33
What Is a Geographic Coordinate System?
and 103 degrees, 27 minutes, 56.46 seconds west of the Prime Meridian
(103°27′56.46″ W longitude).
Negative values can also be used when expressing coordinates using latitude and longitude. When distance measurements are made west of the Prime
Meridian, they are often listed with a negative sign to note the direction. For
instance, the location at Mount Rushmore, being west longitude, could also be
written as Ϫ103°27′56.46″ longitude. The same use of negative values holds
true when making measurements of south latitude (below the Equator).
GCS coordinates can also be expressed in their decimal equivalent, referred to as decimal degrees (DD). In the decimal degrees method, values
for minutes and seconds are converted to their fractional number of degrees.
For instance, 1 degree and 30 minutes of latitude (in DMS) would equate to
1.5 degrees in DD (since 30 minutes is equal to 0.5 degrees). Using the Mount
Rushmore coordinates as an example, the latitude measurement in decimal
degrees would be 43.881494 north latitude (since 52 minutes and 53.38 seconds is equal to 0.881494 of one degree) and 103.465683 west longitude.
Using this system, any point on Earth’s surface can be precisely measured and identified. Keep in mind that these locations are being made on a
3D globe (in reference to a datum), so measurements between points have to
take this into account. When making measurements on a sphere, the shortest
distance between two points is referred to as the great circle distance. Thus,
if you wanted to find the shortest distance between the cities of New York and
Paris, you would have to calculate the great circle distance between two sets
of coordinates (see Hands-on Application 2.1: Great Circle Distance Calculations for more information about using the great circle distance).
The latitude and longitude system of GCS also serves as the basis for breaking Earth up into multiple time zones to account for difference in time around
the world. Time zones set up the concept of a 24-hour day, with each zone having a different time as it relates to the current time in Greenwich, England (also
called Greenwich Mean Time or GMT). The Earth spans 360 degrees of longitude, and dividing that by 24 hours equals 15 degrees of longitude for each
hour. Thus, every 15 degrees away from the time zone containing the Prime
Meridian marks the start of a new time zone (Figure 2.3 on pages 34–35).
decimal degrees
(DD) the fractional
decimal equivalent
to coordinates found
using degrees, minutes,
and seconds.
great circle distance
the shortest distance
between two points on
a spherical surface.
time zones a method
of measuring time
around the world, found
by dividing the world
into subdivisions and
relating the time in that
division to the time in
Greenwich, England.
To find the real-world distance around the planet
between two cities—New York and Paris—you’ll
need the great circle distance between the two.
To quickly calculate great circle distances, use the
Surface Distance application online at http://www.
chemical-ecology.net/java/lat-long.htm. Use the
option for Great Circle Distance Between Cities to
find the real-world distance between New York and
Paris. Use it to find the city closest to you and calculate the great circle distance between sets of cities. Also, use a program like Google Earth to obtain
the GCS coordinates (latitude and longitude) for a
nearby location and find the surface distance from
there to a nearby city.
Hands-on Application 2.1
Great Circle Distance Calculations
What Is a Geographic Coordinate System?
and 103 degrees, 27 minutes, 56.46 seconds west of the Prime Meridian
(103°27′56.46″ W longitude).
Negative values can also be used when expressing coordinates using latitude and longitude. When distance measurements are made west of the Prime
Meridian, they are often listed with a negative sign to note the direction. For
instance, the location at Mount Rushmore, being west longitude, could also be
written as Ϫ103°27′56.46″ longitude. The same use of negative values holds
true when making measurements of south latitude (below the Equator).
GCS coordinates can also be expressed in their decimal equivalent, referred to as decimal degrees (DD). In the decimal degrees method, values
for minutes and seconds are converted to their fractional number of degrees.
For instance, 1 degree and 30 minutes of latitude (in DMS) would equate to
1.5 degrees in DD (since 30 minutes is equal to 0.5 degrees). Using the Mount
Rushmore coordinates as an example, the latitude measurement in decimal
degrees would be 43.881494 north latitude (since 52 minutes and 53.38 seconds is equal to 0.881494 of one degree) and 103.465683 west longitude.
Using this system, any point on Earth’s surface can be precisely measured and identified. Keep in mind that these locations are being made on a
3D globe (in reference to a datum), so measurements between points have to
take this into account. When making measurements on a sphere, the shortest
distance between two points is referred to as the great circle distance. Thus,
if you wanted to find the shortest distance between the cities of New York and
Paris, you would have to calculate the great circle distance between two sets
of coordinates (see Hands-on Application 2.1: Great Circle Distance Calculations for more information about using the great circle distance).
The latitude and longitude system of GCS also serves as the basis for breaking Earth up into multiple time zones to account for difference in time around
the world. Time zones set up the concept of a 24-hour day, with each zone having a different time as it relates to the current time in Greenwich, England (also
called Greenwich Mean Time or GMT). The Earth spans 360 degrees of longitude, and dividing that by 24 hours equals 15 degrees of longitude for each
hour. Thus, every 15 degrees away from the time zone containing the Prime
Meridian marks the start of a new time zone (Figure 2.3 on pages 34–35).
decimal degrees
(DD) the fractional
decimal equivalent
to coordinates found
using degrees, minutes,
and seconds.
great circle distance
the shortest distance
between two points on
a spherical surface.
time zones a method
of measuring time
around the world, found
by dividing the world
into subdivisions and
relating the time in that
division to the time in
Greenwich, England.
To find the real-world distance around the planet
between two cities—New York and Paris—you’ll
need the great circle distance between the two.
To quickly calculate great circle distances, use the
Surface Distance application online at http://www.
chemical-ecology.net/java/lat-long.htm. Use the
option for Great Circle Distance Between Cities to
find the real-world distance between New York and
Paris. Use it to find the city closest to you and calculate the great circle distance between sets of cities. Also, use a program like Google Earth to obtain
the GCS coordinates (latitude and longitude) for a
nearby location and find the surface distance from
there to a nearby city.
Hands-on Application 2.1
Great Circle Distance Calculations