Some time ago the good folks over at the CEF Study Group (www.cefresearch.com) posted material on a new project undertaken by McMaster University Libraries called "World War I Military Maps & Aerial Photography - France & Belgium" (library.mcmaster.ca/maps/ww1/home.htm). As the website notes: "In the early 1970s, McMaster University Library acquired a World War I collection of archival material. Included in this collection were over 400 World War I military maps which now reside in the William Ready Division of Archives and Research Collections." Most of the maps are of the British Ordnance Survey variety with a few German and French maps also available. Most of the maps are 1:10,000, 1:20,000 and 1:40,000 scale and leans towards 1917 and 1918. The aerial photographs, approximately 350 in total, are mostly Royal Air Force products from 1917 and 1918. There is also a wide variety of explanatory material to help with deciphering exactly what it is you're looking at and how to read the maps.