If you're drawing a road on a map, and that road happens to be straight, it's easy to draw its two sides parallel: you just move the ruler. But most roads twist and turn, and drawing their sides perfectly parallel cannot be done with a ruler. And yet even before computers came along, no matter how windy their roads, cartographers appear to have had no difficulty drawing both sides of their roads perfectly parallel (or I should probably say equidistant). I cannot begin to imagine a mechanical way to do this. Can anyone tell me?
Parallel Pens with special split nibs or something similar. Sorta like using two pens taped together into one pen with two tips
An example can be found on the Scribblers calligraphy website:
Airphotos have been the source materials for a lot of maps for a long time. To result in a nice exact machine made look they would use dials to help draw the lines on transparent mylar sheets. The reason those maps had roads that were nice and parallel was the machines and skilled operators. Parallel Pens and parallel ruler are useful for other map making methods.
There is always the parallel ruler for lines that are further apart.
A more modern version would be those with rollers but that is not as good in accuracy as a fixed mechanical design.
Two other methods:
- Engineering drawings (e.g. highway plans) usually use curves with defined radii, so drawing those is mostly a matter of using a compass and creating the geometry from the same instructions for both sides. (Edit: I say "same instructions" because roads are first defined by their centerline path, then parallel stuff is based off of that. So for a highway with a 200-foot right-of-way, you'd copy the straight sections on each side, 100 feet away from the center. For curves you would add 100 feet to the radius when drawing the outside of the curve, and you'd subtract 100 feet from the centerline to do the inside of the curves.)
(Edit: as pointed out in the comments, the following method gives 2 identical curves rather than 2 parallel curves.)
- There's another method using the Lesbian rule (named after the island of Lesbos, where they originally got the material), aka the flexible curve. Descriptions of both are on Wikipedia (here and here respectively). This isn't as handy for small squigglies, but it's great if you can form a gentle/large curve, trace one side, move the rule, and trace the other side.
The answer that @AMontpetit provides shows the pen and ink drawing tools used to draw parallel lines. There are also scribing tools that will draw these lines. The tools have a pivot built into them, allowing you to keep an equal distance between the lines when going around curves. The tools could have a single blade or multiple blades for parallel lines. I used these tools in my first cartographic jobs a few decades ago...
See this video for an example: https://youtu.be/Ovu513papoc?t=1365
Sometimes it's hard to imagine doing things without computers. Especially if you were born into a year when computers were ubiquitous. But somehow, some-way...people got around before them.
As fantasmagorical as it may seem...before computers, cartographers and graphic artists had ADEPTNESS and FACILITY with their hand tools. My own personal experience, for example, in this exact frame: I used an 000 rapidograph with a flexible rule to draw pseudo-parallel lines on town maps in the '80s. Sometimes it took several attempts before a "keeper" was arrived at. The lines would not be strictly parallel, but "parallel enough."