Coordinate systems and map projections are often a stumbling block for lots of people. A simple understanding of what these two things are can save hours in map tools when maps and layers do not look quite right or your centre point is the middle of the ocean.
To start with, let us consider the planet earth.
The earth is not a sphere, it is also not an ellipsoid, it is called an Oblate Spheroid. The earth also is not perfectly smooth, there are mountain regions and huge valleys and depths to the oceans. Some of the problems associated with mapping a 3D object is, unless we carry a 3D object in our pocket, then we have to change 3D coordinates of some kind to 2D coordinates on a sheet of paper or a map. Secondly, the earth is not a solid object. It is a fluid and as such, changes over time. An example being the ice caps on the poles change over hours.
Latitude and Longitude are coordinates and positions represented by angles. The Lines of latitude and longitude always cross at right angles, the same as a cartesian grid but latitude and longitude are on a curved surface.
Longitude lines go from the north to south pole.
Latitude lines go around the globe like the equator.
A way to imagine this is, imagine a centre point in the earth core, draw a mental line from this centre point to the earth surface. Now imagine the earth has a slice through it from the base of your line. Lastly, imagine you are looking down on the planet, draw a line the London.
The angle between your line and London is the first angle and the angle between your line and the plane is the second angle. This will give you a position on the surface.
We all know the earth is a 3D object, and our maps are 2D. This is where the problems arise.
An example of this is to imagine peeling and orange and trying to make a perfect rectangle with no distortion. As we know this is impossible.
What we could do is take a small rectangle section from the orange skin, lay it out on a piece of paper, press it flat but, this would still have little distortion in that one bit of the skin. The smaller the rectangle, the less distortion. The distortion is not eliminated, it has been reduced.
Now we need a way to transform a 3D rectangle into a 2D rectangle, we need a mathematical equation that will convert all the points on out latitude and longitude map into an x and y coordinate.
Mathematicians for hundreds of years have worked on this and now we have hundred of transformations for us to use. Basically, we plug in our latitude and longitude and out pops a grid reference if that is what we are after.
Simply put: A map projection takes (transforms) a section of the earth into a flat rectangle to make a map. We then transform the locations from the earth into locations on a 2D plane.
NOTE: All projections from the earth when put onto a plane will distort.
EPSG: European Petroleum Survey Group.
The EPSG compiled a database, a very comprehensive database of coordinate reference systems, datums, ellipsoids, and other such geodetic parameters. Each one of these entries has a code associated with it. For example: EPSG:27700 is same as the OSGB 1936 British National Grid, United Kingdom.
The proj4 text for this projection is:
EPSG:27700 = proj4.defs("EPSG:27700","+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs");
This guide is a very basic idea and introduction into Map Coordinates and Map Projections. I will follow this up shortly with a more in depth article.