How to determine the appropriate curvature for lines of latitude on a map gore

[RetailPleb]

In this example exported from G.Projector, the lines of latitude are perfectly straight/parallel to the equator. Obviously if I were to print this out and paste it onto a sphere, a line of latitude would be segmented, and not appear to be one seamless ring, as in professional globes.

As you can see here, they've printed the gores with the lines of latitude already with some curvature.

I'm wondering how that is determined, exactly. It would of course have something to do with the size of the sphere it's getting wrapped onto. At the 4:47 mark in this video, the Youtuber can be seen manually warping the map into gores and adding that curvature, but it's a time-lapse and I'm not sure if he did the math beforehand or just eyeballed it really well. I have Photoshop and I'm familiar with the warp tool. I know how he warped it, I just don't know how he decided how much to warp it by.

P.S. Also hello! First post!

[Redrobes]

You can try my Gores app.
https://www.cartographersguild.com/s...ad.php?t=37152

I cant recall whether my latitudes are flat or curved but I had the feeling that I remapped the mercator to gores so they ought to be. If your pasting it onto a globe then the distance from the points of the gores to the latitude line is equal so the line would end up being an arc from the point. The ends of the arcs at the edges of the gores would be perpendicular so that the two gores would match up the line.

I am planning to upgrade my gores app and have it with a web interface but that is some weeks away. But if I do then ill probably post the link at the end of the other gores thread I started above.

[waldronate]

The gores for a globe can be done in a number of different projections, but Stereographic and Sinusoidal projections are the most common, if I recall correctly. Those projections are relatively easy to calculate and are supported in most GIS packages. Sinusoidal will give you more of a flat character to individual lines of latitude when printed, but these lines will get curved slightly when applied to the globe. Wet paper stretches a bit and it also tears easily, which makes getting the spindle-shaped gores onto exactly the right place on the globe a difficult proposition.

One solution to the problem of lots of pieces of paper meeting at the poles is to arrange all of the gores for a hemisphere into a rosette pattern ( https://www.youtube.com/watch?v=tTca9f-0QVc shows how a commercial globe maker does this). This rosette technique requires larger pieces of paper. You can make a smaller polar rosette and partial equatorial gores to try to avoid some of the polar issues, but it's still a messy business.

The hardest thing about getting everything onto a globe is to get a spherical surface to put them on. The second hardest thing is to get your printouts sized correctly to go onto the globe. The simplest solution to the first one is to get a second-hand globe. They are often available with a stand for not too much money at your local thrift store. A little sanding and painting and you'll have a good surface to draw on or to apply gores to. Sizing the gores is more complex and will require some math.