Pavement, buildings, and other sorts of “impervious land cover” seal off soil surfaces, which prevents natural groundwater recharge and rainwater to run off. This leads to decreases in water quality, poor wildlife and fish habitat, heat islands, and lower air quality. The Tahoe Regional Planning Agency (TRPA) regulates the amount of impervious surface allowed on a parcel of land and requires property owners to prepare assessments to quantify impervious surfaces to ensure they are complying with these regulations. This helps TRPA to identify actions that can mitigate negative environmental effects.
SIG was contracted to collect high-resolution imagery for 16 different parcels at South Lake Tahoe. Image collection was performed using a DJI Mavic Pro quadcopter UAS. A quadcopter is a very maneuverable type of UAS, and allowed the team to work in confined areas, with exact lift-offs and landings. It collected at least 20 photos per parcel. The 3 cm/pixel resolution photos were stitched together using cloud-based photo processing to create a 2D image of the parcel.
Once images were combined into a mosaic for each parcel, ArcGIS 10.5 software was used to ensure they were spatially accurate. The software matched three or more points in each image to a base map with known geographic coordinates (in this case it was the ESRI base map, one of several widely-used base maps).
Features in the images were then classified based on 15 different types of landcover. Pavement and building footprints and how they related to TRPA-designated land use types and lake set-back requirements were of most interest to the team, given the project objectives. The SIG UAS Team also used a digital elevation model from high-resolution LiDAR data to develop contour lines and to define Lake Tahoe’s high-water elevation.
This project was the first time that a UAS was used to map and quantify impervious cover for the TRPA. Typically, more costly but traditional land-based surveys are deployed for this application. That’s important because as this technology becomes more readily available, meeting regulatory requirements like this will become quicker and cheaper for property owners, as well as local governments (and therefore local tax-payers) who need to monitor regulations. See a great 360 degree UAS image of South Lake Tahoe here
