Development of Impervious Surface Data

IMC starts by digitizing polygons delineating impervious surfaces from existing digital orthophoto tiles. These orthophoto tiles are geo referenced and the impervious surface polygons are collected on the same horizontal datum as the orthophotos.

Typical collected features:
Building footprints: houses, sheds, garages, barns, commercial buildings, etc.
Paved driveways: concrete, asphalt, or brick. Driveways will be collected all the way to the road, but they will be cut to the edge of the right of way if the Town requests.
Paved private walkways: concrete, asphalt, or brick. Does not include public sidewalks that parallel a public road and are in the road’s right of way.
Paved slabs: pool aprons, athletic courts, paved patios, etc.

Quality Control of Polygon Collection Process

IMC digitizes impervious surface polygons using our VROne mapping software. VrOne has compilation settings that ensure accurate square angles when collecting buildings or other features with uniformity in their shape.

Great care is taken to prevent overlapping polygons (e.g. a roofline polygon overlapping into a driveway polygon) as overlap areas would consequently be double counted. Proper snapping tools will be used to collect polygons that overlay one another at their conjoinment, but do not overlap. The accuracy of the shape and placement of the impervious surface polygons is guaranteed to the same horizontal specifications as declared by the source owner of the digital orthophotos.

GIS Database Development

IMC designs, develops, and populates ArcGIS File Geodatabase (FGDB) for the impervious surfaces project. The FGDB file is populated with digitized polygons, it is designed to place each polygon on its correct sub-type (e.g., building, driveway, and sidewalk) and calculate area and perimeter in feet for each feature.

GIS software is utilized to attach unique parcel data (i.e., PIN) to each impervious polygon. Land use categories are attached to each impervious polygon, based upon the County’s land use data. Date and Time attributes are generated for data creation and maintenance/update tracking. All impervious features are QA/QC’d by observing the feature and updating attributes to indicate surfaces are complete. Any questions or omissions are flagged and revisited to resolve or complete.