{
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  "body": "\n## Summary\n\nA contour line of a function $f$ of two real variables is a curve in the plane along which $f$ has a constant value $c$.\nIt is a [[level set|0B2RZ0778N]] $\\{(x, y) : f(x, y) = c\\}$ of $f$, drawn in the $(x, y)$-plane.\nA contour map draws the contour lines for several equally spaced values of $c$. It shows the shape of the [[graph|0JBJYT69ZV]] of $f$ in the plane.\n\n## Remarks\n\nOn a topographic map, $f$ is the height above sea level, and the contour lines join points of equal height.\nWhere the contour lines of equally spaced values are close together, the function changes fast.\n\n## Explanations\n\n### L1 human\n\nImagine that you cut a hill with horizontal planes at heights $10$, $20$, $30$ and so on, and you look at the cuts from above.\nEach cut gives a closed curve on the map, and these curves are the contour lines.\n",
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