Imagine you’re standing at the exact center of the Earth, looking out at the surface from the underside as your friend stands at some point on the surface. You then draw a straight line from where you are at the Earth’s center to where your friend is standing on the surface; and a second straight line to the equator directly above or below your friend. The angle created by the two lines you’ve drawn is a portion of a circle (ϕ in the diagram at right), and so can be measured in degrees and its fractions, thus creating a north-south coordinate number we call “latitude”. The maximum value of any latitude measure is 90 degrees. The North Pole is 900 North and the South Pole is 900 South. The equator is 00, both north and south.
Now imagine you draw a third straight line; this one to the prime meridian at the point where it crosses the equator. The angle created by this line and your second line above is another portion of a circle (λ in the diagram at right), also measured in degrees and its fractions, called “longitude”. The maximum value of any longitude measure is 1800, or half of the circle. The meridian line directly opposite the prime meridian on the Earth is both 1800 East and 1800 West. Don’t confuse this 1800 meridian with the International Date Line, which diverges from it in several places for political and convenience reasons; for example to keep all islands of a chain in the same time zone.
The combination of these two latitude & longitude components specifies the position of any location on the surface of Earth, ignoring elevation. The origin/zero point of this system (latitude 00, longitude 00) is on the equator in the Gulf of Guinea about 625 km (390 mi) south of Tema, Ghana, Africa.
Coordinates in the latitude/longitude system can be expressed in three ways: a) degrees and decimal-degrees; b) degrees, minutes and decimal-minutes; or c) degrees, minutes, seconds and decimal-seconds. For example, coordinates for the approximate mid-point of San Francisco’s Golden Gate Bridge may be written as any of the following, all equivalent:
a) N 37.81988°, W 122.47848°
b) N 37° 49.193', W 122° 28.709'
c) N 37° 49' 11.58", W 122° 28' 42.54"
Note there are a couple of variations you may see in the way the above coordinates are written:
1.) The direction symbols (“N” and “W” in the above example) may be placed after the number; as in:
37.81988° N, 122.47848° W.
2) The direction symbols may be omitted, and the negative sign (“-“) used for latitudes south of the equator and longitudes west of the prime meridian; as in: 37.81988°, -122.47848°. This method is often used for data entry into a computer formula or database.
The Universal Transverse Mercator (UTM) is another geographic coordinate system you should be aware of. It’s less useful for very long (e.g., multi-state or continental) distances, but can be quite accurate for smaller distances; for example, within the San Francisco Bay Area. Another advantage of UTM is that calculating the distance between two points on UTM-based maps can be performed more easily in the field than by using the trigonometric formulas required under the latitude/longitude system. Further, coordinate designations are given in meters, making them easier to understand.
UTM divides the earth into 60 zones. The zones are imaginary bands that run most of the way between the North and South Poles. (The polar regions south of 80° S and north of 84° N are excluded.) Each zone is 6 degrees wide in longitude, thus covering the full 3600 of the Earth’s surface. Zone 1 begins at longitude 1800 (directly opposite the International Prime Meridian) and ends 60 east of there, at 1740 W longitude. The San Francisco Bay Area and most of northern California are in Zone 10, covering 1260-1200 W longitude.
Easting and Northing
The UTM system uses the concept of “easting and northing” to designate the x & y coordinates within a UTM zone, and so locate a specific point on the Earth’s surface. Although confusing at first, once understood, it can simplify location notation, since it a.) eliminates reference to two of the four compass directions (“S” and “W”); b.) uses easily understood x & y coordinates, with meters as its units, rather than more difficult angular degrees; and c.) eliminates negative coordinate references, as discussed in the Central Meridian and Southern Hemisphere sections below.
In the northern hemisphere, “northing” positions are measured in meters northward from zero at the equator. The maximum "northing" value is about 9,300,000 meters at latitude 840 N. The Golden Gate Bridge’s northing position is 4185959mN; or 4,185,959 meters north of the equator. Similarly, the “easting” coordinate is the number of meters from the UTM zone’s Central Meridian.
Rather than using a “zero reference” as in the latitude/longitude system, each UTM Zone has a “point of origin”. The point of origin is the intersection of the equator and the zone's central meridian. Northing coordinates are measured from the equator and Easting coordinates are measured from the central meridian. The point of origin for San Francisco’s zone 10 is 00 N latitude, 1230 W longitude.
Using the central meridian as a zero reference, however, would create negative coordinates. To avoid that, the central meridian is defined to have a value of 500,000 Easting. So locations west of the Central Meridian have easting coordinates less than 500,000 while those east of the Central Meridian are greater than 500,000. The Golden Gate Bridge’s easterly coordinate is 545899mE, and so is 45,899 meters east of longitude 1230 W (our central meridian).
In the southern hemisphere, the UTM system is modified slightly, again to avoid the use of negative coordinates. Instead of referring to the equator as zero meters northing, as it is for the northern hemisphere; the equator is defined with a value of 10,000,000 Northing; similar to the Central Meridian definition for easting.
Although technically part of the Military Grid Reference System (MGRS) instead of the UTM system, latitude bands are sometimes used in UTM. There are 20 latitude bands, each 80 high, extending from 800 south latitude to 840 north latitude. The northern-most band is extended an extra 40 to encompass all landmass in the northern hemisphere. These bands are designated with the English alphabet letters “C” through “X”; omitting the letters "I" and "O" because of their similarity to the numerals one and zero. Band C begins at 800 S and band X ends at 840 N. (Latitude bands "A" and "B", along with bands "Y" and "Z", cover the western and eastern sides of the Antarctic and Arctic regions respectively.)
When using the latitude band system, the combination of a UTM zone and a latitude band defines a grid zone. The UTM zone is written first, followed by the latitude band. Thus, the San Francisco Bay Area is in grid zone “10S”.