If this is the first time you are joining the Map Reading series, you should start from the first post.
A grid system on a map is usually square and is represented by drawn lines on the map creating those squares. The purpose of the grid system is to give each point in the map an identifier, an address, by which we can refer to it by. Grid maps are inherently very simple and they usually use a cartesian system (X,Y system) that gives a point an X address and a Y address, resulting in an X,Y point.
A grid system is only good if it can be used to describe that address universally, and for that we have coordinate systems. Now this subject can be really, really complicated. There is actually a whole scientific field called geodesy just on coordinate systems, so it can be a huge subject. In the next few paragraphs I will try to explain it, but if you feel that you lose me, just stop, have a break and come back to it. Also this is as complicated as it will get here in this series, to try to stick it out – it will get easier after this, I promise!
Ok, lets dive into it.
Coordinate systems are a way to “cut” the planet to wedges – both horizontal ones and vertical ones – in order to give each location that grid “address” that I mentioned above. The reason that we need to think about coordinate systems as wedges and not plain grid system is the fact that earth is a ball, and a square grid won’t work due to the curvature. Think about the planet as an orange being scored for “easy peeling” – you make straight strips that are closer at the top and bottom but wider apart at the circumference.
There many coordinate systems used on Earth, but I will discuss the two main ones: GCS and UTM.
Geographic Coordinate Systems (GCS)
The GCS is a very complicated system to calculate the exact location of a point on the planet. GCS doesn’t actually measure the surface of the planet, but the whole spherecal shape of it. Each point is measured from the core of the planet and has 3 parts: coordinates (angular measure from the core), a prime meridian and a datum. I know that all sounds complicated, but I’ll try and break it down. Lets start with the prime meridian:
Prime meridian – the agreed “0,0” longitude for the GCS. For most GCSs this Greenwich line.
Corresponding to the prime meridian is the equator – the widest part of the planet when it is observed from north to south.
Coordinates – Longitude and latitude representing the angle of the point with respect to the core. All the longitude lines are equal in length and run from one pole to the other and all latitudes are parallel, creating a grid that is called graticule (a spherical grid). Coordinates are represented by decimal degrees or by a degrees (°), minutes (‘), seconds (“) (DMS) system. Longitudes run from 180° (when traveling west from prime meridian) to -180° (traveling east). Latitudes range from 90° (north to the equator) to -90° (south).
To get precise locations (each square that is created by a longitude and a latitude is pretty big), the degree system is can be broken down to smaller increments. For example: 51.501019° N, -0.093728° W (My favourite pub) in decimal degrees. The DMS system uses the minutes and seconds instead of the increments, such that each degree has 60 minutes, and each minute has 60 seconds (so each degree has 3600 seconds…). That means that the same coordinates above will be 51° 30′ 3.6684” N, 0° 5’ 37.4208” W (still my favourite), as you can see it is DD MM SS system.
I hope that is clear enough that from now on you can look at a set of coordinates and know what they mean. GPSs use the decimal degree system to give a reading, so knowing how to look at coordinates is important.
Datum – these are very tricky since they are here to do nothing but confuse things more (that is how I feel at least). First, it is important to understand there are several GCS systems being used around the world; they all roughly similar, but not exactly – they may vary by up to 200m (218 yards) from point to point. That means that the same coordinates, used on maps that show different GCS will result in an up to 200m error. If you think this is not relevant for you, move on, if it is, stay with me for a little longer.
Datum means a single data (just for your knowledge) and in geodesy (science of mapping) it means the shape and size of earth. There are hundreds of data, as different measurements have been made to earth (starting in the 1700s), but for mapping we are only going to focus on horizontal datum (on the surface). The datum is a reference point for the GCS grid to “place” the longitude and latitude lines). The one most commonly used is the WGS84 (World Geodetic System, updated at 1984), while in the USA you can find NAD83 (North American Datum 1983) and in the UK, OSGB36 (Ordinance Survey of Great Britain 1936). All three GCSs are very similar, so not much of a problem there.
The data are just a reference point to keep an eye on if you are in a less surveyed area.
From all this you can see that the GCS is a very complicated system, so luckily for us, most of the mapping on the planet moved to a much simpler system, UTM:
Universal Transverse Mercator (UTM)
The UTM system is based on a basic concept of meractor projection: ‘pseudocylindrical’ conformal projection (big words for preserving it shape). The UTM is a projection of earth’s sphere using the poles to create a flat representation of that sphere, meaning, the maps you see everywhere. This is done by swathing (creating over-lapping layers) – areas around the equator are over-lapped, resulting in zone boundaries overlapping in some places. This is used in order to avoid curvature in the longitude strips of each zone (which would make it impossible to show on a flat map!).
By creating a flat representation, the world can be divided into rectangles (called UTM zones) that are easy to find and use on maps. Each UTM zone is 6° wide (so there are 60 zones) and it starts at 180° in the GCS system (so the mid of zone 30 is on the Greenwich line). After the break for the 60 zones, we divide each zone to two parts: east and west, with a line called the central meridian – this is the only line in each zone that is not distorted at all due to the projection. This line is used to align the zones. It is important to understand that due to the swathing that happens around the equator, the zones are not actually equal in length throughout the zone. They are much wider around the equator as they are near the poles (due to Earth’s curvature).
Latitude is broken into 20 strips (C to X, A+B and Y+Z represent the poles) with the equator splitting it into 2 parts: northern and southern. Those are not part of the UTM but the MGRS but are still used.
The tricky bit is to actually get a location in the UTM system. First, it is important to understand that the UTM is a meteric system, and can actually give a location down to the meter. Second, we need to look at each location as being a certain distance east (easting) to the central meridian line of the zone, and certain distance north (northing) to the equator. On the opposite side, south and west can be negative numbers, but we don’t like negative distances, so a “falsing” trick is implemented: If any point is south to the equator, it is measured north to the south pole. If a point is west to the meridian line, it is measured from a false line 500km west to the central meridian (false easting). I know this sounds very tricky, but I’ll try to explain.
I think the north/south split is simple. It means that a point in the northern hemisphere can be a maximum of 9,300,000m and in the south 10,000,000m.
The east/west split is also simple if we look at each zone as just one part – east only. For simplicity, the west line is moved 500,000m so that there is only one direction to deal with (false easting).
So, back to my pub – it is in zone 30 (most of England), 5,709,547m North (of the equator), 701,708m East (from the false easting).
That is it for UTM, I hope you got that, as it is very tricky but the more you use it, the easier it becomes.
A couple more points
Just a point to mention – along these lines, there are the USA quadrangle maps that use 7.5 minutes grid (unlike the 60 minutes system in the GSC) , the OS national grid (UK) that uses Transverse Mercator Projection like the UTM but only for the UK, and many others.
It is important to check which grid system your map is using, especially if combined with a GPS device.
Way to go for making it to the end! That was not an easy one, and I hope you made it in one piece all the way here. As I said, it will get easier from here. Coordinates systems only become relevant when communicating a location to some one else, like emergency services. Being able to use the coordinates your map provides is important, just like having a first aid kit – when things go wrong. It is easy to neglect this knowledge and let it rust, but try and give yourself a refresher every so often so stay “in shape”.
If you still have questions about this, just send me an email or comment below and I’ll try to explain more.