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Can anyone explain how to interpret a symbol size of 10 or let's say 100? What kind of units are these? I ask because recently I had to deal with a reference scale and I wasn't really able to explain what symbol size means.

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Usually, this value is a pixel number. Which software do you use? – julien Jan 3 '12 at 9:39
I am using Arcmap – Przemek Jan 3 '12 at 9:52
Normally symbol sizes are given in points, not pixels. 72 points = 1 inch = 2.54 centimeters. (I'm not posting this as a reply because I haven't checked it in ArcMap to confirm and it's quite possible ArcMap uses various--and different--conventions, depending on which committee wrote which piece of the software :-).) – whuber Jan 3 '12 at 16:07

I think this page at ESRI answers your question:

To paraphrase, points are 72 units to an inch. So a 100 point symbol should be roughly 1.4 inches. But just like when points are used to describe font sizes, there is some variability in size depending on how the symbol was designed - the same reason a page in 12 point Arial font will contain a different number of characters from the same page in 12 point Times New Roman. Taking a look at the ESRI fonts in an application like MS Word might be a good exercise to see the difference in relative sizes.

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Thank you for your anwser Jason it helped me a lot! – Przemek Jan 5 '12 at 13:59

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