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poles. The equatorial plane is constructed at the midpoint of this axis at right angles to it (Figure 1.3). A
great circle drawn through the poles is called a meridian, and an infinite number of great circles may be
constructed in this manner. Each meridian is divided into four quadrants by the equator and the poles.
The circle is arbitrarily divided into 360o, and each of these quadrants contains 90o.
1.5.2.1. Take a point on one of these meridians 30o N of the equator. Through this point passes a plane
perpendicular to the north-south axis of rotation. This plane will be parallel to the plane of the equator as
shown in Figure 1.3 and will intersect the earth in a small circle called a parallel or parallel of latitude.
The particular parallel of latitude chosen as 30o N and every point on this parallel will be at 30o N. In the
same way, other parallels can be constructed at any desired latitude, such as 10o, 40o, etc.
AFPAM11-216 1 MARCH 2001 23
Figure 1.3. Planes of the Earth.
1.5.2.2. Bear in mind that the equator is drawn as the great circle midway between the poles and parallels
of latitude are small circles constructed with reference to the equator. The angular distance measured on
a meridian north or south of the equator is known as latitude (Figure 1.4) and forms one component of
the coordinate system.
Figure 1.4. Latitude as an Angular Measurement.
1.5.3. Longitude. The latitude of a point can be shown as 20o N or 20o S of the equator, but there is no
way of knowing whether one point is east or west of another. This difficulty is resolved by use of the
other component of the coordinate system, longitude, which is the measurement of this east-west
distance. Longitude, unlike latitude, has no natural starting point for numbering. The solution has been to
select an arbitrary starting point. A great many places have been used, but when the English-speaking
24 AFPAM11-216 1 MARCH 2001
people began to make charts, they chose the meridian through their principal observatory in Greenwich,
England, as the origin for counting longitude. This Greenwich meridian is sometimes called the prime
meridian, though actually it is the zero meridian. Longitude is counted east and west from this meridian
through 180o, as shown in Figure 1.5. Thus, the Greenwich meridian is the 0o longitude on one side of
the earth and, after crossing the poles, it becomes the 180th meridian (180o east or west of the 0o
meridian).
Figure 1.5. Longitude Is Measured East and West of Greenwich Meridian.
1.5.4. Summary. In summary, if a globe has the circles of latitude and longitude drawn upon it
according to the principles described, any point can be located on the globe using these measurements
(Figure 1.6).
Figure 1.6. Latitude Is Measured From the Equator; Longitude From the Prime Meridian.
AFPAM11-216 1 MARCH 2001 25
1.5.4.1. It is beneficial to point out here some of the measurements used in the coordinate system.
Latitude is measured in degrees up to 90, and longitude is expressed in degrees up to 180. The total
number of degrees in any one circle is always 360. A degree (o) of arc may be subdivided into smaller
units by dividing each degree into 60 minutes (') of arc. Each minute may be further subdivided into 60
seconds (") of arc. Measurement may also be expressed in degrees, minutes, and tenths of minutes.
1.5.4.2. A position on the surface of the earth is expressed in terms of latitude and longitude. Latitude is
expressed as being either north or south of the equator, and longitude as either east or west of the prime
meridian.
1.5.4.3. In actual practice, map production requires surveyors to measure the latitude and longitude of
geographic objects in their area of interest. Local variation in the earth's gravity field can cause these
measurements to be inconsistent. All coordinates from maps, charts, traditional surveys, and satellite
positioning systems are thus tied to an individual mathematical model of the earth called a datum.
Coordinates for a given point may differ between datums by hundreds of yards. In other words, latitude
and longitude measured directly from observation of stars (called an astronomic coordinate) will be
consistent, but it may not match maps, charts, or surveyed points. The theoretical consistency of latitude
and longitude is therefore not achievable in reality. Without knowledge of the datum used to establish a
particular map or surveyed coordinate, the coordinate is suspect at best.
1.6. Distance. Distance, as previously defined, is measured by the length of a line joining two points.
The standard unit of distance for navigation is the nautical mile (NM). The nautical mile can be defined
as either 6,076 feet or 1 minute of latitude.
1.6.1. Sometimes it is necessary to convert statute miles to nautical miles and vice versa. This
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