I use the Historicalaerials site to snip historical aerials of local parks, highway rest parks etc. (dates vary site to site) and save them as jpgs. I then import the jpg file into Google Earth and georeference them.. The georeferenced fiiles are saved as a KML extension and the files copied to my Garmin handheld GPS. The georeference files retain the watermarks from the original website but they are still very usable for detecting. Hope someone can use this...