WEBVTT FILE

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Mapping and surveying professionals
occasionally use the term datum

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but what does it mean and why are datums important?

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A datum is simply a standard reference point,

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set of points or surface from which
survey measurements are based.

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It's similar to the starting point you
use when giving someone directions to your house,

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which usually include a point that they
know, like

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a crossroads or a prominent building. For
surveyors

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that starting point is usually a nearby
survey mark. When adjoining properties

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share these common starting points,

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its easier to align them properly on a
map. And when we all use a network of

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these marks spanning the continent, we
share one national spatial reference system.

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For many decades we have had two types of nationwide datums in the United States:

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horizontal and vertical, and they were
generally separate from one another.

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Horizontal datums allow us to measure
distances

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and directions across the surface of the
earth.

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Most horizontal datums define a zero
line at the equator

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from which we measure north and south.
There is also a zero line at the

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Greenwich Meridian

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from which we measure east and west.
Together

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these lines provide a reference for
latitude and longitude.

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Vertical datum are used to measure
elevations and water depth.

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Sometimes the word height is used to
refer to elevation information.

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Most vertical datums in North America use sea level

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as the basic reference plane from which
we measure elevation changes.

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Because sea level varies along the coast
for a variety of reasons,

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the idea of using multiple tide gauges
to define the vertical datum was

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abandoned in 1988

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in favor of using just one tide gauge.

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Now thanks to the Global Positioning
System commonly known as GPS

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and some other space-age techniques, we are no longer limited to separate

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horizontal and vertical datums.

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Today we have access to geometric
datums, which combine

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latitude/longitude, height and time
information.

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The height information derived from GPS
is relative to something called an

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ellipsoidal model of the earth

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instead of to sea level.

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While there are challenges mixing old and new positioning information,

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overall, new technologies like GPS help
to improve the accuracy of our current datums.

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The North American datum of 1983, NAD 83,

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is the nation's horizontal or geometric
datum. It provides latitude and longitude

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and some height information.

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The North American vertical datum of 1988, NAVD 88,

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is the nation's vertical datum, providing
elevation information relative to a

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reference surface defined by mean sea
level.

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US territories have separate vertical datums.

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Unfortunately NAD 83 and NAVD 88 aren't the only datums you'll encounter.

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Before the current datums were defined,
many maps were created using different

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starting points.

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And even today, people continue to change datums in an effort to make them more accurate.

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When combining data from different users or eras, it is important to transform

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all information to a common datum.

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Let's look at some examples of when and
why using a consistent datum is important.

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Datum are commonly referenced on federal
flood plain and topographic maps.

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Sometimes an older vertical datum
for the United States is used

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rather than NAVD 88.

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In some areas in the country, especially
in the West, differences between some

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vertical datums can be as much as three to four feet.

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This is pretty significant if you're trying
to figure out if your house is in a

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flood plain.

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It's really important know which datum your heights are referenced to.

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For any type of work where it's
important for coordinates to be

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consistent with each other,

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it is critical that the same datum is
used.

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If you are creating flood plain maps,
marking property or land boundaries,

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designing levees or dams, building roads
 or planning for coastal inundation scenarios,

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you must know about and use the correct
datums.