What Are Survey Benchmark Monument Locations?
Survey benchmark monuments are brass or metal disks in the ground that provide latitude, longitude, or orthometric height.
They give surveyors a point of reference because they use them as part of a set of survey observations.
Some benchmarks also provide a reference for elevations. These types of benchmarks indicate the height above or below sea level at that location.
A “benchmark” is a generic term sometimes referred to as survey marks, geodetic marks, and control stations. Each has a slight difference in meaning.
However, survey professionals prefer the terms station or mark rather than “benchmark” to describe horizontal control marks.
Developing a Datum Using Triangulation
Example of a survey benchmark monument. More recently, surveyors have used markers to set up a GPS receiver antenna in a known position for Differential GPS surveying.
In surveying and mapping large areas, surveyors must take into consideration the curvature of the surface of the Earth and sea-level surface. This is why geodesists developed the geodetic survey network extending across North America. They serve as a basis for North America’s horizontal control data.
With 2 benchmark locations, surveyors use triangulation and trigonometry to measure the direction, distance, and elevation between them. Geodetic leveling can help understand the area and volume of the Earth.
Horizontal control data provide a rigid framework for map makers, engineers, and land surveyors in North America. It makes it possible for greater precision for surveys and to locate national, state, county, and private boundaries with greater precision. Benchmarks are important to surveyors because they help accurately measure the positions of all land boundaries and modern infrastructure.
Who is Responsible for Survey Benchmark Monuments?
In the United States, NOAA’s National Geodetic Survey (NGS) is responsible for managing approximately 240,000 stations gathered over the last two centuries. The National Geodetic Survey Data Explorer makes this data available to the general public.
NGS developed the North American Datum of 1927 which used the Clarke spheroid of 1866 and a point of origin at Meade’s Ranch. Then, NGS developed the more accurate North American Datum of 1983 (NAD 83). The United States, Canada, Mexico, and Central America use this horizontal control datum based on a geocentric origin and the Geodetic Reference System 1980.
What Types of Benchmarks Exist?
Two main types of benchmarks exist – “vertical control points” and “horizontal control points”.
- Vertical control points contain a precisely measured orthometric height that is measured as height above sea level.
- Horizontal control points simply contain latitude and longitude values.
Within these two broad types of survey benchmarks, there are different types of categories for horizontal control markers as described in NOAA’s Horizontal Control documentation.
INTERSECTION STATIONS are horizontal control marks with landmarks that can be seen from a distance. Examples of intersection stations are water or radio towers. These points can be observed with a telescope using trigonometry to determine their position on Earth.
TRIANGULATION STATIONS are markers with positions determined by measuring distances and angles from other stations. Reference marks help keep triangulation stations from being lost with arrows on their disks pointing in the direction of their referenced triangulation station. Azimuth marks, coupled with its triangulation station, provide a compass direction of the true north (which is different from the magnetic north).
NOAA Geodetic Control Points Data Explorer
Did you know that surveyors have gathered over 240,000 stations over the last two centuries in the United States? The National Geodetic Survey Data Explorer makes this data available to the general public.
From coast to coast dating back to 1832, surveyors have installed benchmarks using a range of different techniques. Surveyors installed survey benchmarks systematically across the region. They have recorded new distances and angles between points.
The NGS database stores each benchmark location with its own unique personal identifier (PID), six-character alphanumeric code, latitude, and longitude. Many have scaled positions from maps.
Types of NOAA Control Points
The National Geodetic Survey Data Explorer shows geodetic control stations maintained by the National Oceanic and Atmospheric Administration (NOAA), National Geodetic Survey.
Here are the various types of NOAA control points:
CONTINUOUSLY OPERATING REFERENCE STATIONS (CORS) NETWORK are control stations with permanently operating Global Positioning System (GPS) receivers.
COMBINED CONTROL STATIONS are horizontal and vertical control stations.
LOW QUALITY STATIONS have low-quality latitudes, longitudes (horizontal), and/or low-quality Orthometric Heights (vertical).
HORIZONTAL CONTROL STATIONS only contain precise latitudes and longitude.
VERTICAL CONTROL STATIONS have precise Orthometric Heights which measure elevation in meters indicating the height above the Geoid.
Note: A single station can be classified in multiple ways – for example having low-quality Orthometric Height and being part of the CORS network.
Geocaching for Survey Monuments (Benchmarking)
The term “benchmark hunting” refers to the hobby of geocaching for historical survey benchmarks. Survey benchmark hunting has carved out its own niche in this exciting hobby.
Why do geocachers seek survey benchmark points?
When you search for a survey benchmark, you really get to see a piece of history. Surveyors haven’t visited these points for decades, even centuries.
When you find these hidden gems placed into the ground years ago, the geocaching community documents and shares them with each other. And you have bragging rights in the community.
There’s a lot of buzz around benchmarks in the geocaching community because there’s almost a hidden history behind them. When you find a survey benchmark, you may be looking at a benchmark surveyed more than a century ago.
Seek out the nearest survey monument to you. Geocache and discover the history of classical surveying.
Geodetic Survey Benchmark – Image Courtesy of National Oceanic and Atmospheric Administration and the Department of Commerce