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Terrain and slope: reading the lay of the land before you buy

Listing photos rarely show how steep a lot is. Slope affects where you can put a house, how long and steep the driveway gets, whether a conventional septic system fits, and how water drains across the land.

Degrees vs percent grade

Slope is given either in degrees or as a percent grade (rise over run x 100). They are not the same number: 5° is about 9% grade, 8° about 14%, 15° about 27%, and 45° is 100%.

As a rough guide, slopes under about 8° (14%) are usually straightforward to build on. Between about 8° and 15°, plan for more grading, retaining walls or a stepped foundation. Steeper than that, many sites need engineering, and some local codes limit building, driveways or septic fields. These are rules of thumb, not code limits. Check your county's rules.

How the elevation data is made

The USGS 3D Elevation Program (3DEP) publishes a seamless elevation model for the US at about 10 m resolution, much of it from lidar. Slope calculated from it is reliable for the general lay of the land. It can miss small features like a ditch, a bank cut or a building pad, and it describes one grid cell, not the whole parcel.

Soils go with slope

USDA soil surveys (SSURGO) describe each mapped soil's drainage class, shrink-swell potential, ponding and depth to a restrictive layer. Poorly drained soils, shallow bedrock and high shrink-swell can each add cost for foundations, septic systems and roads. A soil survey map unit is a general description of an area, not a test of your exact spot.

Questions to follow up

  • Walk the land and look at the site you'd build on, not just the road frontage.
  • Ask the county health department about septic requirements, and schedule a perc test if you need septic.
  • For steep or unusual sites, get a topographic survey or ask a civil engineer to look.

What a Groundtruth land screen checks

  • Elevation, slope and the direction the slope faces at the pin (USGS 3DEP)
  • Soil map unit, drainage class, hydrologic group, shrink-swell, ponding and depth to a restrictive layer (USDA SSURGO)
  • Modelled depth to bedrock
  • Landslide susceptibility (USGS) and whether the point is in a mapped karst area

What it can't tell you

  • Slope across the whole parcel or at the exact building site
  • Whether the soil will pass a percolation test or support a septic system
  • Fill, drainage problems or erosion on the ground
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This guide is general information, not legal, engineering or insurance advice. Rules vary by state and county.