Abdul, could you describe, or link to good description of, how these portolan charts were made? Is it something based on location via stars, and speed via knots (the device), or something more complex?
It's not fair to ask me a big question like that. You totally know I am going to take the opportunity to go unhinged on the topic and stray all over the place.
Trying to keep a long story short (really trying).
The 13th-15th C. portolan charts were all based on ship pilot notes, and thus all dead reckoning (bearing & distance between points). They had no idea how to localize themselves by stars yet.
There wasn't even a way to express location - there were no coordinates, latitude/longitude grids were as yet unknown (had to wait until Ptolemy was rediscovered in the late 15th). And even then it wasn't easy. Cosmographers had some idea how to roughly localize latitude on land (with a stable, land-based astrolabe), but astrolabe readings were pretty impossible on bouncing, billowing ships. Had to wait until the development of the maritime astrolabe in early 16th C. So astronomical navigation only really began being introduced in Europe in the mid-16th C.
(All the turn-of-century navigators - e.g. Columbus, Gama, Cabral - still used dead reckoning. However, they also had begun to take cosmographers on board by this time to experiment, but they invariably had to get off on beaches to do readings (indeed, that was what Amerigo Vespucci's job was, if you ever wondered). You will notice Columbus's journal records only bearing & distance on the journey; his only astronomical reading is made on land, and even so gets it grievously wrong).
So portolan charts were made with only two parameters: bearing and distance. There are no latitude/longitude lines (latitude lines only begin to appear in 16th C.). Instead they are marked by compass rhumblines.
If you are wondering about portolan construction, time for a detour.
Rhumbline construction
(1) Portolan charts are usually drawn on a single piece of vellum (calfskin).
The mapmaker started off by drawing a gigantic outer circle on the map with a pair of dividers, and then used a ruler to inscribe the great circle with a
hexadecagon (a 16-sided polygon, with 16 equidistant points). (In case you were wondering how:
inscribing animated gif).
(2) These sixteen points were then all connected to each other with straight lines. As a result, you ended up with a simplex like this:
Pretty ain't it?
This is the rhumbline pattern present on
all portolan charts.
(On a given chart, it is sometimes tricky for your eye to "see" the pattern in the jumble of crazy lines - too much sensory overload. A simple visual trick is try looking for the center of the simplex - the simple "wheel with spokes" - and then let your eye zoom out.)
(3) Why a hexadecagon? Because as a result, when you draw the simplex lines,
each of the sixteen equidistant edgepoints becomes automatically a 32-point compass rose. That is, the lines emanating from the other points converge on each edgepoint at a specific angle. Each edgepoint "collects" lines drawn to them and becomes the center of an independent compass rose. That is, each of the sixteen edgepoints now looks like this:
This is a 32-point compass rose. Unadorned perhaps, but still a rose. It is only one of sixteen roses present on any portolan chart.
You will notice the lines are color-coded.
- The eight principal wind lines are black (N, NE, E, SE, S, SW, W, NW)
- The eight half-wind lines are green (ENE, NNE, etc.)
- The sixteen quarter-wind lines are all red (North-by-east, Northeast-by-north, etc.)
The color code was followed in all portolan charts. So a reader can immediately deduce the bearing they're talking about when he looks at any line.
(4) To summarize, squint at the 1325 Vesconte portolan below. You can see the implicit roses - the "gathering points" of the rhumblines. There are sixteen such roses, arranged around an implied great circle, whose center is the Bay of Biscay. All conventionally-color coded.
All portolan charts have rhumblines constructed with this pattern.
[Side note: Later maps, especially Portuguese maps of the 15th-16th C., took to "adorning" each of these sixteen little roses on the edges with fancy designs. e.g Pedro Reinel map of 1504:
Same set-up. A hexadecagon with sixteen roses (rhumbline gathering points) equidistant on the edges of a large implied circle centered just east of the Azores. Unlike the Italians, Reinel decides to be fancy and draws them as ornate compass roses. Not all sixteen - some are too close to the coasts that if he drew an elaborate rose there it would obscure important coastline details. But the ones out at sea are "elaborate". And he draws a real nice big ornate one in the center of the implied circle. But that's just bling - all quite unnecessary. Italian charts were all soberly unadorned.]
Navigation by chart
Prior to the 16th C., European navigation used only two parameters: bearing & distance. Both are easily deducible from the chart.
Bearing
Navigation by portolan chart is quite easy. Say you want to sail from Barcelona to Palma de Majorca using the 1325 Vesconte chart above. There is no direct rhumbline connecting the two cities, but using a pair of parallel rulers, you can figure out the bearing you need pretty quickly by setting one ruler connecting the points you want, and shifting the parallel ruler to the nearest tangent rhumbline to find the implied bearing.
(Do it visually with a couple pencils on the screen - mark one from Barcelona to Palma, raise it parallel and you'll adjoin a red rhumbline coming from the nearest rose to your east, which you can see is three winds above west, that is, a nw-by-w line. So, you need to sail steadily the opposite bearing, southeast-by-east, or, in the lingo of the time, "scirocco per levante", to get from Barcelona to Palma.)
Distance
Bearing, of course, is not enough. How
long do you need to sail to get to Palma? For that you need to know
distance. As it happens, every portolan chart has a distance scale. (It is barely visible on the 1325 chart, but the scale is that little rectangle at the very bottom of the map, with vertical lines and dots.) It has no numbers, but the spacing of the lines and dots in the scale bar is standardized across charts (I forget exact numbers, but something like 50 miles between lines.) So get your pair of dividers, measure distance between Barcelona and Palma, place those dividers on the scale bar and you got your distance - 130 miles or so.
So to get from Barcelona to Palma you need to sail 130 miles at bearing southeast-by-east. Done.
(If your route is more complicated, e.g. Barcelona to Cadiz, where you need to change bearing several times, that's no big deal; just determine the route you want in segments - SE for 100 miles, S for 200 miles, WSW for 80 miles, etc.)
(Notice there are three distance scale bars in the 1504 Reinel chart, one of them unusually drawn diagonally).
Charts vs. Rutters
Notice the result from the chart: "to get from Barcelona to Palma, sail 130 miles southeast-by-east" That phrase is the kind of sailing instruction that you usually find written in pilot notebooks, i.e. rutters.
So, by backwards engineering, you can realize how these charts were constructed. Collect a bunch of rutters, with written sailing instructions like these, and construct the maps accordingly. The result is the portolan chart.
The chart and the notebook are interchangeable. One contains the same information as the other. It just happens to be visually depicted and neater.
The advantage of the chart over the notebook is that you can
innovate routes much easier. That is, you can construct and combine routes visually, and translate them into a set of sailing instructions, rather than scouring through notebooks to look for the appropriate instructions and then trying to combine them.
Which brings up another point - who used these charts? There is a long debate as to whether such charts were actually used by pilots aboard ships at this stage. There is some evidence they were taken aboard. But these are very elaborate and expensive maps, which you don't really want to expose to the sea elements. Moreover, we know that, for the most part, pilots continued to compile and rely upon rutters, long after charts were invented. So charts didn't actually displace them.
My personal theory (and this is just a hunch, not generally accepted) is that these early portolan charts were originally made to stay on land. That is, they were made for commercial houses, for the houses to plot itineraries for their ships, in advance of sailing. The pilots themselves would only be given the written sailing instructions of what they were to do.
Pilots don't need charts unless they're innovating. And pilots aren't supposed to innovate. They're supposed to follow the instructions of the ship owners. To the letter. Not go off on side trips of their own invention.
So when charts were taken aboard, it was likely not for the benefit of pilots but for use by traveling ship owners and masters, or their representatives aboard,
in case they decided to change their itinerary and issue new instructions.
In short, charts were invented for ship owners. Pilots only started to be given charts when they got cheaper to make and easier to supply. For the most part, and for a long time, pilots still relied primarily on notebooks and their own long memory.
Navigation in practice
Pilots need specific sailing instructions of bearing & distance. These can be deduced from a chart. Or from a rutter. Or drawn from memory.
Once given the instruction - "sail 130 miles SE by E" - they just need to point the ship in that direction and go that set distance.
Bearing is easy with magnetic
compass. And European pilots sailed firmly with eye on the needle. Not scanning the starry sky, like Africans and Asians did.
(The exact date when the magnetic compass was introduced aboard ship is shrouded in legend. But assumed sometime in the 13th C., right around the same time as portolan charts came into existence.)
What about distance? As you know:
Distance = Speed x Time.
Time is measured by a simple on-board
sand-glass. Every ship had a boy charged with watching and turning it every half-hour.
Speed at this time (13th-16th C.), was measured by throwing a
log or piece of debris into the water and measuring the time it takes to pass the length of the ship. The time is measured by crew engaging in a rhythmic chant as it floats by (singing stops when it passes the ship, then the lines chanted are counted).*
(* - It was only later (c.17th C.) that they began using a knotted log to mark speed, i.e. tie the piece of wood to a long coiled rope (rope marked off with intermittent knots at set intervals), throw it overboard, and let the rope unwind for one minute or so (marked by a small sand glass), then just count the number of knots that were unwound in that time interval.)
Problem with calculating speed of the ship this way is that, of course, the floating log is also affected by the speed of the sea current. Sailing in known waters, you might have some prior idea of the current and make adjustments according. But speed calculations could be off, sometimes quite off. And that messes up your distance calcs and can throw you completely off. Lots of terrible accidents can and did happen as a result. Dead reckoning is an imperfect art.
When astronomical navigation was introduced in the 16th C., it was not as a complete substitute, not as something to be relied upon precisely (and never was - sky may be cloudy). Rather it was merely something to "correct" miscalculations. You set your route as usual, by compass-and-chart, and sailed by dead reckoning (bearing & distance) as usual. But intermittently you could check your latitude (and you could only check latitude at this stage - longitude being a problem only solved much later) to see if you were where you were supposed to be at this time, and if not, then adjust it.
Correcting course
From your chart or rutter you get instructions: "sail southeast-by-east for 130 miles". Done.
Done? Yes, done as far figuring out
intended route. But
actual route might be complicated by the fact that you get caught by bad winds, or storms or something like that that throw you off your track.
Here arose one of the big practical challenges of Medieval navigation: if you lose your course, how do you fix it? How do you return to your original course. How do you figure out where you are?
You may
think you need charts for this. Or use stars or astronomical instruments to localize. But you don't. You just need to do a little math.
This "traverse problem" is a mathematical problem. In principle, easy to solve. The intended route is a line. The actual route is a deviation from that line. So returning to the original line is a matter of "completing the triangle".
Say your intended route is to sail east from point A to point B for 100 miles. Bad winds catch you at A and force you to sail northeast for 30 miles. To get back to original route, you can sail southeast for 30 miles, then straighten the ship and continue east. You are back on your original course.
If it was only so simple. But there are added questions:
- (1) you went northeast by 30 miles, and southeast by 30 miles. Fine. You're back on the original route again. OK. But
where on the original route exactly? During your deviation, you've "made good" some of the easterly distance between A and B. But how much east? Exactly how far are you along on the A-to-B route when you rejoin the original route? How distant is your destination now?
- (2) what if winds don't let you sail southeast, but must sail by another bearing, say, it is forcing you East-southeast? That is a slenderer gradient. 30 miles sailing that way will bring you short of the original route. How long exactly must you sail on this bearing to recover the original route?
Sounds complicated, but this is all triangle math, a.k.a. trigonometry.
Note: trigonometry wasn't known to Europeans yet (it was known in Arab math, but Western math didn't get it until late 15th C.)
Well, sorta. Sailors knew trigonometry long before the academics did. Or rather they had recourse to the
toleta di marteloio, a simplified trigonometric table. It was compact enough to be routinely memorized by pilots, and copies could be found in their notebooks and in portolan atlases. Looks like this:
(above is from flyleaf of Andrea Bianco's portolan chart of 1436).
The Toleta is a brilliant device, known to have been around and used in Mediterranean navigation since the 13th C. I could explain it to you in more detail, if you'd like. But all you really need to know is that this table is enough to correct any course. Just plug in the angle by which you deviated from your original course (expressed in terms of "quarter winds" rather than degrees), and then choose the angle by which you intend to be return to it. The table will tell you exactly how long you must remain on that return bearing to recover the course (
ritorno) and, in addition, it also tells you how many miles of the original course you made good (
avanzar) during this whole detour.
With this data, you know
exactly where you are at all times and how to get back on course, no matter how off-course the winds blow you.
In other words, you need no longer worry about losing your course. Just keep track of your bearing & distance at all times, and you can find it again pretty quickly and accurately, and go on happily to your destination. No need for stars or GPS devices. No need for charts either. Just memorize the table (and all pilots had it memorized) and do some basic multiplication and division.
(Note: the toleta gives the answer per 100 miles of deviation, so you do have to do a little extra multiplying and dividing to scale the table's answer to your particular case, but it's pretty straightforward. It may seem incredulous that Medieval folks could manipulate this kind of "high math". But keep in mind Arabic numerals had come shortly before, making math a lot easier. And pilots and navigators were drawn from the same socio-economic class as merchants and clerks. The required math for this - multiplication, division, fractions - was the day-to-day business math of any Medieval merchant, and taught in such families from a young age.)
If you lose track of your deviations, then, well, you're kinda screwed. So keeping track was
very important, and of utmost priority regardless of what else is going on. The sand-glass turner would demand the bearing from the pilot every half-hour, and mark it on a little pushpin traverse board.
Well, I could go on. But I think the practicalities of it are clear enough. This is basically how navigation was done between 13th & 16th C. It's all about bearing & distance - i.e. dead reckoning. And that's exactly what the charts capture.
If its merely dead reckoning, then why are terrestrial maps getting worse as you get away from the sea? I know, speedxtime over land is not distance as the crow flies, but I have seen some maps from 16th century Hungary that are shockingly bad when it comes to even simple things like river flows and city locations.
Nautical charts are accurate because navigation at sea depends critically on bearing & distance, and so that is obsessively recorded by pilots and sailors.
I am less familiar with land maps. But intuitively, land travel just follows long-established roads. And nobody - really, nobody - keeps track of bearing and distance when traveling on land. Well, distance maybe, but not bearing. They just follow the established road by its twists and turns and get there eventually. So these parameters are not as obsessively recorded.
(And even overland distance is probably more usually recorded in time (days, the relevant measure for most travelers) than mileage proper (as speed is probably not routinely measured)).
So I can totally see land maps being a lot more inaccurate because they begin with a lot more inaccurate or missing data.
But there is also the element of interest. Nautical charts (by my theory) were needed by the commercial houses and they invested in them, and made sure they were accurate. Can't see who would be keenly interested in land maps. You can't really deviate from standard paths and roads, merchants followed the same well-worn paths of yesteryear. Military maybe?
