Hi Stuart - it's a pad of dotted Fabriano graph paper I stole from an office I worked at a few years back. I think Rhodia do some similar exercise books.
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Fabriano are most well known for their artists/watercolour papers, but the pad in question isn't anything extra fancy. Likewise Rhodia basically make somewhat fancy looking notebooks/pads - not in the ridiculously lavish Moleskine bracket though.
The maps are drawn on A4 paper, and then the contrast on the pencil was tweaked a little bit in Photoshop.