31
What Is a Geographic Coordinate System?
mapping arises because there isn’t just one datum to use for all measurements
of Earth’s locations. In actuality, there are hundreds of datums being used.
Some datums are global and measure the entire world, while some are more
localized to a particular continent or region. Some of the more common datums you’ll likely encounter with geospatial data are:
a NAD27: The North American Datum of 1927. This datum was developed
for measurements of the United States and North America. It has its center point positioned at Meades Ranch in Kansas.
a NAD83: The North American Datum of 1983. This datum was developed
by the National Geodetic Survey and Canadian agencies and is used as
the datum for much data for the United States and the North American
continent as a whole.
a WGS84: The World Geodetic System of 1984. This datum was developed
by the U.S. Department of Defense and is used by the Global Positioning
System (GPS) for locating points worldwide on Earth’s surface (see Chapter 4 for more information about GPS).
Measurements made with one datum don’t necessarily line up with measurements made from a second datum. Problems would arise if you take one
set of data measured in one datum and your friend takes a second set of data
measured from a different datum and you place both sets of data together.
You have data of the same place, but your datasets wouldn’t match up because
you’re both using different forms of reference for measurement. For instance,
data measured from the NAD27 datum could be off by a couple hundred
meters from data measured from the NAD83 datum. This problem is compounded with the availability of several local datums, or reference surfaces,
designed to better fit a smaller geographic area. With the measurement differences that may arise, the best strategy to use when dealing with geospatial
data is to keep everything in the same datum. A datum transformation is
required to alter the measurements from one datum to another (for instance,
changing the measurements made in NAD27 to NAD83). Datum transformation is a computational process, but a standard one in many geospatial software packages. A datum can be used to set up a geographic coordinate system
(GCS) for measuring coordinates.
What Is a Geographic Coordinate System?
A geographic coordinate system (GCS) is a global reference system for determining the exact position of a point on Earth (see Figure 2.2 on page 32
for an example of GCS). GCS consists of lines of latitude and longitude that
cover the entire planet and are used to find a position. Lines of latitude (also
referred to as parallels) run in an east-to-west direction around the globe. The
Equator serves as the starting point of zero, with measurements north of the
Equator being numbers read as north latitude and measurements south of
NAD27 the North
American Datum of
1927.
NAD83 the North
American Datum of
1983.
WGS84 the World
Geodetic System of
1984 datum (used with
the Global Positioning
System).
datum transformation
changing
measurements
from one datum to
measurements in
another datum.
geographic
coordinate system
(GCS) a set of global
latitude and longitude
measurements used as
a reference system for
finding locations.
latitude imaginary
lines on a globe north
and south of the
Equator that serve as a
basis of measurement
in GCS.
Equator the line of
latitude that runs
around the center of
Earth and serves as the
0 degree line to make
latitude measurements
from.
What Is a Geographic Coordinate System?
mapping arises because there isn’t just one datum to use for all measurements
of Earth’s locations. In actuality, there are hundreds of datums being used.
Some datums are global and measure the entire world, while some are more
localized to a particular continent or region. Some of the more common datums you’ll likely encounter with geospatial data are:
a NAD27: The North American Datum of 1927. This datum was developed
for measurements of the United States and North America. It has its center point positioned at Meades Ranch in Kansas.
a NAD83: The North American Datum of 1983. This datum was developed
by the National Geodetic Survey and Canadian agencies and is used as
the datum for much data for the United States and the North American
continent as a whole.
a WGS84: The World Geodetic System of 1984. This datum was developed
by the U.S. Department of Defense and is used by the Global Positioning
System (GPS) for locating points worldwide on Earth’s surface (see Chapter 4 for more information about GPS).
Measurements made with one datum don’t necessarily line up with measurements made from a second datum. Problems would arise if you take one
set of data measured in one datum and your friend takes a second set of data
measured from a different datum and you place both sets of data together.
You have data of the same place, but your datasets wouldn’t match up because
you’re both using different forms of reference for measurement. For instance,
data measured from the NAD27 datum could be off by a couple hundred
meters from data measured from the NAD83 datum. This problem is compounded with the availability of several local datums, or reference surfaces,
designed to better fit a smaller geographic area. With the measurement differences that may arise, the best strategy to use when dealing with geospatial
data is to keep everything in the same datum. A datum transformation is
required to alter the measurements from one datum to another (for instance,
changing the measurements made in NAD27 to NAD83). Datum transformation is a computational process, but a standard one in many geospatial software packages. A datum can be used to set up a geographic coordinate system
(GCS) for measuring coordinates.
What Is a Geographic Coordinate System?
A geographic coordinate system (GCS) is a global reference system for determining the exact position of a point on Earth (see Figure 2.2 on page 32
for an example of GCS). GCS consists of lines of latitude and longitude that
cover the entire planet and are used to find a position. Lines of latitude (also
referred to as parallels) run in an east-to-west direction around the globe. The
Equator serves as the starting point of zero, with measurements north of the
Equator being numbers read as north latitude and measurements south of
NAD27 the North
American Datum of
1927.
NAD83 the North
American Datum of
1983.
WGS84 the World
Geodetic System of
1984 datum (used with
the Global Positioning
System).
datum transformation
changing
measurements
from one datum to
measurements in
another datum.
geographic
coordinate system
(GCS) a set of global
latitude and longitude
measurements used as
a reference system for
finding locations.
latitude imaginary
lines on a globe north
and south of the
Equator that serve as a
basis of measurement
in GCS.
Equator the line of
latitude that runs
around the center of
Earth and serves as the
0 degree line to make
latitude measurements
from.