Location on maps
Most maps allow us to
specify the location of points on the Earth's surface using a coordinate
system. For a two-dimensional map, this coordinate system can use simple
geometric relationships between the perpendicular axes on a grid system to
define spatial location Figure 2b-1 illustrates how the location of a point can be defined
on a coordinate system.
Figure 2b-1:
A grid coordinate system defines the location of points from the distance
traveled along two perpendicular axes from some stated origin. In the example
above, the two axes are labeled X and Y. The origin is located in the lower
left hand corner. Unit distance traveled along each axis from the origin is shown. In this coordinate system, the
value associated with the X-axis is given first, following by the value
assigned from the Y-axis. The location represented by the star has the
coordinates 7 (X-axis), 4 (Y-axis).
Two types of coordinate systems are currently in general use in geography:
ii. rectangular (also
called Cartesian coordinate system
)
Geographical Coordinate System
The geographical coordinate system
measures location from only two values, despite the fact that the locations are
described for a three-dimensional surface. The two values used to define
location are both measured relative to the polar axis of the
Earth. The two measures used in the geographic coordinate system are called latitude and longitude.
Figure 2b-2:
Lines of latitude or parallels are drawn
parallel to the equator (shown in red) as circles that span the Earth's
surface.
i.
These parallels are measure in
degrees (°).
ii.
There are 90 angular degrees
of latitude from the equator to each of the poles.
iii.
The equator has an assigned
value of 0°.
Measurements of latitude are also defined as being either north or south of
equator to distinguish the hemisphere of their location.
Lines of longitude or meridians are circular
arcs that meet at the poles. There are 180° of longitude either side of a
starting meridian which is known the Prime Meridian. The
Prime Meridian has a designated value of 0°. Measurements of longitude are also
defined as being either west or east of the Prime Meridian.
Latitude
Latitude measures the north-south position of locations on the Earth's surface
relative to a point found at the center of the Earth (Figure 2b-2). This
central point is also located on the Earth's rotational or polar axis. The equator
is the starting point for the measurement of latitude. The equator has a value of zero degrees. A line of latitude or parallel of 30° North
has an angle that is 30° north of the plane represented by the equator (Figure
2b-3). The maximum value that latitude can attain is either 90° North or
South. These lines of latitude run parallel to the rotational axis of the
Earth.
Figure 2b-3:
Measurement of latitude and longitude relative to
the equator and the Prime Meridian and the
Earth's rotational or polar axis
Longitude
Longitude measures the west-east position of locations on the Earth's surface
relative to a circular arc called the Prime Meridian (Figure
2b-2). The position of the Prime Meridian was determined by international
agreement to be in-line with the location of the former astronomical
observatory at Greenwich, England. Because the Earth's
circumference is similar to circle, it was decided to measure longitude in
degrees. The number of degrees found in a circle is 360. The Prime Meridian has
a value of zero degrees. A line of longitude or meridian of 45° West
has an angle that is 45° west of the plane represented by the Prime Meridian (Figure
2b-3). The maximum value that a meridian of longitude
can have is 180° which is the distance halfway around a circle. This meridian
is called the International Date Line.
Designations of west and east are used to distinguish where a location is found
relative to the Prime Meridian. For example, all of the locations in North
America have a longitude that is designated west.
Direction on Maps
Like distance, direction is difficult to measure on maps because of the
distortion produced by projection systems. However, this distortion is quite
small on maps with scales larger than 1:125,000. Direction is usually measured
relative to the location of North or South Pole. Directions
determined from these locations are said to be relative to True North or True South. The
magnetic poles can also be used to measure direction. However, these points on
the Earth are located in spatially different spots from the geographic North
and South Pole.
The North Magnetic Pole is
located at 78.3° North, 104.0° West near Ellef Ringnes Island, Canada. In the
Southern Hemisphere, the South Magnetic Pole is
located in Commonwealth Day, Antarctica and has a geographical location of 65°
South, 139° East. The magnetic poles are also not fixed overtime and shift
their spatial position overtime.
Topographic maps
Topographic maps normally have a declination diagram drawn on them (Figure
2b-8). On Northern Hemisphere maps, declination diagrams describe the
angular difference between Magnetic North and True North. On the map, the angle
of True North is parallel to the depicted lines of longitude. Declination
diagrams also show the direction of Grid North. Grid North
is an angle that is parallel to the easting lines found on
the Universal Transverse Mercator
(UTM) grid system (Figure
2b-8).
Figure 2b-8:
This declination diagram describes the angular difference between Grid,
True, and Magnetic North. This illustration also shows how angles are measured relative grid, true,
and magnetic azimuth. In the field, the direction of features is often determined by a magnetic
compass which measures angles relative to Magnetic North.
Using the declination diagram found on a map, individuals can convert their
field measures of magnetic direction into directions that are relative to
either Grid or True North.
Compass directions can be described by using either the azimuth system or the bearing system. The
azimuth system calculates direction in degrees of a full circle. A full circle
has 360 degrees (Figure 2b-9). In the azimuth system, north has a direction
of either the 0 or 360°. East and west have an azimuth of 90° and 270°,
respectively. Due south has an azimuth of 180°.
Figure 2b-9:
Azimuth system for measuring direction is based on the 360 degrees found in a full
circle. The illustration shows the angles associated with the major cardinal
points of the compass. Note that angles are determined clockwise from north. The bearing system divides direction into four quadrants of 90 degrees. In
this system, north and south are the dominant directions. Measurements are
determined in degrees from one of these directions. The measurement of two
angles based on this system are described in Figure 2b-10.
Figure 2b-10:
The bearing system uses
four quadrants of 90 degrees to measure direction. The illustration shows two
direction measurements. These measurements are made relative to either north or
south. North and south are given the measurement 0 degrees. East and west have
a value of 90 degrees. The first measurement (green) is found in the
north - east quadrant. As a result, its measurement is north 75 degrees to the
east or N75°E. The first measurement (orange) is found in the south -
west quadrant. Its measurement is south 15 degrees to the west or S15°W.
