The Wisconsin Historical Society maintains the Wisconsin History and Architecture Inventory (AHI), consisting of approximately 120,000 properties surveyed by architectural historians or preservation consultants. The inventory has been assembled over a period of more than 25 years from a wide variety of sources. In many cases, the information is dated and properties may be altered or no longer exist. The majority of properties included in this inventory are privately owned and not open to the public. After 25 years of data collection, there are numerous difficulties with inaccurate addresses. Nearly 10,000 properties have errors, which makes it difficult to map out using a geocoder and have to be entered manually and validated with primary and secondary online sources. I will share how Historic Preservationist are using GIS and what online resources I use to validate and find historic sites from the past.