The Elements of Geodesy: The
Horizontal Datum |
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Brass monuments permanently affixed in
concrete or surrounding
bedrock indicate
accurate geodetic
reference positions
within the National
Geodetic Survey's
horizontal and/or
vertical datums. Click
on the image for larger view.
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At its most basic level of definition, the horizontal
datum is a collection of specific points on the Earth
that have been identified according to their precise
northerly or southerly location (latitude) and easterly
or westerly location (longitude) (National Geodetic
Survey, 1986).
To create the horizontal datum, or network of horizontal
positions, surveyors marked each of the positions they had
identified, typically with a brass, bronze, or
aluminum disk or monument. These markers were placed so
that surveyors could see one marked position from
another. To maximize the line-of-sight between monuments, they were
usually set on mountaintops or at high elevations. When monuments were
set on flat land, towers were built above them to aid surveyors in
locating them.
To "connect" the
horizontal monuments into a unified network, or
datum, surveyors have used a variety of methods,
including triangulation. As technology has improved,
surveyors now rely almost exclusively on the Global
Positioning System (GPS) to identify locations
on the Earth and incorporate them into existing datums.
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When positioning monuments are located on flat ground, large towers such as this were often erected above them allowing geodesists to observe the monument's accurate position over large distances. Click
on the image for larger
view.
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In 1927 the U.S. Coast and Geodetic Survey, the predecessor of the National Geodetic Survey, "connected" all of the existing horizontal monuments together and created the North American Datum of 1927 (NAD 27). This datum was used extensively during the next 60 years as the primary reference for horizontal positioning. In 1983, NAD 27 was adjusted to remove inaccuracies and to correct distortions. The new datum, called NAD 83, is the most commonly used horizontal positioning datum today in the United States.
One application of the horizontal datum
is monitoring the movement of the Earth's crust.
This type of monitoring is often used in places like
the San Andreas Fault in California where many earthquakes
occur. Extending from northern California south to
San Bernardino, the San Andreas Fault is where two
plates of the Earth's crust meet. The fault is approximately
800 miles long and extends 10 miles below the Earth's
surface. By looking at the movement of monuments
in the horizontal datum, geodesists can determine just how much the
surface of the Earth has moved after an earthquake (Schultz and Wallace,
1997).
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