How Impervious Area on a Property is Calculated
The amount of impervious area on a property is determined by using a
geographic information system (GIS) to analyze aerial photographs and
satellite imagery. These images are taken in the winter and early spring
when trees have lost their leaves. This makes it easier to determine
where impervious areas are on each property. These pictures are put into
a GIS that identifies the impervious area on each property.
Residential Property Example
In these examples the GIS program identified roofs (orange),
driveways (yellow) and private sidewalks (also yellow) as impervious
A technician then calculates the area for each of these surfaces. He or she also identifies what type of impervious area they are. Once the total impervious area is calculated for a property, it is assigned to a billing tier as outlined in the city’s ordinance. In the 1st example, the total impervious area is 1,906 square feet. As a residential property it would be assigned to the 1st tier and charged a yearly utility fee of $39.12 (or $3.26/month).
The next 2 photos are examples of tier 2 and tier 3 properties respectively. Learn more about the stormwater utility fee tiers and rates for residential properties.
Non-Residential Property Example
The impervious areas on commercial properties are identified in a manner very similar to that used for residential properties. A geographic information system (GIS) is used to analyze aerial photographs or satellite imagery to identify impervious surfaces. A technician then calculates the area for each of these surfaces. He or she also identifies what type of impervious area they are. In the example to the left, the roof of the building is shown in orange and the parking lot and walkways are shown in yellow. The stormwater utility fee is then determined by dividing the amount of impervious surface into Equivalent Residential Units or ERUs. An ERU in Durham is 2,400 square feet. For each ERU of impervious surface, the property owner is billed $6.75/month.