Creating a Map Layout from Start to Finish - Selecting an Appropriate Display Projection

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Selecting Appropriate Map Projections

Selecting an appropriate map projection to properly display your data is both an art and a science. ArcMap is capable of "projecting on the fly", or displaying data is a projection it's not stored in. While it's important to remember, for accuracy and precision reasons, to keep you data all your data in the same projection for analysis reasons, it's not always necessary to re-project your data just to display it.

In order to select a projection, one must first define the parameters of the project. Asking questions such as:

  1. What is the map output?
  2. What is the overall purpose of the map? #Which of the six distortions (shape, area, distance, direction, bearing, scale) am I trying to reduce? Which of the six can I give up?
  3. What is the desired accuracy and precision of the map?
  4. What is the extent of the map?

By understanding what it is you need in a map and what you overall goal is, making a projection selection will be much easier.

Map Output

With so many options of map outputs (digital, GeoPDF, hardcopy, web maps, interactive web maps), the projection must first and foremost match the output. For digital, hardcopy, and static web use, the following guidelines are most likely appropriate, while maps designed for interactive web use might be best used with a special "Web Mercator" projection. The Web Mercator projection is a special version of the Mercator projection designed for use at all scales, allowing users to change the extent of the final product.

Overall Map Purpose and Distortions

One of the most important questions you can ask in regards to choosing the appropriate map projection is how will the output map be used for ''this project''. Defining the end use of the map will help you decided which of the six distortions you are willing to give up and which of the six are a definite keeper. For example, maps that will be used for navigation need to preserve distance, direction, and bearing and could probably do without shape or size, since the overall purpose is the get from point A to point B with the best travel plan. However, thematic maps, for which the purpose is to display attribute data and compare one geographic region to another, do not really need to preserve distance, direction, and bearing and would benefit much more from preserving shape, size, and scale.

Projections to Reduce Specific Distortion

Certain projections are designed to reduce specific error throughout a map. Starting with the developable surfaces, we will look at five of the more common methods used to reduce specific distortion: equal area, conformal, equidistant, true direction, and compromise.

Equal Area
The goal of equal area maps, as the name suggests, is to create a map where each of the land masses represented is given an equal amount of area. Equal area projections are useful where relative size and area accuracy of map features is important (such as displaying countries / continents in world maps), as well as for showing spatial distributions and general thematic mapping such as population, soil and geological maps. Looking at the distortion ellipses, we see the shape is distorted but the area remains constant throughout.  
Conformal
Conformal maps serve the purpose of preserving shape, distance, and bearing, at the expense of area and scale. Just like in the West Wing clip and the BuzzFeed "Maps Lie", it is explained continents away from the Equator are larger in size. When we understand that it is impossible to preserve all six characteristics and conformal maps, such as the Mercator projection, aim to preserve shape and distance, we then understand that Mercator had no intentions of "lying" to anyone, nor did he want to create social inequality. He just wanted a quality map to navigate with. Preservation of shape, distance, and bearing makes conformal map projections suitable for navigation charts, weather maps, topographic mapping, and large scale surveying. In the image, we see the distortion ellipses as circles. This tells us the shape is preserved, but area is distorted away from the Equator. Looking at this image, is the Mercator projection a tangent or secant developable surface? 
Equidistant

Equidistant, similar but different then conformal projections, aim to preserve distance, but only from the line or lines of tangency. This means that when you use an equidistant map, the measured distance from the place where the developable surface came into contact with the globe will be correct, but distances measured between other points will be incorrect. Equidistant projections are used in air and sea navigation charts, as well as radio and seismic mapping. They are also used in atlases and thematic mapping. In the image, we see the ellipses as circles which are not distorted in shape or size at the equator, but become increasingly so as you move away. When compared to the conformal example, we see the continents becoming distorted in shape, but the distortion ellipses do not stop far from the north and south edges. 

 
True Direction

Similar to the equidistant projection, which starts with a cylindrical developable surface, true direction starts with an azimuthal developable surface. Much like your oblique gnomonic projection, all directions and bearing away from the Washington Monument are preserved, but if you were to measure between Los Angeles and New York, the measurement will be incorrect. True-direction projections are used in applications where maintaining directional relationships are important, such as aeronautical and sea navigation charts.

Compromise

Compromise{img type="fileId" fileId="240" thumb="y" rel="box[g]" width="300" desc="Robinson projection" responsive="y" align="right" stylebox="border"} projections attempt to balance all of the distortions in one map. This means that none of the six are "perfect", but each one is is balance with the others, the idea being that no one place is grossly distorted in comparison to any other place on the map. Compromise maps are used to preserve the look of the finished product, a wall or book map, for example. Two common types of compromise maps are the Robinson and Winkel Tripel projection (both of which we will look at in lab). In the image, which is a Robinson projection, we see none of the ellipses are terribly distorted in size, shape, or distance from each other. But because they are all distorted in all six ways, this map wouldn't be perfect for navigation, nor preserving area for measurements, nor comparing the shapes to a globe.

 

Accuracy and Precision (and Distortions)

Second, and related to the overall map purpose, is the desired accuracy and precision of the output map. Again, the purpose must be known in order to define the desired accuracy and precision. If the map is for navigation, selecting a projection which preserves the accuracy of distance and direction measurements is of the utmost importance while a thematic display map has a much larger tolerance of error. Since we know that precision in mapping is how exactly a feature or attribute represents the best-known real world measurement, for a particular thematic map, the precision of the features and attributes being represented may the most important factor. Selecting a projection which preserves the distortion in a way that meets the project's demanded level of accuracy and precision is extremely important. !!!Mapped Extent (and Distortions) --- To review:

  1. The extent of a map is the maximum area displayed as defined by the geographic coordinates of the four corners.
  2. Distortion is increased the further away from the tangent or secant line or line one moves across the developable surface))
  3. Specific{img id=35 imalign="right" height="200"} projections are best used with large scale (small extent) areas while others are best used with small scale (large extent) areas. By determining the desired extent of the map, combined with the established distortions to reduce, selecting a projection becomes easier, since some projections are designed for large scale areas while other are designed for small scale areas.