Signals: Flags, Banners, Cones, and Lights Help Direct Surveys

Since the early 1800s, the National Geodetic Survey and its predecessor organizations have been working to establish a consistent and accurate framework from which to conduct land and coastal surveys in the United States. An integral part of completing these surveys was the measurement of angles, through a process known as triangulation. Signals—used to help surveyors sight on distant survey marks—were integral to obtaining high-quality angles.

Did you know that activities such as transportation, navigation, communication, delineation of property boundaries, mapping and charting efforts, defense operations, and many others all rely on a consistently and accurately defined network of survey points? For nearly 200 years, NOAA's National Geodetic Survey (NGS) has defined and managed this network, known as the National Spatial Reference System (NSRS).

Here, we'll take a look at the evolution of one of the crucial tools used by early surveyors to establish the NSRS: signals.

Some Information about Signals

Most of the survey work accomplished to establish the nation's survey network by the U.S. Coast and Geodetic Survey was done by observing the angles of triangles. This process, known as triangulation, involved observing the angles of thousands of triangles, all across the U.S. All angles were measured multiple times to help detect errors.

While excellent instruments were available to measure the angles, the key to high-quality angles was to have the angle measuring instrument centered over the survey point, and to have some sort of signal or target centered over the distant survey marks. This signal gave the observer looking through the telescope of the instrument something to sight upon.

Ferdinand Hassler, the Survey's first superintendent, had used spheres covered in white cloth as signals in his native Switzerland. However, on the U.S. East Coast, with its high humidity and much poorer visibility, this type of signal did not work well. Thus, surveyors had to take a different approach.

During the early days of the Survey, several types of signals were used. These signals included wooden poles or towers topped by tin reflectors, flags, one or two cloth cross-banners, or wooden targets. Also, systems of mirrors, called heliotropes, were used to reflect sunlight once or twice, depending on the position of the sun, along the line-of-sight to distant survey stations.

In the 1870s the C&GS experimented with the light beam from lamps burning whale oil, kerosene, and magnesium, none of which worked well for long survey lines. Acetylene lamps were introduced in the late 1890s. Around 1900, bicycle acetylene lamps were used by the C&GS for the survey along the 98th meridian and were found useful for surveying distances up to about 34 miles. Larger acetylene automobile headlights were also eventually used successfully. Two of these automobile lamps stacked one above the other were observed at a distance of 133.9 miles at Pilot Peak, Nevada.

The development of the electric automobile headlight resulted in an excellent signal lamp. A bulb with a special filament was made at the request of the C&GS. In 1920, the light from two of these headlights was visible at a distance of 152.9 miles to an unaided eye (no telescope). By the mid-1920s, the electric signal lamp was the standard. Over the years, electric lights were made smaller and smaller, as more efficient bulbs and reflectors were produced.

After the development of electronic distance measurement instruments, special prisms (mirrors) were used to reflect a light signal from a distant survey mark back to the observer. These prisms also needed to be mounted so that they were centered directly over the distant survey mark.

Soon after NGS began performing surveys using Global Positioning System (GPS) technology in 1983, angle observations and the need for visual signals ended. The signals described above allowed accurate triangulation methods to be used to survey the nation. GPS now enables surveyors to determine positions faster, cheaper, and more accurately.

The images below include a variety of the signals used throughout the history of the Survey, with a brief explanation on the usage of each.

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


Works Consulted

Bowie, W. (1922). Geodetic Operations in the United States January 1, 1912 to December 31, 1921. Special Publication No. 81, USC&GS.

Bowie, W. (1926). Long lines of triangulation. Geographical Review, 16 (4): 638-641.

Hodgson, C.V. (1921). Utah-Washington Arc of Precise Triangulation. Special Publication No. 74, USC&GS.

Hodgson, C.V. (1926). Manual of First-Order Triangulation. Special Publication No. 120, USC&GS.

U.S. Coast and Geodetic Survey. (1880). Annual Report of the USC&GS for 1880, Appendix No. 8 "Report on Geodesic Night Signals."

U.S. Coast and Geodetic Survey. (1921). Annual Report of the USC&GS for 1921, pp109-110.