AP Human Geography

What a map is for

A map is a selective picture of part of the Earth. The mapmaker decides what to show and what to leave out, so every map answers one kind of question better than others. Geographers sort maps into two broad groups according to the question they answer.

A reference map shows where places and features are. Road maps, atlas maps of countries and cities, and maps of rivers and mountain ranges are reference maps: their job is to let you find a location, a boundary or a route.

A thematic map shows the spatial pattern of one theme or variable, such as population density, average income, religion or rainfall. The place names and features are only there to anchor the pattern. Choosing a thematic map type depends on the kind of data and on what pattern the author wants a reader to see.

Types of thematic map

A choropleth map is best used with rates or densities rather than raw totals. Shading by total population would make a large, thinly settled state look more important than a small, densely settled one, because the shading hides the difference in area. Boundaries on a choropleth also imply that a value is the same across a whole region, which is rarely true inside it.

Elevation is shown on a topographic map by isolines called contour lines. Lines that are close together mean a steep slope, and lines that are far apart mean gently sloping land. Each contour line joins points at the same height above sea level.

Spatial patterns that maps reveal

Geographers describe what they see on a map using a small set of ideas.

• Absolute distance is the measured distance between two places in units such as miles or kilometers.
• Relative distance is the distance expressed as the time, cost or effort of getting between places. Two towns 60 miles apart can be close in relative terms if a fast highway links them, and far apart if the only route is a slow mountain road.
• Absolute direction uses the cardinal points (north, south, east and west) and the angles between them, such as northeast.
• Relative direction describes direction from a point of reference: left, right, toward the coast, down the valley.
• Clustering is when features are bunched together, as when stores gather around a highway junction. Dispersal is when they are spread out, as with farmhouses across a plain.
• Elevation is height above sea level, shown with contour lines or by color shading.

Describing a pattern means saying how the features are arranged (clustered or dispersed), where the high and low values lie, and what that arrangement suggests about the processes behind it. A dot map of a town's convenience stores might show a cluster along one road. That pattern points to the road as the factor drawing them, and a geographer would then test that explanation against other evidence.

Scale on a map

The scale of a map is the relationship between a distance on the map and the same distance on the ground. It can be written as a ratio such as 1:24,000, as a fraction, or as a bar scale drawn on the map.

The terms can confuse because they refer to the fraction. A large-scale map shows a small area in great detail, such as a city street map, and its ratio has a small number on the right (1:10,000 is a larger fraction than 1:1,000,000). A small-scale map shows a large area with little detail, such as a map of a continent. A world map has to be small scale.

Projections and distortion

The Earth is nearly a sphere, and a flat map cannot show a curved surface without stretching or tearing something. The method used to transfer the globe onto a flat surface is a map projection. Every projection distorts at least one of shape, area, distance and direction, so all maps are selective in what they preserve. A mapmaker picks the projection that keeps what matters for the map's purpose and accepts distortion in the rest.

• The Mercator projection keeps directions and the shapes of small areas accurate, which made it useful for sea navigation, but it greatly enlarges areas near the poles. Greenland looks about as large as Africa, although Africa is roughly fourteen times bigger.
• Equal-area projections, such as the Gall-Peters projection, keep the relative sizes of areas correct, but shapes are stretched, so continents look elongated.
• Compromise projections, such as the Robinson projection, give up perfect accuracy in any one property to keep all of them reasonably close, and they are common in atlases and classrooms.
• Interrupted projections such as Goode's homolosine split the oceans so that continents keep their shapes and sizes quite well, at the cost of cutting the map apart.

The choice of projection also carries a message. A map centered on one country, or one that makes some regions look larger, shapes how a reader thinks about their importance. A geographer reading any map therefore asks who made it, for what purpose and what the projection has done to the pattern.

Where geographic data comes from

Geographers use data about places that is either gathered directly or collected by others. Data may be quantitative (numbers such as population counts, rainfall totals or incomes) or qualitative (descriptions such as interviews and observations), and a good study usually uses both.

Information is gathered in the field by organizations or by individuals. A government agency may count a population, a company may survey customers about where they travel, and a researcher may walk a neighborhood recording land uses. Spatial information also comes from several kinds of account: notes made in the field, reports in the media, the narratives travelers wrote, government and organizational policy papers, and interviews with people. A geographer can also read the buildings, signs and land uses of a place as evidence of what goes on there, and interpret aerial and other photographs to see land use from above.

The census

A census is an official count of a population, in the United States carried out every ten years, which records characteristics such as age, sex, household size and housing. Census results are published by area, so they can be mapped and compared across places and over time. Surveys of a sample of the population fill in the years between counts.

Geospatial technologies

Modern mapping relies on technologies that locate, record and analyze information tied to a position on the Earth.

Geographic information systems (GIS) are computer systems that store, combine and display data in layers, each layer holding one kind of information for the same area, such as roads, rivers, land ownership and population. Layers can be switched on and compared. A planner might lay a map of flood-prone land over a map of homes to find which households are at risk, a question that would be slow to answer from separate paper maps.

Satellite navigation systems (the Global Positioning System in the United States, and equivalents run by other countries) use signals from satellites to fix a receiver's position. A phone giving directions, a delivery driver's route and a farmer's self-steering tractor all depend on them.

Remote sensing is gathering information about the Earth's surface from a distance, usually from satellites or aircraft, without touching the ground. It records land cover, crop health, city growth, deforestation and the spread of floods. Because a satellite passes over the same place repeatedly, images from different dates can be compared to show change.

Online mapping and visualization tools put these data in front of the public. Interactive maps can show traffic in real time, let users add their own information, and allow people with no training to explore patterns, although the quality of crowd-supplied information varies.

How geographic data is used in decisions

Geospatial and geographic data, including census data and satellite imagery, are used at every scale for personal, business, organizational and government decisions, and the decisions have effects on the ground.

• Individuals use maps and satellite data to choose routes, compare neighborhoods before they move, or check flood risk before they buy a house.
• Businesses use customer location data and census figures to choose where to open a store, how large a delivery area to serve, or where to advertise. A store may avoid a district because the data show few households of its target customers, and that choice affects local access to goods and jobs.
• Governments use population counts to draw voting district boundaries, decide how many representatives each area elects, allocate funding for schools and hospitals, and plan roads and transit lines. Emergency services use satellite images to direct aid after a hurricane or earthquake.

Because decisions follow from the data, a mistake or a gap in the data can lead to unequal results. A neighborhood that is undercounted in a census may receive less public funding than its real population needs. A route planned from traffic data may favor areas with more recorded trips and leave quieter neighborhoods with poorer service.

Limits of data

Data should always be read with its limits in mind.

• The data may be out of date. Census figures describe the day they were collected, and a fast-growing city changes quickly.
• The area chosen shapes the pattern seen. Averages for a large region can hide sharp differences between neighborhoods inside it, and results can change when the same data are grouped into different boundaries.
• Some people are missed or choose not to respond, so counts and surveys may under-represent particular groups.
• Qualitative sources such as interviews and media reports reflect the views of those who produced them and may be biased.
• Data collected for one purpose may not fit another, and the method of collecting it (what was counted, and how) may differ between places, so comparisons need care.

A reader assessing any map, table or image therefore asks who collected it, when, at what scale, and what it leaves out.