Section 1.3 Map projections and geographic coordinate systems
Whenever we put something on a map, we need some sort of system to pinpoint the location. A coordinate system allows you to integrate any dataset with other geographical datasets within a common framework. With the help of coordinate reference systems (CRS) every place on Earth can be specified by a set of numbers, called coordinates. In general CRS can be divided into projected coordinate reference systems (also called Cartesian or rectangular coordinate reference systems) and geographic coordinate reference systems.
A geographic coordinate system is a three-dimensional reference system that enables you to locate any location on Earth. This coordinate system is round, and so, records locations in angular units (usually degrees). The use of geographic coordinate reference systems is very common. The most popular is called the World Geodetic System (WGS 84) which refers to locations using "Longitude", "Latitude", and in some cases "Elevations". For example, to communicate where is the University of Manchester, we would say it is Longitude: -2.2360724 degrees and Latitude: 53.466853 degrees.
Projected coordinate systems, or map projections, try to portray the surface of the Earth or a portion of the Earth on a two-dimensional flat piece of paper or computer screen. Therefore, location is recorded in linear units (usually meter). Working, for example, with data in the UK, we would use British National Grid (BNG). In this case, points will be defined by "Easting" and "Northing" rather than "Longitude" and "Latitude". It basically divides the UK into a series of squares, and uses references to these to locate something. The most common usage is the six-figure grid reference, employing three digits in each coordinate to determine a 100 m square. For example, the grid reference of the 100 m square containing the summit of Ben Nevis is NN 166 712. The more digits are included, the more precisely we can refer to the location of interest.
All projections of a sphere like the Earth in a two-dimensional map involve some sort of distortion. You can’t fit a three-dimensional object into two dimensions without doing so. Projections differ to a large extent on the kind of distortion that they introduce. This will be important later on when we are linking data from different projections, or when you look at your map and you try to figure out why it might look "squished".
The decision as to which map projection and coordinate reference system to use, depends on the regional extent of the area you want to work in, the analysis you want to do, and often the availability of data. Knowing the system you use allows you to translate your data into other systems whenever this may be necessary. Often you may have to integrate data that is provided to you in different coordinate or projected systems. As long as you know the systems, you can do this, and we will be demonstrating this throughout the book.
You will often see the notation epsg when referring to different coordinate systems. This refers to the EPSG registry or EPSG Geodetic Parameter Dataset. It is a collection of geodetic datums, spatial reference systems, Earth ellipsoids, coordinate transformations, and related units of measurement. All standard coordinate systems will have one of these codes assigned to it. So, for example, the WGS84 coordinate system corresponds the epsg key 4326, whereas the British National Grid has the key 27700. Having this common framework to identify projections makes things much easier when we want to change our data from one system to another. You can query the website https://epsg.io/ for the different keys and information associated to each of them. This EPSG dataset was originally developed by the European Petroleum Survey Group (EPSG), thus the name. You can also define the coordinate system in R using ways other than an EPSG code (for details see [115]).
A word of caution about WGS84. Although it is very popular (global positioning systems rely upon it) and we will be using it a lot, it has a problem: it is a dynamic reference frame. Due to plate tectonic motion, there are changes over time and the coordinates are adjusted to account for this. The implications are that WGS84 coordinates derived by users from GPS measurements may appear to move over time. This means that we need to know the time of our spatial feature (say, the time of a criminal event) if we want the correction needed as a result of plate tectonics. The problem is not simply of data availability (say, you don’t have the exact timing of the event that you want to place in a map), but of data structures. Traditionally, the file formats that we have used to store this information do not lead themselves easily to record this information so that we can then easily address these inaccuracies. Progress has been made in recent times, but you need to be aware that WGS84 is not perfect despite its popularity. Some European guidelines suggest, for example, the use of ETRS89 in Europe as a consequence of this.
