In mid-September 2004, Pennsylvania was hit by Tropical Depression Ivan, or as it became known, Ivan the Terrible. This storm resulted in damage across 56 counties primarily because of flooding along streams and rivers. More than six inches of rain fell across parts of the state, with more than nine inches in some areas.

PEMA authorized aerial imagery photography missions to document the damages and impact of flooding. This included the area of the Susquehanna River basin shown on this map. The digital orthophotographs were downloaded from PASDA and processed with GDAL to display on this web map.

For reference and comparison, imagery from USDA's NAIP dataset from 2005 was used, also downloaded from PASDA.

The basic imagery workflow was:

orthos ➡️ gdalbuildvrt ➡️ gdalwarp ➡️ gdal2tiles.py ➡️ AWS S3

The tiles are displayed on a leaflet map with a swipe control adopted from this Mapbox blog post which provides a nice, simple way to visualize the difference between the two layers. Additionally, there's a river boundary GeoJSON simplified from the data available from the USGS's National Hydrology Dataset to give a sense of where the river banks were pre-flood.