What Are Those Intersecting Lines on Medieval Maps?

Modern maps show places in relationship to the lines of lattitude and longitude. However, if you look at older maps, you see a network of intersecting lines, which go off at all angles.
What are these ines called? And what was their function?

Example image?

If you mean Portolan maps intended for navigation, they are compass headings drawn from several points scattered about the map. Example:

http://www.geogr.muni.cz/ucebnice/kartografie/images/K2/Portolan.jpg

But how were they used? What is the significance of the places in the sea where a dozen or more of these lines originate?

I would guess those are where lines from cities on the map intersect, rather than a point of origin.

Ley Lines?

That’s a good question. The ones originating from ports have an obvious utility - you are, say, in Genoa, so you look at the ray originating from there which most closely approximates direction to your destination, and guesstimate what compass heading you want. I’ll WAG that the ones in the middle of the sea are intended for mid voyage course changes, should you think you are near that point - perhaps they are near small islands useful as navigation aids, or are simply to accommodate positions obtained from dead reckoning. If you wished to sail to a destination which requires you to sail around a land mass, you could sail to that point, and change course.

No. Bad Enola Gay! No Nuke for you!

No, if you look at the map in the link they’re not random intersections - you couldn’t have so many straight line links between real cities crossing at one spot, even on a less than accurate map like this one. Also, they don’t just appear in the sea, they continue across land which seems to suggest that they’re not recommended compass bearings.

It doesn’t even look to me like these lines are coming from ports; if they were you would expect many lines to radiate from one spot on the coast but I don’t see that happening. Cyprus on this map does not have any spots on the coast which have more than one line coming from it. There’s a spot in western Greece which has many lines radiating from it, but it appears to be slightly inland, and why isn’t there a similar spot for Athens? Crete has a spot on its south coast, when you’d expect the main ports to be on the north coast.

I’ve seen these lines, and these points with several lines radiating from them, on many such maps, not just this one.

I’m an no expert cartographer (or um, any cartographer at all) but looking at this map the conjunctions all appear at places where the compass heading intersects with a line of lattitude. Latitude can be measured using a sextant *; longitude could not be measured without a ship’s chronometer (explanation of why, and story of invention told, in the book “Longitude” by Dava Sobel).

So, they would (presumably), sail on a particular heading from a port until they reached a particular latitude, that would be where they turned, using the new heading provided from that junction on the map.

ETA:
The junctions on yabob’s map are also at a line of lattitude; it’s just hard to see because of the orientation of the map (the lattitude line is shown at an angle) and because the map is very busy with headings. But look close and you’ll note a series of parallel lines drawn in blue; these are latitudes.
*and before that a “Cross Staff” was used. This required the navigator to look directly at the sun; over time they would go blind from this, hence the image of old grizzled sailors to wear an eye patch.

That makes a great deal of sense.

But it’s even clearer on that map that the diagonal lines don’t originate from ports (such as Alexandria) or points on the coast which are actually significant.

The idea of what a map was for was rather different when portalan maps were in use - it wasn’t like AAA Triptik through the ocean; a map system that kinda sorta got you where you were going was a hell of a sucessfull map system. Potolan made sense to them based on the tools they had, which was a compass (which only kind of worked at sea, before the gyrocompass) and the sextant, which was a tool which took a great deal of skill, and required at least occasional view of the sun.

I imagine that practice was to follow a heading to a patch of coastline nearish to where you want to go, using the nearest relevant compass heading, and follow the coast from there. Understand that the entire process was estimation, sailors would not reasonably expect to follow the portalan directly to their destination, it would be kind of pointless to even try for a direct heading to, say Alexandria. Instead the map markers appear to be centered on some objective reference point apparent to sailors, probably celestial in nature like latitude. I don’t know enough to know what they mean, but there are a series of parallel lines, very hard to make out but at about 20 Degree angle in either direction from the latitude, that are obviously significant.

If your destination is between two of the lines, just estimate the angle between them you need to head.

Modern charts have a compass rose somewhere, from which you can take a direction and parallel-transfer it to where you actually need it using two set squares or one parallel ruler. Or conversely, take a line through two points and transfer it to the compass rose to get the bearing. Perhaps these sets of lines radiating from these points served the same purpose.

I have just re-read the opening chapter of Mark Monmonier’s excellent Rhumb Lines and Map Wars - A social history of the Mercator Projection. Portolan maps were wholly utilitarian maps for navigation, mainly across the Mediterranean. The maps pre-dated Mercator’s famous projection and it would not have been possible to just draw a straight line from A to B and read off the heading. Instead the lines mentioned in the OP are a representation of special lines known as Rhumb Lines or alternatively Loxodromes. Rhumb lines have a constant heading and so are easy for mariners to follow. However, if drawn ‘correctly’ on the map they would not be straight at all. If drawn on a globe they would actually be spirals!

A typical portolan map would have a central point from which which radiated the rhumb lines. Some of the lines would point directly to major ports whilst others would form go to a network of secondary points which radiated their own set of rhumbs. Often there would be 16 such secondary points arranged equally in a circle around the primary one representing the major compass points, though this may not be so obvious from smaller maps covering only a section of such a network. To navigate you would follow a rhumb to one of these points, which don’t represent any real ‘place’, then change course to follow another rhumb, either to your destination port or to another one of the rhumb-line convergence points.

They do seem to be laid out regularly, with a center point, and sort of a ring of points around it. They all fall at least on a latitude or longitude line, more or less corresponding to N (mainland, to the right of “GRAECIA”), NNE (mainland across the Bosporus, left of “ET BITHI”), NE (the compass rose, mainland Turkey), ENE (nothing of particular importance, southern mainland Turkey), E (the middle of Cyprus), ESE (in the middle of the Mediterranean, south of Cyprus), SE (off the coast of Egypt), SSE (hopefully the pattern is obvious by now), S, SSW, SW, WSW, W, WNW, NW, NNW.