The projection you choose for your particular mapping application should be driven by your map’s purpose and the size and shape of your area of interest. You can begin by asking yourself a set of questions that can help you narrow down which projection to choose (see Figure 7.cg.9, below). Below, we discuss each of these questions in detail.
What type of activities will the map be used for?
It is important to ask this question to determine whether there are particular map properties that you want to try to preserve. For example, if you are creating a thematic map that people may use to compare relative areas, you should probably choose a projection that preserves area (i.e., an equivalent projection). If you are creating a map for reference or navigation purposes (e.g., nautical or aviation map), where the angular relationships between features are important, it is best to choose a conformal projection (i.e., one that preserves angles). Other maps may be used to calculate or show true distances or directions (e.g., flight distances from one city to other cities around the world).
How much space do I want to map?
The larger the area you need to map, the more you need to think about where the distortion will fall on your map, as the total amount of distortion present on the map will be greater. Cylindrical or pseudo-cylindrical projections are often the best choice for very small-scale maps (i.e., those that aim to show the entire world). Conic projections are often a good choice for hemispherical or continental-sized areas. For example, the United States, Australia and Canada all commonly use Albers Equal Area and Lambert Conformal Conic (two different conic projections) for maps that show the entirety of each country. Azimuthal projections are best used for smaller areas, as they can only show one hemisphere at a time, and even then the distortion at the edges of these projections can be quite extreme.
Where is the area I want to map and how is it shaped?
You may make different choices about which projection to use depending on the location and shape of the area you would like to map. Projection aspect refers to the orientation of the projection’s aspect. A ‘normal’ projection is the projection orientation that produces the most symmetrical geometry for a particular class of projections. The ‘normal’ aspect for cylindrical projections is one that is centered along the equator, while the polar aspect (centered at the pole) is the normal aspect for azimuthal projections and an oblique aspect (a non-equatorial, non-polar aspect) is the normal aspect for conical projections. The term ‘transverse’ is reserved for cylindrical projections that are oriented at a 90° angle to their normal (equatorial) aspect.
Let’s look at the example of Chile because it has a somewhat unusual shape (i.e., it is longer and skinnier than most countries we might map). If Chile was oriented East-West instead of North-South, and was located near the equator, we might choose to simply use a standard Mercator projection for our map, as there would be minimal amounts of distortion of the country’s shape and area. However, because Chile lies roughly perpendicular to the equator (i.e., it runs North-South rather than East-West), a normal Mercator projection would significantly distort the country’s area in the south. One way to reduce this distortion is to use a different projection aspect (i.e., orientation), such as the Transverse Mercator (see Figure 7.cg.10, below).
The shape of the area you are trying to map may also influence your choice of projection case. There are two projection cases: tangent and secant. These terms refer to the number of places that the globe touches the ‘flattenable’ surface onto which a globe can be projected. In the tangent case, the globe touches the surface at one point or one line (depending on what surface (cylinder, cone or plane) the projection is based upon). In the secant case, the globe touches the surface at two points or two lines. If you recall that projection distortion increases away from the standard point or line (i.e., the point or line of tangency between the flattenable surface and the globe); it should be clear that using two standard lines will decrease the total amount of area that is badly distorted, because more of the map area will be close to the standard point or line (see Figure 7.cg.11, below). In the example of Chile that we used earlier, it is not really necessary to use more than one standard line, as most of the country will not be far from that line. However, in countries that have a more compact shape, such as the United States, cartographers often choose a secant projection.
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