St.Oz - what is the distance represented by the squares on the map? i.e. one square equals 100 miles or kms or whatevs...
So. This is a complicated question to answer, and many other cartographers don't even bother learning about this and simply slap on that "one inch = 225 KM!" bullshit. This might work on very localized maps, but definitely not on fullworld map
A map is a 2d representation of a 3d object, there will never be a perfect map for some one pixel to X km sort of projection. It's just impossible. Unless you want some weird fugly map that bubbles out and such, but those are hardly helpful and distort things as well.
You can see here how equidistant maps are incredibly unhelpfulThe example I show there, is the visualization of a mercator projection. A line at angle θ travels through the sphere, which is the earth, and then reaches out and touches the paper. I may have positioned the paper out too far now that I think of it, it should be touching the circle, nonetheless, this is what's happening with distortion with a mercator map. It also perfectly explains why you cannot have a mercator map from 0-90 degrees or 0-89 degrees, and why you will often see mercator maps at 0-75 or 0-80.
Where am I getting at with this? The distance between the squares are degrees. Since it's equirectangular, all degrees are spread out the same as others. However the represenation of 15 degrees longitude at say 70 degrees latitude will be different than 15 degrees longitude at 0 degrees latitude.
Imagine your globe, all the lines meet at the top. The gap between them stretch out from where they meet to the maximum at the equator, then condense again at the other pole where they meet. Same concept for a basketball, the lines of a basketball max out at the halfway point, then minimize the gap at the poles.
Taijitu is equivalent to earth, well, not really.
This is where it gets
really complicated. The earth isn't a perfect sphere, NASA has some complicated ellipsoid model of the earth, but since things are so complicated enough, I just use the simple spherical model of the earth.
The best way to knowing the distances, is seeing where this target piece of land is on the map, finding it's latitude position on the map, then comparing it with other earth countries or pieces of land on that same latitude. I also suggest downloading
http://www.giss.nasa.gov/tools/gprojector/ gprojector, a program that lets you overlay the earth over images, such as say... taijitu. Save the taijitu image and load it up. It will help you see relative distances with it meshed up.
