Dr Walker starts to unravel the twine again, and notices that the twine, after the several foot-long or so marked sections have given way to clear lengths, that the first clear length does in fact match the distance to the second gravestone mentioned at the back of the photo. It doesn't take a lot of intelligence to realise that the twine was wrapped around the first gravestone a few times to secure it in place.
Dr Walker then trails the twine to the next one mentioned, and again is unsurprised to see that after wrapping the marked sections around the gravestone, that the clear length again matches. The third is the same, and leading back to the first gravestone from where the photo was taken, also matches up, leaving the twine wrapped around four of the gravestones, with clear lengths between them, and four knots in total in the clear lengths.