Chapter 13 figure for orthophotography

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There's a reference in chapter 13 on line 147: \@ref(fig:13-ortho) (which I've temporarily removed the \@ to keep it from throwing a LaTeX error), and a description of a figure comparing images. There's also an image to go with it: images/13-orthorectification.png. But we'll need to determine a caption, get a citation, and assess permissions before adding. Or we leave it out and edit the text.