The Foundations of the National Spatial Reference System

NOAA Heritage Deep Dive:
An in-depth look back at the National Spatial Reference System.

This story was originally published on NOAA's 200th Anniversary website in 2007.

In 1807, Ferdinand Hassler, the first Superintendent of the Survey of the Coast, laid the foundation for the first consistent and accurate framework from which to conduct land and coastal surveys in the United States. Today, this survey network is called the National Spatial Reference System (NSRS). The NSRS includes a set of points on the Earth's surface whose locations have been accurately determined; it is the consistent, national coordinate system that specifies latitude, longitude, height, shoreline, scale, and gravity throughout the nation.

In 1807, the Survey of the Coast, a predecessor organization of NOAA, was formed and given the mandate to map the shoreline and coastal features of the new United States, as well as to measure water depths along the coastline. However, before these topographic (land) and hydrographic (water) surveys could begin, a consistent and accurate survey network needed to be established.

A survey network is a group of survey points interconnected such that they are all on a common coordinate system. The nation's first surveyors used a process called triangulation to establish known positions on land from which survey vessels at sea could be positioned.

Building a consistent and accurate survey network ensured that all future surveys (land and water) in a given area fit together correctly. Today, this survey network is called the National Spatial Reference System (NSRS), the foundation for surveying and engineering projects dependent on precise spatial information.

This article tells the story of how the first survey network was established by Ferdinand Hassler, the first Superintendent of the Survey, and later extended across the United States. Later, the network also included other types of measurements such as magnetism, gravity, and elevation.

Establishing the Network

On February 10, 1807, President Thomas Jefferson signed the act establishing the "Survey of the Coast." Shortly thereafter, the Secretary of the Treasury, Albert Gallatin, issued a call for proposals for a plan to conduct a national survey of the U.S. coast. Gallatin asked applicants to propose how they would determine astronomic positions at selected points along the coast, connect these points with the triangulation method of surveying, and then perform the hydrography (surveying the depths of the coastal waters).

Ferdinand Hassler, who had conducted triangulation surveys in Europe, responded to Gallatin's call. Hassler's plan to survey the coast was selected in 1807. After many delays in obtaining instruments and financial support, Hassler conducted the first survey in 1816-17.

The First Survey

The first field survey performed by the Survey of the Coast in 1816-17 was a land survey along a portion of New York Harbor, supervised by Ferdinand Hassler.

As Hassler's survey team did not have the automated data collection and storage technology available today, this first survey took a considerable amount of time and effort. To preserve the work, Hassler's team established marks on the ground at each of the points (i.e., the points of each triangle) where they made survey observations. As it is still today, these marks were required so that future surveyors could find and use the same points for surveying needs far into the future. Hassler probably used buried earthenware cones to mark his first survey points. Unfortunately, none of the survey marks from this first survey have survived to today.

The area around New York Harbor is rather hilly and because surveyors need a clear line-of-sight between surveyed points, some survey points were located on hill tops or bluffs. Often, these points were not at ideal locations, and the end result was a series of overlapping triangles, rather than the ideal neat set of adjacent triangles.

After Hassler's 1816-17 survey, administration changes caused the Survey's fieldwork to completely stop until 1833. Work restarted with surveys connecting to and overlapping with the 1816-17 survey. Starting in 1833, work progressed rapidly up and down the East coast and continued through the middle of the 19th century.

Surveys to extend and increase the density of coverage of this first survey network continue today.

The First U.S. Transcontinental Arc of Triangulation

In the late-1800s, the first "arc of triangulation" that stretched across the United States was surveyed. This was a major accomplishment; at 4,224 kilometers, it is one of the longest arcs of triangulation ever surveyed.

Constructing this arc of triangulation often involved building tall survey towers. The tallest tower was 275 feet high. Such towers were needed to obtain clear lines of sight over extensive forests in the eastern portion of the country or to see from forested mountain top to mountain top in the West. As they attempted to observe angles between the peaks of tall mountains in the West, some over 14,000 feet high, early surveyors often suffered from altitude sickness and poor weather conditions, including blizzards.

Arcs of triangulation were first constructed along the coastlines, then across the country, and later the areas in-between them were surveyed with "area triangulation," a surveying method in which adjacent triangles completely filled the area between the arcs. This process allowed surveyors to build a survey network that covered an increasingly larger area of the U.S., expanding the NSRS.

Early Technology

The primary instrument for measuring angles is the theodolite. The technology for measuring the angles has changed very little since the 1816-17 survey, although the instruments are now smaller and more accurate. One of Hassler's first theodolites weighed in at a hefty 300 pounds; compare that to the 25-pound instrument of recent times.

The technology for measuring distances has changed several times over the past 200 years, from chains, to bars, to invar tapes, to today's electronic distance measuring instruments. Electric distance measuring instruments provided dramatic improvements over previous technologies.

In the 1980s, a major change in surveying occurred. Positioning changed from line-of-sight surveys for measuring angles and distances to using global positioning system (GPS) technology, which only requires clear lines of sight to GPS satellites in the sky.

Extending the Network

In the mid- to late-1800s, the Survey began incorporating other types of measurements such as magnetism, gravity, and elevation, into the NSRS.

Magnetic Surveys Begin

In 1840, the Survey began making magnetic observations. By 1843, these observations had become part of the regular functions of the Survey and were used for magnetic studies and to determine magnetic declination. Magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north. This value can vary from location to location due to the presence of iron ore in the ground or other magnetic disturbances.

At first, magnetism was observed primarily to provide mariners the correction between magnetic north and true north, which is important to safe navigation. Later, magnetic observations were extended to areas in the interior of the country to assist land surveyors and engineers in their work.

A comprehensive magnetic survey of the country was begun in 1899 and completed in 1925. This survey included investigations into some of the areas in the nation with unusual local magnetic anomalies or disturbances. Many magnetic observations were done at county seats so these data would be available to local surveyors.

Many of the magnetic survey locations from this study were marked by special MAGNETIC STATION disks with a six-pointed star in the center. Magnetic observations were also made at sea from Survey vessels.

Today, magnetic information may be obtained from NOAA's National Geophysical Data Center.

Elevation (or Leveling) Surveys Begin

The traditional method for setting vertical benchmarks (indicating elevation or height) is called geodetic leveling. In 1856, the first geodetic leveling was done by the Survey beginning at a benchmark set in the base of the Sandy Hook Lighthouse in New Jersey.

During a 20-year period, beginning in 1877, a "level line" was surveyed across the entire United States, primarily following the arc of triangulation conducted along the 39th parallel.

This method of determining height uses leveling instruments consisting of a telescope with cross-hairs and a level vial, arranged such that the centerline of the telescope can be precisely leveled. After leveling, readings were taken in opposite directions on level rods (which resemble long yard sticks), producing the difference in elevation between the two points where the level rods were located.

As crews continue constructing this "level line," they install a survey mark (called a benchmark when it marks elevation) in the ground, typically about every kilometer along the level line. The benchmark, with its known elevation, serves as a starting point for future surveys. This process of the establishing survey points that future surveyors can use is referred to as "establishing geodetic control."

This leveling method is slow, but accurate, and is still used today. Today, NOAA's National Geodetic Survey also uses GPS surveys to determine elevations, but this method is slightly less accurate than geodetic leveling.

Gravity Observations Begin

In 1872, the Survey began making gravity observations. Initially, the Survey developed its own gravity instruments, but later purchased state-of-the art instruments. Gravity measurements were made at selected locations all across the country. Some observations were made on existing benchmarks and some on special gravity survey disks and gravity reference mark disks.

This extensive network of gravity observation stations is used in combination with other data to model the geoid (an equal gravity surface, approximating mean sea level) and geologic structures. Some of these gravity stations serve as reference stations for relative gravimeter surveys. Another subset contains the absolute gravimeter stations, where very precise and repeated measurements are made to determine geologic changes (e.g., elevation, water-table, magma movement).

Conclusion

Superintendent Ferdinand Hassler's forward-thinking 200 years ago created the nationwide, accurate, and consistent survey network that led to today's NSRS. The NSRS, which today is defined and managed by NOAA's National Geodetic Survey, is a consistent national coordinate system that provides latitude, longitude, shoreline, height, scale, gravity, and orientation throughout the nation. The network also allows us to see how these values change with time.

Contributed by Commander George E. Leigh, NOAA Corps (Ret'd.)

Works Consulted

Dracup, Joseph F. (Date Unknown). Geodetic Surveys in the United States - The Beginning and the Next One Hundred Years, 1807 - 1940. Retrieved October 24, 2006, from: http://www.history.noaa.gov/stories_tales/geodetic1.html.

Dracup, Joseph F. (1977). A Potpourri of Geodetic and Other Surveying Items. National Geodetic Survey.

Studds, Robert F.A. (1951). Technical Services, Coast and Geodetic Survey. Author's personal experience and photographs.

The Hassler Legacy: Ferdinand Rudolph Hassler and the United States Coast Survey. Retrieved on November 3, 2006, from: http://www.lib.noaa.gov/edocs/HASSLER1.htm.