Guides

How to make a site base plan from an address

Find the parcel in your state or county parcel layer, download USGS 3DEP elevation for the area and generate contours, add FEMA flood zones, NWI wetlands, SSURGO soils and an aerial, then bring everything into CAD in the local State Plane coordinate system. Done by hand this takes one to three hours; the result is good for feasibility and concept work, not for construction documents.

What a base plan needs

• Parcel line and neighboring parcels, from the state or county assessor GIS layer (see free parcel data by state).
• Topography: contours and slope from USGS 3DEP elevation data (how to get contours).
• Regulated areas: FEMA flood zones and National Wetlands Inventory polygons.
• Soils: NRCS SSURGO map units and their drainage and hydrologic soil group.
• Context: road centerlines and names, and a recent aerial photo.

Steps, by hand

1. Locate the site. Geocode the address and note the county. Pick the State Plane zone for that county; every layer has to end up in it.
2. Get the parcel. Query the statewide or county parcel service for the polygon at the address, plus the parcels around it. Save as GeoJSON or shapefile.
3. Get elevation. Download the best 3DEP DEM covering the parcel plus a buffer (100 ft is a good default) from the USGS National Map Downloader, reproject it to State Plane feet, and generate contours.
4. Get the overlays. Clip FEMA's National Flood Hazard Layer, the NWI wetlands layer and SSURGO soils to the same extent.
5. Get an aerial. Export a georeferenced image (World Imagery or USDA NAIP) for the extent.
6. Assemble. Bring it all into CAD or GIS on sensible layers, add a north arrow, scale and a note that it's planning-level, and write down where each layer came from and how old it is.

Where it goes wrong

• Layers in different coordinate systems or units (meters vs US survey feet vs international feet) that don't line up.
• Assessor parcel lines treated as surveyed boundaries. They're often 5 to 50 ft off (why).
• Contours drawn tighter than the elevation data supports, which implies accuracy that isn't there.
• No record of sources and dates, so nobody can tell later which layer was stale.

Everything here is planning-level public data. Confirm boundaries, topography, wetlands and flood status with a licensed surveyor and the right specialists before design decisions depend on them.

Related

• How to get contour lines for a property
• What is a base sheet?
• Public-data base plan vs a boundary survey

Updated October 2026.