Projecting the Earth
Unlike Earth, maps are flat. Though Earth can also be represented as a globe, globes are inconvenient, expensive, and challenging to design. Maps are much more convenient: they are easier both to produce and to reproduce, and are better suited for displaying detailed data. The process of transforming latitude and longitude values from the 3D earth onto a 2D surface (a map) is called map projection.
In the past, cartographers were tasked with projecting maps by hand. Fortunately, GIS software such as ArcGIS is now able to perform this task of projection for us mathematically. Though manual map projection is uncommon today, terms from this era of map production are still in use and are helpful for conceptualizing how the process of map projection works.
To create a map, cartographers transfer a model of the earth as it appears on a reference globe to a developable surface.
A reference globe is a model of Earth (including landmasses, oceans, and the graticule) at some chosen scale, which is the final scale of the map to be created (Slocum et. al 2009). This projected map is thus modeled from an imaginary scaled-down version of Earth.
A developable surface is a mathematically-definable surface onto which landmasses and the graticule (lines of latitude and longitude) are projected (Slocum et. al 2009). In simpler terms, a developable surface is any surface that can be “unrolled” flat (and thus, create a 2D map). Typically, either a cone, a plane (flat surface), or a cylinder is used. In this next section, we discuss how the choice of a developable surface—among other factors—influences a map projection's characteristics.
Imagine the cone developable surface as a party hat placed on top of Earth. After projection, which locations do you imagine would appear the least distorted on the resulting map? Which would appear the most distorted?
- Chapter 9: Elements of Map Projections. Slocum, Terry A., Robert B. McMaster, Fritz C. Kessler, and Hugh H. Howard. 2009. Thematic Cartography and Geovisualization. Edited by Keith C. Clarke. 3rd ed. Upper Saddle River, NJ: Pearson Prentice Hall.