Environmental Engineering Reference
In-Depth Information
FIGURE 5.2 Transect sampling when two or more distinct soil types are
present. Note that both surface and subsurface (dots) samples in all
three soil types are taken.
altitude, and if it is moving, the direction and speed of movement. The GPS location
given is a map location, and so this system is directly linked to maps of the Earth's
surface. The maps must have the same units of geographical reference as displayed by the
GPS unit for both to be used together. Such units can also show the direction one needs to
go to get to an identified place on a map or on the Earth's surface.
5.2.1. Coordinates and Time
Global positioning systems depend first on a datum, second on such coordinates as
longitude and latitude, and third on time. There are different data for different parts of the
world, but in North America, the 1927 North American datum is used (commonly
referred to as NAD-27). (There is also an NAD-83.) In addition to the usual longitude
and latitude used on maps and globes, a coordinate system called the universal transverse
mercator (UTM) is also in use and must be available in the GPS unit. Before starting to
use a handheld GPS unit, make sure that it is reading the correct datum and is giving
locations in the appropriate format for the maps being used.
Time is kept using atomic clocks in satellites, and, in turn, time is used for determining
the position of the GPS receiving unit (i.e., GPS time). Using this very precise time
satellites can be kept synchronized and GPS units convert GPS time to the universal time
coordinate (UTC), which is the same as Greenwich mean time (GMT). From this point,
the local time can
 
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