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Parcel data is the map of land ownership: one polygon for every piece of land, with the records that describe it. Each parcel carries a boundary, a parcel number, an owner, a land use code, acreage, and an assessed value. Counties maintain parcel data to tax property and record who owns it. Insurers, utilities, developers, and AI systems use the same data to tie an address, a building, or a satellite observation to a legal unit of land.
Parcel data has two parts: geometry and attributes. The geometry is a polygon, or several polygons, that outlines the parcel. The attributes are the columns of the assessor's record. The State of Connecticut defines parcel data as property and land attributes such as acreage, usage, ownership, and boundaries.
Fields vary from county to county. Regrid, which compiles parcels nationwide, maps over 100 county-provided columns into one standard schema and keeps each county's extra columns alongside it.
A parcel number is the identifier a local taxing authority assigns to each parcel. Counties that use the term assessor's parcel number abbreviate it APN. Other jurisdictions say parcel ID, property identification number (PIN), assessor's identification number (AIN), or tax ID. The number is unique within its jurisdiction and repeats across jurisdictions, so nationwide datasets pair it with a county code or a provider key.
The format encodes how the county maps land. San Francisco combines a block and a lot: parcel 3590-035 is lot 035 in block 3590. New York State uses section, block, and lot, so its tax bills call the identifier the SBL number. Other counties concatenate book, page, and parcel from the assessor's map books.
A parcel and a lot are different units. A parcel is the unit the assessor taxes. A lot is a subdivision of land with a recorded legal description, and one parcel can contain several lots.
Parcel data starts at the local government that assesses property. In most states that is the county assessor, recorder, or GIS office. In New England, cities and towns hold the records. Each office maintains its own parcel map, often linked to a computer-assisted mass appraisal (CAMA) system that stores the values.
Three layers of publishers sit on top of those local sources:
Freshness depends on the source, since each county updates on its own schedule. Regrid reports that on average 94% of its parcels were refreshed in the last 12 months.
A parcel boundary shows where a parcel sits. A property line is the legal boundary in the deed. The two often differ by a few feet, because tax maps are compiled from deeds, plats, and older maps for assessment.
Parcel boundaries also shift against imagery. A parcel layer and an aerial photo come from different sources with different errors, so lines can cross roofs or fall into streets. Regrid does not align parcels to imagery for this reason. It sends large shifts back to the county to correct. For a fence, a dispute, or a building permit, a licensed land surveyor sets the legal line.
Counties publish parcels as Shapefile, File Geodatabase, GeoJSON, or CSV with a geometry column. Nationwide providers add GeoPackage, KML, and SQL exports. Shapefile truncates column names to 10 characters, which breaks long schema names such as parcelnumb_no_formatting.
County data usually arrives in a local projection, such as a State Plane coordinate system in feet. Nationwide datasets reproject everything to WGS84 (EPSG:4326) longitude and latitude so counties line up. Analysis at state or national scale converts parcels to a columnar format such as GeoParquet or Apache Iceberg, with a spatial index for joins.
Parcel data answers the question "what piece of land is this, and who owns it?" for any location. Common uses:
Parcels gain value when joined to other spatial data. A spatial join matches records by location: each building footprint to the parcel it sits on, each flooded pixel to the parcels it touches. One query can count the parcels inside a Sentinel-2 flood extent and total their assessed value, and an elevation model adds how low each parcel sits.
Wherobots runs these joins at state and national scale. In one project, WherobotsDB scored every building in Colorado, 2,771,126 Overture buildings, against the Regrid parcels they sit on across five perils. The result is the Colorado property risk explorer in the Havasu catalog. A Regrid parcel sample is ready to query in Wherobots open data as wherobots_open_data.partner_samples.regrid_parcels: 7,935,620 parcels in Los Angeles, Orange, Riverside, San Bernardino, and San Diego counties in California and Maricopa County in Arizona, with over 130 columns.
wherobots_open_data.partner_samples.regrid_parcels
Parcels are also context for AI. The models people use every day were trained on text, documents, databases, and the internet. They can describe a neighborhood, and they cannot compute which parcel contains a point. An AI agent connected to parcel data through the Wherobots MCP server can answer "which parcels over 15 acres sit within a mile of a substation?" with a spatial query.
Join parcels to buildings, imagery, and hazard layers on the Wherobots free tier at cloud.wherobots.com.
Parcel data is the map and records of land ownership units. Each parcel has a boundary polygon and attributes such as a parcel number, owner, land use, acreage, and assessed value, maintained by county and city governments for property tax and land records.
Parcel data starts at the local government that assesses property: usually the county assessor, recorder, or GIS office, and in some states the city or town. State GIS portals combine county data, and companies such as Regrid collect it from counties, states, and municipalities into one nationwide dataset.
Parcel records are public records in most US jurisdictions, and many counties and states publish parcel boundaries for free download. Terms vary by county: some charge for bulk files, and some withhold owner names. Nationwide, standardized parcel data is a commercial product.
A parcel number is the unique identifier a local tax authority assigns to a piece of land, often called an assessor’s parcel number (APN). Other names include parcel ID, PIN, AIN, and tax ID. The format differs by county, for example San Francisco’s block and lot number or New York’s section, block, and lot (SBL) number.
Yes. APN stands for assessor’s parcel number, and it is the parcel number in counties that use that term. A property ID or tax ID usually refers to the same identifier, though some counties keep a separate tax account number alongside the parcel number.
The parcel number appears on the property tax bill and on the deed. The county assessor’s website and the county GIS parcel viewer let you search by address and return the parcel number with the boundary.
County GIS parcel viewers and nationwide parcel maps draw parcel boundaries over aerial imagery. Those lines come from tax maps and can sit several feet off the legal boundary. For an exact property line, hire a licensed land surveyor.
Counties publish parcel data as Shapefile, File Geodatabase, GeoJSON, or CSV. Nationwide providers add GeoPackage, KML, and database exports. For analysis at scale, converting parcels to GeoParquet or Apache Iceberg tables keeps geometry and attributes together in a columnar format.
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