rhumb line placement

[Coyotemax]

Just to back up Ghostman's post, one of the maps that inspired me most when trying to figure this out for myself is this one - with the same pattern shown by Ghostman.. Another thing if you want some accuracy to it is to notice the line weights - thick/thin and dashed for particular angles

(and i created a PSD with the lines drawn out as well so I could just copy the layer into new files, but the website won't let me upload it - it gets to the end of the file and tells me it's not a valid image file.. grr.)

[Ascension]

Yeah, my rhumb line image is 5000 x 5000 so it won't upload either. But for those of you who want one just draw a line on a layer, duplicate the layer, rotate it 90, merge down, duplicate and rotate 45, merge down and duplicate then rotate 22.5.

[Bill Hooks]

I've just done some quick research and Ghostman is essentially correct, except that there are sixteen points of origin rather than twelve. Each set of parallel lines indicates a wind direction, or rather two wind directions (it might take some Magic Eye-style brain gymnastics to see the chart as overlapping parallels rather than an upended bag of uncooked spaghetti, but once you figure it out it's easy).

http://upload.wikimedia.org/wikipedi...ado_1571-1.jpg

EDIT: Oops, didn't notice there was a page two -- this post is probably redundant then.

[Ascension]

Looking at those maps I notice that the rhumbs are placed at intersections of a grid. The grids are not set to latitude or longitude (not sure about that first one). So, to me, it looks like a random placement...put your grid anywhere and put rhumbs on top. As far as the parallel lines thing, as soon as you place two sets of radiating lines you'll get a parallel somewhere. Put enough of them on a grid and they'll all be evenly spaced. Interesting to see that the lines use 4 different colors in the one posted by Bill.

[Bill Hooks]

In fact, in almost every example I can find -- here's another -- the basis of the system is a circle whose center is the center of the map, and whose radius touches the top or bottom edge of the map. It's hard not to conclude that the lines are mostly, or even entirely, decorative.

[Ascension]

Good eye on that circle thing - wherever the unseen circle hits the grid you place the rhumbs (in these examples). Now to just figure out how the grids are situated, random or reason?

[Bill Hooks]

The sixteen rhumb line origins are equidistant from each other, spaced by 22.5 degree increments around the circumference of the circle. The grid is drawn after the fact and connects origin points across the circle. Bottom line, it's not random, but nor is it related to actual geographical measurements in any way that I can discern.

[waldronate]

The map that Coyotemax posted shows a classic pre-printed base map that navigators would use as the basis for drawing during explorations. The basic layout of a portolan chart included lines from the center out at N, NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW, WSW, W, WNW, NW, and NNW. Each of these basic lines has another set out near the edge to allow for simple course changes using a compass at given locations.

[Bill Hooks]

Okay, with Waldron's post and having read up a little on early navigation I'm starting to get the idea of how these were used. Thanks, Waldron.

[Ghostman]

Looking at those maps I notice that the rhumbs are placed at intersections of a grid.

The "grid" you're seeing seems to be rather the result of the North-South and East-West going lines, than anything that was put in place before the rhumbs. The rectangular boxes of these grids are of differing sizes and proportions. In a proper grid of square (or even just identically rectangular) boxes it would be impossible to place the rhumbs so that they'd connect to each other forming a circle while at the same time matching the grid intersections.