Longitude in AP Human Geography

Meridians run pole to pole yet measure east-west position

The lines and the measurement point in different directions. A meridian is drawn north to south, but its number tells how far east or west a place sits. Moving along one meridian changes your latitude while your longitude stays the same.

The values split at the Prime Meridian. Places in the Americas have west longitudes, and most of Asia has east longitudes. East and west meet at 180°, on the opposite side of the globe from Greenwich, and the International Date Line roughly follows that meridian.

All meridians meet at the poles, so the ground distance between them shrinks as you move away from the Equator. One degree of longitude covers about 111 km at the Equator and zero at the poles. Lines of latitude stay about the same distance apart everywhere. That difference becomes important when maps get flattened.

Longitude as the east-west half of absolute location in Topic 1.1

A coordinate like 40.7°N, 74°W (close to New York City) is an absolute location. It names one spot on Earth no matter who reads it, and the 74°W is the longitude half. That idea comes from Topic 1.1 (Introduction to Maps and Types of Maps) in Unit 1: Thinking Geographically, which asks what maps can show, including absolute and relative distance and direction.

Relative location describes a place by its connection to other places, such as "at the mouth of the Hudson River, south of Albany." Both kinds of description can be correct for the same city. They answer different questions. Coordinates say exactly where a place is. Relative location helps explain why the place grew there, through its access to rivers, ports, or trade routes.

By convention, latitude is written first and longitude second, each with its direction letter. A value like "74°" with no E or W could be on either side of the Prime Meridian, so it can't serve as an absolute location.

How map projections draw meridians

Topic 1.1 also expects you to know that every projection distorts shape, area, distance, or direction. On a globe, meridians come together at the poles. Different projections draw them differently. Some keep meridians straight and let them meet at a pole, some curve them, some draw them as parallel lines, and some cut the map into pieces. Even when meridians stay straight and meet at a pole, the flat map still distorts other spatial relationships.

The Mercator projection draws meridians as parallel vertical lines that never meet. It's built so that a route with a constant compass bearing appears as a straight line, which is useful for navigation. It also greatly enlarges land near the poles, which is why Greenland looks about the size of Africa. Goode's homolosine projection is an equal-area projection, so it keeps the relative sizes of landmasses accurate. It cuts the map through the oceans, and those interruptions help limit shape distortion over the continents, but the oceans end up broken apart.

When a question shows an unfamiliar projection, the meridians give you clues, but they don't tell the whole story by themselves. On a Mercator map, parallel meridians together with latitude lines that spread farther apart toward the poles show that high-latitude areas are enlarged. Curved meridians alone don't tell you whether a projection preserves area, so check the projection's stated properties or label.

Latitude-longitude coordinates behind GPS and GIS in Topic 1.2

Topic 1.2 (Geographic Data) lists geospatial technologies as one way geographic data are collected. Coordinates connect them all:

• Satellite navigation systems such as GPS calculate a receiver's position and report it as latitude and longitude.
• GIS stores data in layers and uses coordinates to line those layers up on the same base map.
• Remote sensing images are matched to coordinates so each pixel lines up with a real spot on the ground.
• Online mapping tools turn a typed address into coordinates before placing a pin.

If an answer choice describes recording exact locations during fieldwork with a GPS unit, longitude is part of the data being collected.

Picking the coordinate answer and avoiding the latitude swap

In a multiple-choice question, longitude can appear as part of an absolute-location description. One style of question gives several descriptions of a city and asks which one uses absolute location. The correct choice has coordinates. Choices built on landmarks, travel time, or "near" and "between" describe relative location. Another style describes using compass directions and latitude-longitude coordinates to pinpoint a place and asks for the term, which is absolute location.

In free response, Question 2 (One-stimulus question) and Question 3 (Two-stimuli question) can use a map as the stimulus. A strong answer uses "absolute location" and "relative location" accurately and gives coordinates with direction letters when the map has a grid.

A common mistake is swapping the two coordinates. Longitude measures east-west from the Prime Meridian and goes up to 180°. Latitude measures north-south from the Equator and goes up to 90°. A value over 90° must be longitude. A related mistake is assuming meridians measure north-south because they run north-south.