http://doi.acm.org/10.1145/248603.248610 db/journals/sigmod/Gardels97.html Kenn Gardels Open GIS and On-Line Environmental Libraries. 32-38 sigmodR/26-1/P032.pdf 1997 26 SIGMOD Record 1 db/journals/sigmod/sigmod26.html#Gardels97 http://www.acm.org/sigmod/record/issues/9703/Special/gardels.ps