The data aren't perfect. Instead of painstakingly checking everything I released the map to so the public could take a look and notify me of any errors they might find
Causes/sources of errors can be attributed to the following:
Incorrect data from government resources
Old houses/structures have "estimated" constructions years based on criteria the Assessors use (see comment here). I adjusted for this by using the median build year of surrounding structures for each building with a build year of "1900". You can read more about how I approached this issue here
Missing data points. School construction years aren't in parcel or assessor data, and aren't availble on one nice web page like UI data.
Spatial join/proximity issues. I geocoded some data scraped from web pages. Buildings with no data were assigned a construction year based on the nearest point, or the median build year of those surrounding it.
You may notice building footprints aren't 100% accurate. I simplified the JSON to reduce load time and improve performance - which resulted in the building footprints changing in some cases.