Bias in maps is inevitable. The world is round, more or less, and maps are flat, so no map can truly reproduce the shape of land or sea. The larger the area the map covers, the greater will be the physical distortion.
Other factors introduce different kinds of bias. Anyone who makes a map has a way of looking at the world that influences the way he or she makes the map. So anyone who uses a map must take into account its author, their circumstances and their purpose. Mapmakers can introduce bias in the way people who use their maps think about the world.
Projection is the process of transferring the shapes of landforms from an oblate spheroid globe to a flat sheet of paper.
Some map projections show the world as if a cylinder of paper were wrapped around the globe and all the landforms were projected onto it. Though it is reasonably accurate near the equator, the common Mercator projection makes Greenland look bigger than Australia. It is not-Australia has three times the area of Greenland.
Other projections show the proportions of the continents fairly well, but distort the distances between them. Mapmakers must choose whether to distort size, shape, distance or relationships between features. Since a map is flat (and much more useful that way) something has to be distorted.
Purpose and place
The author or publisher of a map has a particular purpose in mind, and purpose distorts some maps. A map showing a new real estate subdivision, for example, may show the development well placed, near freeways and quite close to town. Somewhere on the map will appear the words 'not to scale.' The map has been distorted to make the subdivision more appealing to commuters.
The place a map was produced can cause bias, too. Some maps are strongly nationalistic, centering on a particular country or continent perhaps, and relegating other countries and cultures to the edges and corners of the map. Of course, every map has to center somewhere, but the best maps introduce as little bias as possible.
A political map made to show the states and their capitals to U.S. schoolchildren will be simplified in a particular way. It will look very different than a relief map a highway planner would use, or a weather map a meteorologist might study.
Since maps are smaller than the territory they show, all maps leave out something. Maps must be simplified, because there isn't room for everything and a map becomes confusing if too much is included. Therefore, it is important to notice what is left out of a map and what is emphasized. Mapmakers sometimes distort a user's idea of a location when they leave out information.
Be aware of bias
Users must consider who made a map, as well as when and where it was made. They should also consider what users the map was designed for, and where the mapmaker got the information to make it. Finally, users should consider what was left out of the map. What a mapmaker chooses to leave out tells a lot about a map.