The National Spatial Reference
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A
United States triangulation network map from 1937. This system was a precursor
to today's modern National Spatial Reference System. Click
on the image for
larger view.
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All of the elements of geodesy are joined together in the National Spatial Reference System (NSRS). For almost 200 years, the National Geodetic Survey (NGS) and its predecessors have been using geodesy to map the U.S. shoreline, determine land boundaries, and improve transportation and navigation safety. NGS evolved from the Survey of the Coast, an agency established by Thomas Jefferson in 1807. The creation of the United States' first civilian scientific agency was prompted by the increasing importance of waterborne commerce to the fledgling country. As the nation grew westward, NGS's mission began to include surveys of the North American interior.
With numerous surveys
being conducted simultaneously across the growing nation, the surveyors
needed to establish a common set of reference points. This would insure
that surveyors' maps and charts, which often covered hundreds of miles,
would align with each other and not overlap. The common set of reference
points they used were the benchmarks from the horizontal and vertical
datums. Today, the complete set of vertical and horizontal benchmarks
for the United States is known as the NSRS. This defined group of reference
points acts as the foundation for innumerable activities requiring accurate
geodetic information.
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The
National Spatial Reference System
(NSRS) is as the foundation for
many activities requiring accurate
spatial information. Click
on the image to see
an animated example of how the NSRS works. |
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Think of it this way: When construction workers
begin to build, they have to be sure that the area
where they are building is free from dangerous
power lines. The construction team will have to
find out where the power lines are and make sure
they are not building on top of them. To ensure
success, the team needs to know the coordinates
of the building site and of the local power lines.
The NSRS provides a framework for identifying these
coordinates. The team can then compare the two sets of coordinates
and make sure they do not overlap.
To identify the benchmarks
in the NSRS, NGS has traditionally
placed markers, or permanent monuments, where the
coordinates have been determined. These markers are
brass or bronze disks (metals that sustain weathering) and are set
in concrete or bedrock. Each marker is about 9 centimeters wide and
has information about NGS printed on its surface.
With the advent of
the Global Positioning System (GPS), NGS began
to use different kinds of markers. These are made
from long steel rods, driven to refusal (pushed
into the ground until they won't go any farther.)
The top of each rod is then covered with a metal
plate. This method ensures that the mark won't
move and that people can't destroy or remove it.
After tying these marks into a specific horizontal
or vertical datum, the mark can be included in the
NSRS database. Once the coordinates of the mark
are entered into this database, they are available
for anyone to use.
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