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Q&A

How do mathematicians measure shape perimeters?

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When I create a circle on a computerized plane with a graphic editing program (such as Window's Paint), I can see a group of dots circling around an imagined central point.

if the perimeter is comprised of just one layer of dots we could always add another layer of dots on top of that first layer, and then a third one on top of the second layer, and so forth.
In CSS code it might be: border: 1px solid black and then 2px and then 3px.

In classical mathematics, how do mathematicians set or measure the perimeters (border thickness) of circles and other shapes that they draw on paper, or in a more daily life manner, if one has a pipe with the diameter of 3cm, how could that person measure its perimeter (circumference)?

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2 comment threads

"Thickness," not length of perimeter (4 comments)
Using formulae? (5 comments)

2 answers

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A main idea mathematicians use to define lengths of curves, areas of flat shapes, volumes of solids etc is to divide them into smaller and smaller parts such that

  1. The finer and finer subdivisions approximate the shape better and better.
  2. The measure of the small parts is already defined.

However, one needs to be careful that this approach creates a definition that is unique and that the resulting definition agrees with what we expect in cases where we can define the measure in a different way or measure them on physical objects.

Your example of computer graphics using a grid of pixels to approximate shapes is both an illustration of this method and also an illustration of some of the difficulties.

Example 1: Measuring the area of a disc

Subdividing a circular disc into squares and adding up the areas of the squares will give a good approximation of the area of the circle, when the squares are small enough.

Example 2: Measuring the circumference of a circle

If you just count the number of squares on the perimeter of the disc to get an approximation of the length of the circumference, you can get a wrong result. Approximating a circle with pixels in a grid (own work, CC BY-SA) Somehow the result is too short, and it would remain too short even if the subdivision was made finer and finer.

The method could be improved, for example by noting that some of the pixels are only connected at a corner and thus their centers are further away than the side length of the square.

But I expect it to be difficult to describe mathematically the location of all the pixels on the circumference in a finer and finer grid.

Example 3: Inscribed polygons (Archimedes)

We could approximate our circle of radius 1 by a regular hexagon inscribed in the circle, this would give an approximate perimeter of 6. We can also define a process for better approximations: Given an inscribed regular polygon we double the number of sides and calculate the perimeter of the new polygon. We get a process like this:

number of sides   length of one side   circumference
---------------   ------------------   -------------
              6                1.000           6.000
             12                0.518           6.211
             24                0.261           6.265
             48                0.131           6.279

see for example Regular 12-gon The circumference of the polygons quickly approach the circumference of the circle.

This Video (Youtube) shows how the approximations can be calculated using Pythagoras' formula.

Further Reading

Going from these better and better approximation to the true value is called "taking the limit" Wikipedia: Limit (mathematics)

The field of mathematics defining these limits rigurously and studying their properties is called calculus. Wikipedia: Calculus and for these problems of adding up "infinitely small" parts the tool of integration is used. Wikipedia: Integral

This article on arc length (Wikipedia) shows formulae for calculating the length of more general curves using calculus.

There is a demonstration of the process of approximating a circle with polygons available on Wolfram Alpha, however they are using it for the area of the circle rather than the circumference.

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The perimeter doesn't work because your pixels are incorrect (2 comments)
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A physical circle would be measured in centimeters or other applicable unit.

A circle in a coordinate system is measured by the unit implied by the coordinate system.

A circle in a more general Euclidian space is measured by building a measure, which is done by assigning length one to some line segment and thus using it as a unit.

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