Grounded in Philly gets its data from a number of city data sources, including but not limited to:

Where an API is available, Grounded in Philly updates the data on a rolling basis to keep the data up-to-date.

Once we have data for a lot, we give a weight to each piece of information and only show the lots that exceed a certain threshold. By doing this we avoid putting properties on the map that are not actually vacant. For example, a property that only has a vacancy violation but is not very permeable might not exceed the threshold.

On each lot's page we have a section containing the data points that we think indicate that the lot is vacant. We also have a way to report lots that aren't actually vacant to correct the map.

We are still working on finding the most appropriate threshold, and we appreciate you letting us know when we're wrong, both through our contact form and the "Have we made a mistake?" button on each lot's page. We look forward to making the map more accurate together!