• Question: what is the opisometer?

    Asked by kellym to Shane, Karen on 15 Nov 2013.
    • Photo: Karen McCarthy

      Karen McCarthy answered on 15 Nov 2013:

      An opisometer is a tool used for measuring maps, or more correctly, for measuring lengths of arbitary curved lines.
      A basic opisometer has a toothed wheel of a known diameter on a handle. The wheel is placed in contact with the curved line to be measured and run along its length. By counting the number of teeth passing a mark on the handle while this is done, the length of the line can be calculated: line length = wheel diameter × teeth counted/teeth on wheel.

      They’re normally used for calculating distance on a map, so normally the results are in kilometers or miles.

    • Photo: Shane Mc Guinness

      Shane Mc Guinness answered on 18 Nov 2013:

      Strange question Kelly, but good one! I haven’t used one of these little things in a long time. Yep, Karen has it right, it’s used to measure curved lines.
      All rulers are straight, and most aren’t bendy so its hard to measure things that are curved. This little tool allows you to do this, using a little wheel which adds up the total distance as you push it along the paper. Some just click every time it turns around and you count this. Other ones have little dials which tell you how far you’ve travelled.
      I’ve only ever used one of these once though. You can also use a piece of string, if you lay it along the line, then straighten it out and put it against a ruler.

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