this guy has been doing it very well for years:
http://www.flickr.com/photos/excusemysarcasm/
you are late to the party, but have at it if that's what blows up your dress.
will you be documenting all graf in the D or just Detroit writers? taking the project to the next level [[for let's say, a Blurb book) would include making a google map and some narrative/bio on each artist. if you get to know the writers, you will know the spots. where, how will you draw the line betw masters and toys? all inclusive? tags, throwups, burners, pieces? what about street art -- in or out? lots of decisions if you plan to make significant contribution to documentation.