What am I missing?

[Lalaithion]

Well, the graticule (grid) is a bit problematic. Being square like that implies that the map is either, large scale ("zoomed in") and centred on the equator, or it's in Plate Carree projection, in which case the features on the map would be distorted, unless it's a large scale map centred on the equator.

Because the earth is a sphere, it actually doesn't matter where it is in the world, because it is an regional map. Plate Carreé projections (also called "equirectangular") were also invented around 100 BC, so having one exist in the middle ages actually is completely valid. In fact, the Mercator map, one which is usually imagined when you think about old maps, was invented in the mid 16th century. Latitude and Longitude were also invented by the ancient greeks, as evidence by this map made in the second century.

Although this map is inaccurate, Latitude and Longitude were, in fact, used.

There is, however, one important thing to take away from this discussion: if you make a world map, you will be forced to decide on a projection, or else have your world be literally flat. Wikipedia is a great place to compare what different projections look like- mercator vs equirectangular, robinson vs mollweide....

[Hai-Etlik]

Because the earth is a sphere, it actually doesn't matter where it is in the world, because it is an regional map.

Yes, it does matter. At the equator, the "box" bounded by a small equal angles of latitude and longitude is approximately a euclidean square. As you move into higher latitudes, it narrows and becomes more like a trapezoid, until you get a very narrow triangle at the poles.

A map in a projection suitable to a restricted extent is going to try to minimize distortion within that extent so those shapes will be approximately preserved: At mid latitudes, a lat-long box with equal angles would be approximately a rectangle with an aspect ratio determined by the latitude. In a normal cylindrical projection it will be projected as precisely a rectangle.

In tangent normal equidistant cylindrical, all such boxes are squares, but this means that they, and all other shapes, are being stretched out east-west. Which is a rather noticeable form of distortion. So my point stands, to get a square lat-long graticule, you can either be covering a small area near the equator in a projection that minimizes distortion within your extent. Or you can be in Plate Carree, in which case you can be anywhere, but would have significant and ugly distortion unless you are covering a small area near the equator.

Plate Carreé projections (also called "equirectangular") were also invented around 100 BC, so having one exist in the middle ages actually is completely valid. In fact, the Mercator map, one which is usually imagined when you think about old maps, was invented in the mid 16th century. Latitude and Longitude were also invented by the ancient greeks, as evidence by this map made in the second century.

Although this map is inaccurate, Latitude and Longitude were, in fact, used.

Yes, I know the ancient greeks had an idea of latitude and longitude and tried to use it in map making. My point was that maps from the Medieval period, which high fantasy is generally based on, didn't. In fact they rarely used maps at all let alone ones with graticules. When they did draw maps, they were at best about giving a crude idea of topology: what was connected to what. That's why I specifically said "a bit anachronistic" I admit that wasn't exactly the clearest way to express my point. Dropping the graticule (admittedly somewhat harder given it's hand drawn) would sidestep the whole projection issue and better give a pseudo-medieval, high fantasy feel.

[Lalaithion]

Yes, it does matter. At the equator, the "box" bounded by a small equal angles of latitude and longitude is approximately a euclidean square. As you move into higher latitudes, it narrows and becomes more like a trapezoid, until you get a very narrow triangle at the poles.

A map in a projection suitable to a restricted extent is going to try to minimize distortion within that extent so those shapes will be approximately preserved: At mid latitudes, a lat-long box with equal angles would be approximately a rectangle with an aspect ratio determined by the latitude. In a normal cylindrical projection it will be projected as precisely a rectangle.

In tangent normal equidistant cylindrical, all such boxes are squares, but this means that they, and all other shapes, are being stretched out east-west. Which is a rather noticeable form of distortion. So my point stands, to get a square lat-long graticule, you can either be covering a small area near the equator in a projection that minimizes distortion within your extent. Or you can be in Plate Carree, in which case you can be anywhere, but would have significant and ugly distortion unless you are covering a small area near the equator.

The equator is geographically significant, yes, but it mainly just is another invisible line on our map. You could center a equirectangular map anywhere in the world, effectively treating any great circle (or really, any circle at all!) as the equator and any two points as poles- here we see the cassini projection of our world:

Which is basically the equirectangular projection with the prime meridian as the equator (it is called a cassini projection, just like how a plate careé is just an equirectangular projection centered on the equator).

(I agree with you that it is anachronisctic, but we can make exceptions in order to provide our readers/players/others clearer maps.)

[Hai-Etlik]

The equator is geographically significant, yes, but it mainly just is another invisible line on our map. You could center a equirectangular map anywhere in the world, effectively treating any great circle (or really, any circle at all!) as the equator and any two points as poles- here we see the cassini projection of our world:

Which is basically the equirectangular projection with the prime meridian as the equator (it is called a cassini projection, just like how a plate careé is just an equirectangular projection centered on the equator).

(I agree with you that it is anachronisctic, but we can make exceptions in order to provide our readers/players/others clearer maps.)

My comments specifically regard a projection that produces square graticules. That was the entire point of my post. If the graticule hadn't been a square grid I wouldn't have said anything.

I suppose you might have some other coordinate system. I even mentioned that in my original post. But that would be extremely anachronistic and out of place on a map like this.

If you took that transverse projection and plotted a graticule on it, you'd get something very wonky looking. If you come up with some other angular coordinate system not aligned to the poles, and then align a tangent equidistant cylindrical projection to it, and the area you are mapping happens to be near the circle of tangency (effectively the equator of the new non-rotation defined coordinate system). Then sure, you could make this sort of work out. This is however such an incredibly bizarre and unlikely thing to do, and so much more complex and unintuitive to explain compared the already hard to grasp concept of map projections in the first place, that I stuck with the simple "equator" rather than trying to explain what a "circle of tangency" was so that I could then use the term to be more precise and cover this insane possibility.

[Zipster]

For not having any map-making background or anything, that's quite impressive.