Weird gravestone

This odd gravestone is in Greyfriars Kirkyard, Edinburgh. There are a couple of weird things about it. Firstly at the top is carved:-

"16 R.L 87
Repaired 1815"

And right enough there is a great big crack right across it. But the first burial detailed on the stone isn't until 1828. Then there is the script at the bottom:-

"From west to east eight feet
From south to north twelve feet"

It could be the size of the lair but I have never seen this before and can't imagine why it would be necessary. Very puzzling.

There is a photograph of the full stone here but the text is a little hard to read.