I don't know if this will help me make better maps, but it's something about map making/using, that I've never understood, so I thought I would ask.

I've seen a lot of old world maps (and even some of the maps here on this site) that use the navigation lines. Now, I understand about Latitude and Longitude, those are the navigational lines based on the poles and the equator. Those lines create coordinates that help you find a specific point on a map/globe. That part I understand.

But I have seen a lot of maps that have navigational lines that start in places other than the poles and the equator. http://www.cartographersguild.com/sh...539#post299539

This is an example of what I'm talking about. This mapper has 3 separate points of origin for navigational lines.(sorry to hijack the thread but I wanted an example of what I'm asking about)

I want to understand why this is done. I realize there is a reason, I just have no clue as to why. I don't really understand navigational lines beyond latitude and longitude, and I want to understand.

So here are my questions. Why use origin points beyond latitude and longitude? What do these other origin points represent? How and why would a map maker use more than the poles and the equator for navigational lines? I also notice that sometimes the poles and equator are NOT used, and other origin points are used instead. What would be the reasoning behind this?

Any assistance in furthering my map making education would be greatly appreciated!