Latitude and Longitude

What latitude and longitude are

Latitude and longitude use angular measurements to describe a position on the surface of the Earth. The system has been in use, with little change, since Ptolemy used it in his first world atlas in A.D. 150.

Lines of latitude measure north-south position between the poles. The equator is 0°, the North Pole is 90° north, the South Pole is 90° south. Lines of latitude are all parallel to each other, which is why they're often called parallels.
Lines of longitude, or meridians, run between the North and South Poles. They measure east-west position. The prime meridian is assigned the value of 0° and runs through Greenwich, England. Meridians to the west of the prime meridian are measured in degrees west; those to the east, in degrees east.

A point like 40.0150° N, 105.2705° W puts you in Boulder, Colorado.

How big is a degree?

Latitude is straightforward because a degree of latitude represents a constant distance on the ground:

1° of latitude ≈ 60 nautical miles, 69 statute miles, or 111 km.
1 minute of latitude ≈ 1 nautical mile, 1.15 statute miles, or 1.85 km.

Longitude is trickier. A degree of longitude represents a varying distance on the ground, depending on the latitude — meridians converge as they head toward the poles, so a degree of longitude shrinks as you head north or south from the equator.

Two memory aids that help:

"Tropical latitudes improve my attitude" — to remember that latitude indicates north-south position.
"Lines of LONGitude are all just as LONG as one another" — visualized on a globe, all meridians run pole-to-pole and are the same length, even though the distance between meridians varies with latitude.

Who uses it

Mariners and aviators have been the primary users of lat/lon. The system is used worldwide and many different types of maps carry lat/lon markings.

Most GPS receivers ship configured for lat/lon as their default factory setting. That means most new GPS users start out using lat/lon coordinates whether they intended to or not. There are other geographic coordinate systems — see UTM Coordinates, MGRS Coordinates, USNG Coordinates — that may be better suited to your task and easier to use, but lat/lon is the universal lingua franca that every other system can convert to.

Three formats

The same point on the ground can be written three ways. Students need to recognize all three and convert between them.

Degrees, Minutes, Seconds (DMS)

Written as DDD° MM' SS.S". Example: 32° 18' 23.1" N 122° 36' 52.5" W.

This is the most common format used to mark maps. It's also the most cumbersome to work with: there are sixty seconds in a minute (60" = 1') and sixty minutes in a degree (60' = 1°), so arithmetic is base-60.

Quarter-minute conversions worth memorizing:

15 seconds is one quarter of a minute, or 0.25'.
30 seconds is one half of a minute, or 0.5'.
45 seconds is three quarters of a minute, or 0.75'.

Degrees and Decimal Minutes (DDM)

Written as DDD° MM.MMM'. Example: 32° 18.385' N 122° 36.875' W.

This is the format most commonly used when working with electronic navigation equipment.

Decimal Degrees (DD)

Written as DDD.DDDDD°. Example: 32.30642° N 122.61458° W, or with signed values: +32.30642, -122.61458.

This is the format you'll find most computer-based mapping systems displaying.

Sign conventions in decimal degrees

Positive latitude is north of the equator, negative is south.
Most programs use negative longitude for west of the prime meridian — but a few do it the other way around. Always check.

A working preference: many users (including John) reach for degrees and decimal minutes even when working from USGS maps that are marked in degrees, minutes, and seconds. DDM is the sweet spot — accurate, no base-60 arithmetic, and matches what GPS receivers display.

Plotting on a topo map

The ticks at the corners of a USGS topo map mark the lat/lon grid. Plotting a coordinate is the operation of finding where a given lat/lon falls on the map sheet.

Latitude (the easy direction)

Lines of latitude are parallel and evenly spaced, so a degree of latitude is a constant distance on the map.

Place a ruler spanning the latitude lines, where the point falls between them.
Orient the ruler north-south.
In the northern hemisphere, position the ruler's zero-minute end on the southern latitude line.
Read the value at the point and add it to the southern line's latitude.

Example: a point marked X reads at the 4' mark on a 37° line → 37° 34' N.

Longitude (the harder direction)

Because meridians converge, you can't just lay the ruler horizontally between two longitude lines — the ruler's spacing won't match the map's spacing.

Place the ruler diagonally spanning the longitude lines, with each end touching a marked longitude line.
Slide the ruler up or down vertically while keeping both ends on the longitude lines.
Slide until the ruler's edge touches the point you're measuring.
Extend the longitude lines above or below the map if you need to.

Example: point X reads at the 3.5' mark → 122° 3.5' W.

The point of interest is located where the plotted lines of latitude and longitude cross.

A custom ruler eliminates the diagonal

Plotting longitude diagonally works but is awkward. MapTools' custom MapRulers carry a longitude scale adjusted for a particular latitude — within a 1° to 2° band of latitude, the ruler can be placed perpendicular to the longitude lines and read directly. Easier to use, harder to teach with, since the diagonal method makes the convergence problem visible.

Choosing a coordinate system

For guidance on when to use lat/lon vs. UTM vs. MGRS vs. USNG, see Selecting a Coordinate System.