How thick in ground units is this line on a printed map?

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.

• 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
• @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