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Sometimes in discussing how accurate this that or the other thing needs to be someone will make the observation that "a line shift of 30m at 1:1million scale is less the width of the line itself when printed, so, no worries". That's great! For that map that is, but what about this map which will be printed at 1:25,000?

This got me thinking that it would be useful to have a quick reference chart of "how thick is an Xpt line on the ground at Y scale at Z dpi?". A bit of searching reveals no such beast readily available online, so let's fix that!

I suggest a base line of 0.2 pt and 300 dpi since in offset printing that's the thinnest reasonable colour line and standard dpi.

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Making a chart would be about as useful as printing a multiplication table. After all, by definition of scale, an X point line at a scale of 1 : Y represents X * Y points on the ground which is X * Y / 72 inches or X * Y / (72 * 12) feet or X * Y / (72 * 39.37) meters, etc. Those who prefer to measure line thicknesses in, say, mm, would either need a completely different set of tables or would need to convert the units appropriately, anyway, so having to multiply is no hardship. All anyone has to remember, then, is to multiply the map distances by the reciprocal scale. –  whuber Dec 6 '11 at 1:00
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@whuber, point taken. That said, I would still find such a table of common scales useful. I guess 'cause I'm not very handy with math (given the up-votes, I have company). For example while composing this post, which took 30 minutes or so, I had a spread sheet open in which I started putting together a scale table. My table didn't arrive at the same number a dmahr's answer below. I also had at least twice as many calculations involved, which likely contributed to my wrong results. ;-) –  matt wilkie Dec 6 '11 at 6:50
    
For reference, matt, I posted a spreadsheet image (not the file itself) so you can see one way to do it. It contains one formula almost identical to that in @dmahr's answer. The value cells are given the names next to them using Excel's Insert|Name|Create tool. –  whuber Dec 6 '11 at 15:08
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up vote 6 down vote accepted

As per dmhar's answer, here's an automated solution that you wanted. Since I don't know much about typography or maps, I have no idea what are the commonly used line widths and map scales. If you can provide me with a list, I'll add it to the page.

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(+1) Wonderful! –  whuber Dec 6 '11 at 15:08
    
very nice, thanks! It would nice to have it at a stable location. If you're amenable to the idea I'll look into what it would take to host it on the GIS.se blog (I'm not sure if we can put in arbitrary javascript). –  matt wilkie Dec 6 '11 at 18:36
    
If you can, no problem. Though my private site has been there for years and I don't intend for it to disappear for many more years. Maybe I should buy my own domain finally. :P –  Vilx- Dec 7 '11 at 8:44
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One PostScript point is one seventy-second of an inch (1/72"). One inch 0.0254 meters. So one point is 0.000352777778 meters. Multiply this by the denominator of a map scale to get the scaled size:

[scaled distance ] = [line width in points] * 0.000352777778 * [map denominator]

By this math, a 0.2 point line is equivalent to 1.76388889 meters on a 1:25,000 map.

EDIT: It should be noted that on most printed paper, ink will bleed away from the paper fibers to which it was initially applied. So while you might set a line to be a specific width, it will be slightly larger when printed. In typography, ink traps are used in some typefaces to counteract this effect, especially with small fonts and/or when printing on bleeding-prone newsprint paper.

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