Ordnance Survey's chiseled "crow's foot" arrow and its metal flush brackets.

What is the round brass disk in the sidewalk?

What you see: A coin-like brass or aluminum disk, about the size of a drink coaster, set flush in a curb, sidewalk, bridge abutment or rock, stamped with letters, numbers, maybe a triangle or an arrow, and sometimes a warning about a fine.

It is a survey mark. Classic US benchmark disks are brass or aluminum, about 3.5 in. (9 cm) across, set in rock or concrete, and usually stamped with a station name, a year, sometimes an elevation, and the agency that set it. The US Geological Survey gives the example "C9 1923 7337.580": station C9, set by the US Coast and Geodetic Survey in 1923, at an elevation of 7,337.580 ft.²⁸

Not every mark is a disk. Some are rods driven until they will go no deeper, under hinged covers; crosses or squares are occasionally chiseled into rock.²⁸ NOAA lists disks, stainless steel rods, drill holes and even landmarks like water towers as survey marks, and counts more than 1,200,000 marks in the US.²⁹

Federal disks are the best known, but state transportation departments, counties and cities set their own, with their own stamping. Whoever set it, the point is the same: a fixed spot whose position or height was measured carefully once, so that later surveys can start from it instead of from scratch.

What do the triangle and arrow on a survey disk mean?

What you see: A disk with a small triangle in the center, or one with an arrow stamped across it, often a few feet from another disk.

The design tells you what the mark is for.

Reference marks exist because main stations get buried, paved over or knocked loose. If you find a disk with an arrow, the station it points to may be a short distance away along that line.

NGS's own manual, Geodetic Bench Marks (NOAA Manual NOS NGS 1, 1978), describes the standard disk as brass or bronze, about 9 cm across, with a slightly domed top to support the foot of a leveling rod and a shank about 7.5 cm long underneath. The setter stamps the mark's designation and year on top with 3/16 in. (4.75 mm) steel dies, and the agency's recessed logo and legend identify the mark and tell the finder where to get its data.³¹

Is it illegal to remove a survey marker?

What you see: Disk text warning of a fine, such as "250-dollar fine or imprisonment."

Yes for government marks in the US. Some older disks carry that warning in the metal itself.²⁸ Federal law, 18 U.S.C. 1858, penalizes anyone who "willfully defaces, changes, or removes any monument or bench mark of any Government survey."³⁰ NOAA's advice to hunters is never to disturb a benchmark.²⁹ If a mark is in the way of construction, the agency that owns it is the one to contact; the mark recovery process below is how you tell NGS.

How do I look up a survey benchmark?

What you see: A disk you want to identify, or a curiosity about what is near your street.

Every mark in the NGS database has a six-character PID (Permanent Identifier).³² Look up the PID and you get the mark's datasheet: its coordinates, its elevation where known, and a "Station Description" giving directions to it from nearby landmarks.²⁹

To work backward from a disk on the ground, note the stamped name and year, the agency and where it sits, then search the NGS records for that name in your area. NOAA's survey mark hunting activity walks through the process for beginners.²⁹

Anyone can report what they find. NGS takes recoveries from the public through its Mark Recovery Form, using the mark's PID; individuals enter agency code "M" and agency "INDIV," and questions go to ngs.mark.updates@noaa.gov.³³ A report that a mark is still there, in good shape, or destroyed is useful to the surveyors who depend on it.

The sources disagree on how many marks there are. NGS says it keeps records of more than 800,000 survey marks;³³ Wikipedia gives about 740,000 benchmarks in the NGS database;³² NOAA's activity guide counts more than 1,200,000 survey marks of all kinds in the US.²⁹ The figures likely count different things, but none of the sources reconciles them.

Who goes looking for benchmarks?

What you see: Someone crouched over a curb with a phone, a printout and a brush.

Benchmarking is a hobby: finding survey marks from their datasheets, photographing them and reporting their condition. Geocaching.com hosted a benchmarking section until 2023; its retirement notes are dated October 17, 2022.³² NGS's mark recovery form remains the official way to report a find.³³ In the UK, hobbyists hunt trig pillars, of which the T:UK site lists 6,550, along with benchmarks.³²

The etiquette is the same everywhere: look, photograph, report, and leave the mark exactly as you found it.²⁹

What is a PK nail in the road?

What you see: A small, flat nail head in asphalt, sometimes with a metal washer under it, often circled or flagged with pink or orange paint.

These are surveyors' nails, driven as temporary control or reference points for a project: the spot a crew sets its instrument over, or a reference tied to a corner. Surveyors treat the PK nail and the MAG nail as the standard pavement nails; alternatives include Ramset nail-and-washer combinations, concrete nails and spikes, chosen partly for how easily a metal detector picks them up.³⁴

What is the X or square chiseled into the curb?

What you see: A small cross or square cut into a granite or concrete curb, sometimes with a dot of paint or a number beside it.

A chiseled cross or square is a recognized survey mark form; the USGS notes such marks cut into rock, though rarely.²⁸ On a curb, a cut cross can serve as a durable control point or a reference for a nearby property corner, because a curb is heavy, hard to move and easy to find again. Without a record from whoever cut it, the meaning of any one cross is hard to tell from the street.

Some local standards make the curb cross the official corner mark. Yolo County, California's 2008 improvement standards for survey monuments require that "cut crosses in the concrete curb or sidewalk shall be used instead of iron bar or pipe bar at the front of the property."³⁵

What is the metal rod with a cap at my property corner?

What you see: A steel rod or pipe flush with the ground, often topped with a colored plastic or aluminum cap stamped with a name and number, sometimes with a wooden stake and bright tape beside it.

It is a property pin, a boundary monument set by a licensed land surveyor. The cap identifies who set it. Gig Harbor, Washington, for example, required in a 1986 ordinance that lot corners be staked with "an iron pin topped with a plastic cap marked with the Registered Land Surveyor's name and license number."³⁶ Some surveyors use different sizes of rebar for boundary monuments and for control points to avoid confusing the two.³⁴

Each state sets its own monument rules. If you are building a fence or a shed near a line, a pin is useful evidence, but the surveyor whose number is on the cap, or the recorded plan, is the authority on where the line runs. Pins sit below the surface where digging happens, so the rule in the paint section applies: in the US, contact 811 before you dig (Dig Safe in Massachusetts).

What does pink spray paint on the road mean?

What you see: Hot pink dots, crosses or numbers on pavement, or pink flagging tape on a stake.

Under the APWA color code used for locate marks, pink means temporary survey marks, and also unidentified facilities.⁶ Pink on its own therefore often means a surveyor was there, not a utility locator. The same color is used when a locator finds something buried that it cannot identify, so pink next to dashes and arrows reads differently from a pink circle around a nail.

In Boston

The NGS lookup, the federal law on government marks and the pink-paint convention all apply in Boston as anywhere in the US. Massachusetts requires white premarking before any dig, so in Boston a survey crew's pink can share the pavement with a contractor's white.² The sister guide's section on roads covers the street reconstruction that survey crews are usually measuring for.

Federal disks are easy to find downtown. NOAA's bench mark sheet for its Boston tide station (8443970, published 2018) lists a disk stamped "3970 C 1991" in the granite bulkhead cap by the southwest corner of the Northern Avenue Bridge, a Coast and Geodetic Survey disk stamped "FED 2 ELEV 20.186" in the bottom step of the Devonshire Street entrance of the US Post Office, and an NGS disk stamped "3970 D 2000" in the seawall cap at Christopher Columbus Park.³⁷

Elsewhere in the world

What is the arrow carved into old buildings in England?

What you see: A horizontal line cut into a wall, church, bridge or gatepost, with a three-line arrow beneath it pointing up at the line.

It is an Ordnance Survey cut benchmark, marking a height above Ordnance Datum Newlyn.³⁸ Cut marks are chiseled into stone, brick or wood as a horizontal leveling line over a three-line arrow, usually pointing up.³⁹ The arrow is the broad arrow, nicknamed the "crow's foot," which became the mark of the Board of Ordnance by 1597.⁴⁰ Cut marks are "the commonest and least highly-ranked grade" of benchmark.⁴¹

Variants include rivets (cut marks on horizontal surfaces with a brass rivet), bolts engraved "OSBM," and pivots made to take a 16 mm ball bearing.³⁹

About 500,000 of these "lower order" benchmarks remain, but only about 190 fundamental benchmarks are still maintained. Ordnance Survey says the rest have not been maintained for decades (the page gives both 30 and 40-plus years) and that their heights "should not be relied upon." The full list is searchable online and downloadable as an archive under the Open Government Licence.³⁸ The bench-marks.org.uk site maps them too.⁴¹

What is a flush bracket on a wall?

What you see: A rectangular metal plate about 6 by 3 in. (15 by 7.5 cm) on a wall or a concrete trig pillar, cast with "OS," the broad arrow and a number like "S1755."

A flush bracket is a higher-order OS benchmark. One example reads "OS," the broad arrow and "BM S1755," with two square sockets above the arrow that once held a leveling staff support.⁴⁰ Flush brackets rank below fundamental benchmarks, and trig pillars usually carry one.⁴¹ Their numbers date them: plain numbers 1 to 3000 were used from 1912 to 1921, the S prefix from the 1920s and the G prefix from 1936. Brackets not on pillars are known as NPFBs (non-pillar flush brackets).³⁹

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