There are several ways to estimate impervious cover citywide. For most properties, we are using aerial photography and LIDAR data obtained in 2012. For properties developed or modified afterwards, we need other sources of information. Originally, we based impervious cover estimates on:
Construction site plans and building permits
The impervious cover values already in use for assessing the drainage charge
Of these options, the first is most reliable. We are gradually converting impervious cover estimates to construction site plan and building permit data for properties developed after 2012. Account holders may see a change in their drainage charge as these updates are made. In the vast majority of cases, the changes to the drainage charge have been less than $10 per month.