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Figure 5.3. Global geodetic datum

Geodetic datum types include horizontal, vertical datums.
The Horizontal Datum define the relationship between the
physical Earth and horizontal coordinates such as latitude and

longitude. To create a horizontal datum, surveyors marked each of the
positions they had identified, typically with a brass, bronze, or
aluminum disk (referred to as a 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 mountain tops 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. One

application of the horizontal datum is monitoring the movement of the
Earth's crust.
The Vertical Datum is a collection of specific points on the
Earth with known heights either above or below the mean sea level.

Near coastal areas, the mean sea level is determined with a tide gauge.
In areas far away from the shore, the mean sea level is determined by
the shape of the geoid.

Similar to the survey markers used to identify known positions in
the horizontal datum, round brass plates mark positions in the vertical
datum. The traditional method for setting these vertical benchmarks is
called differential leveling. This method uses a known elevation at one

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