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Geocode vs Parcel
- previous - Main data-fetching explanation
Geocoding means getting information about an address by running a process that accepts an address string or another identifying feature that the geocoder accepts (like an OPA number). The City of Philadelphia's geocoder is called Address Information System (AIS) - here's its Github location. It has a parser, so it can guess which exact address you mean when you search an imperfect address string. The info returned by AIS is
- a single [longitude, latitude] geometry point
- an extensive set of info from all city departments.
You can't search AIS using a [lng, lat] location, because it only knows a single [lng, lat] for any address. The data returned includes PWD parcel id and DOR parcel ids, if AIS knows that the address corresponds with parcels from each dataset.
The parcel layers (PWD and DOR) are sets of polygons (built from many [lng, lat] points) showing the exact locations of every building in the city. There is a small amount of data about the parcel in each of these polygon features, including an id and an address string. You can do a spatial query of the polygon layers using a [lng, lat], because a [lng, lat] point can fall within a parcel polygon. You can't reliably search the parcels by address, because you would need an exact match.
There ARE some parcels in the PWD and DOR parcel datasets that are NOT associated with any address in AIS. They are usually old "inactive" parcels. There are people (real estate researchers) that need to be able to research parcel info that includes parcels not associated with any AIS addresses, and providing this kind of difficult esoteric data is one of the goals of atlas.phila.gov.