The Elements of Geodesy: Datums |
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One way to think about a datum is as a
set of spatial information
that acts as a foundation
for other data, just
like concrete acts
as a foundation for
a building. Click
the image for
an animated view
and large images.
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Datums (sets of data) are the basis for all geodetic
survey work. They
act as reference points in the same way that starting
points do when you give someone directions. For
instance, when you want to tell someone how to get
to your house, you give them a starting point that
they know, like a road or a building. Geodesists and surveyors use
datums as starting or reference points when they create maps, mark
off property boundaries, and plan, design and build roads, bridges,
and other structures.
Another way to think about a datum is as a set
of information that acts as a foundation for other data. For example,
when a skyscraper is about to be built, the construction
team must first pour the foundation. Without this element, the skyscraper
would be unstable and unsafe. This is the same concept as a datum. While
a datum is a mathematical and geometric concept, it acts like the concrete
foundation of a skyscraper. Once the foundation is set, the construction
workers can build on top of it, creating the building's structure. After
the building is complete, offices or apartments can be created inside
the building. If the structure is an apartment building, its tenants
can bring in furniture and decorate as they please. Although the foundation
of the building probably isn't the first thing
on the minds of the tenants, without it, the building would not be a
safe place to live.
In geodesy, two main datums create the foundation
for navigation and transportation in the United States.
These datums --
called the horizontal and vertical datums -- make
up the National Spatial Reference System (NSRS).
Geodesists, surveyors, and people interested in precise positioning
use the NSRS as their foundation for reference.
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