Impervious Surface

Impervious surfaces are defined as those polygonal features that significantly (but not
absolutely) prevent the draining of storm water into the ground. Typically, these features
include buildings, building foundations, storage tanks, parking areas, roads, driveways,
runways, taxiways, aprons, hardened athletic courts, patios, sidewalks (concrete or
asphalt), concrete slabs surrounding swimming pools, and any other hardened (packed)
surface.  Note all features are captured as one single impervious feature within the impervious layer. 
Features that are typically not considered impervious include:

  • fine gravel/sand surfaces not used for vehicular traffic or parking
  • rock gardens or other areas landscaped with rocks
  •  junk automobiles or scrap piles
  • railroad beds



The following specifications are based on source imagery captured at a scale of 1:1200 -1:2400 and containing a pixel resolution of 0.5' -1.0'.

  • Minimum Mapping Unit (MMU) of 100 sq ft. (MMU is a target unit.  Some polygons captured as part of an impervious data layer might be less than 100 sq ft.)
  • Sidewalks 5' or greater in width should be captured (with imagery resolution of 0.5' to 1.0')
  • Circular portions of polygons should be captured so that the feature appears to be circular when viewed at a scale of 1:1200
  • All adjacent impervious polygons should be merged to form a single impervious polygon
  • Attempts should be made to fully captured features hidden by tree canopy, building shadow, or other obstacle by interpolating portions of hidden impervious surface areas
  • All features captured are based on aerial imagery interpretation. Field visits are not typically used to delineate impervious features.

The scope of any Impervious Surface mapping project is dependent upon the needs of that year’s participating entities.

Sample Impervious Surface File

For more information about impervious surface mapping, send us an email.