Projections are perhaps the most complex piece of the GIS puzzle.  To effectively use GIS in your work it is important that you have at least a very rudimentary understanding of map projections, how they work and why they matter.

Map projections are very complex and you don't want to know much about this whole area, really. The easiest type of system you will come across are those described as Geographic- this means that the model actually wraps around the globe and so has three dimensions- horizontal measurements (the X value) called longitudes, vertical measurements (the Y values) called latitude and height (the Z values). Your car GPS unit and your fishing GPS use these types of systems as they are fast, easy to calculate and don't really change in how they can be used around the globe. When you see the name WGS1984 it means it is a geographic coordinate system and the numbers will relate to latitudes (the distance from the equator) and longitudes (the distance from London- London has the honor of hosting the zero point). These numbers look like 37.45415, 120.541254 if you are in the northern hemisphere and like -28.541364,40.654154 if you live below the equator.

The following image shows the world represented by a geographical projection- it looks right because we are used to seeing this type of map. What you don't see easily is that it significantly distorts shapes in an east-west fashion. We use this sort of system heavily in GIS.

When we look at a map we are viewing a 2-dimensional version of what is really wrapped around a 3-dimensional globe. This is not a problem for small areas in most lines of work but for large areas like states and in precise fields, this obviously means something is broken- you can't peel an orange and make it a perfect flat rectangle- it gets all messed up. In many situations geographers define a way to project the round earth onto a flat page or sheet of paper. These are the map projections we use in our GIS. They use a cartesian system that best suits our local region- each state has it's own version of this projection and some have many different projections, say California for example. They use good ol X & Y coordinates albiet ones with a lot of digits. If you see just two or three digits before the decimal in the coordinate display you are looking at and using geographic coordinates- lats and longs, if they are long numbers before the decimal they are X,Y coords on a projected dataset.

The following image shows how a map projection like UTM looks when it is applied to the whole earth- similar to what you would do to an orange to slice it up and get it flat right? Messy and not so useful at a global scale but good for small regions because it does a better job of preserving the shape and area or countries and regions.

Images courtesy Planetary Visions

The projection you should choose to use depends on the scale you work at most often- if you operate a global focused advocacy group then using geographics for your data makes sense. If you only work in a certain city then most data you get from your local municipality will be in a locally projected system and hence it will be smoother to use that projection as your standard one. WGS84 is the most typical global system and is used in GPS units worldwide.  In North America many organizations use NAD83 which is a geographic projection that happens to suit the USA/Canada slightly better than WGS84. For many uses the two are interchangeable unless you care about more precise measurements and analysis. Your GIS can handle both at once if you open two files with different projections but it may cause some slight errors in finer detail and in larger areas not matching properly. In the USA each state has its own cartesian style projection, called the State Plane Systems. For local work consider using your relevant one- some states have many!

For more reading on coordinate systems and how they are defined and used have a look at some of these- I assure you it's not easy and not particularly fun!

• Wikipedia on coordinates
• Coordinates explained
• State plane systems- very cool, actually potentially helpful too