[11.1] - Back to the park
The following is a map showing contour lines for a region of Orangerock National Park.
Estimate (numerically) the average elevation by sampling the elevation at the mid-point of each sub-rectangle.
Going across the the rows, I estimated the heights of points in the middle of each box and added them together, then divided by 16 to get...
$$\begineq
\text{average height}=&(50+92+121+109\\
&+83+101+95+62\\
&+76+94+89+58\\
&+31+70+94+90)/16\\
=& 1,315/16=\color{blue}82.2
\endeq
$$