LOSHO (the League of Snohomish County Heritage Organizations) is taking an inventory of historical assets across unincorporated Snohomish County. They maintain a mapping system that allows users to overlay a variety of vintage maps accurately scaled to match modern survey references.
The maps allow users to easily compare the historical maps (including homestead lines) and modern parcel surveys and aerial photographs.
While the mapping system is comprised of over 330GB of data (too much for them to conveniently share online), much of the capability has also been streamlined for online access:
1) An interactive online map allowing the user to explore Snohomish County ca. 1910 can be accessed on a PC, tablet, or smartphone at: https://loscho.maps.arcgis.com/apps/StorytellingSwipe/index.html?appid=408cad1ea69943bb8aa15dc87d6cd50a#
2) An interactive online map allowing the user to overlay maps from 1910 through 1975, original homesteads, early railroads, and “County Corners”, and to explore them on a PC, tablet, or smartphone at: https://loscho.maps.arcgis.com/apps/PublicInformation/index.html?appid=d88679dd9d04410f806546b0d7f776f2
3) A map of 465 significant items, comprised of a subset of the 2015 Historic Resource Scan contracted by Snohomish County augmented with LOSCHO additions, can be found at https://loscho.maps.arcgis.com/apps/StorytellingSwipe/index.html?appid=93618e0d7d94433ca2740bebbd684f55
You can find more information on their website.