HWM Depth Grid Process Overview

Data Acquisition Terrain Data - Raster Best available dataBest available data USGS National Map (NED) National Map (NED) Available nationally at resolutions of 1 arc-second (about 30 meters), 1/3 arc- second (about 10 meters)Available nationally at resolutions of 1 arc-second (about 30 meters), 1/3 arc- second (about 10 meters) 1/9 arc-second resolution data is very limited in coverage for most areas of the U.S.1/9 arc-second resolution data is very limited in coverage for most areas of the U.S. LiDAR available through local municipalitiesLiDAR available through local municipalities High Water Mark Data - Vector Multiple collection methodsMultiple collection methods Field Survey’sField Survey’s Tape Measure and GPSTape Measure and GPS Must be collected in the fieldMust be collected in the field SLOSH – Coastal FloodingSLOSH – Coastal Flooding ***NGVD 29 vs NAVD 88*** ***NGVD 29 vs NAVD 88*** Make sure all data are in the Make sure all data are in the same vertical datum same vertical datum

Add Terrain and HWM data

Interpolate the Water Surface

Subtract WSE from DEM

Remove Negative Values

Convert to Polygon

Extract Flood Boundary

Extract Depth Grid

Process Overview

Useful Links USGS NED - NED - Vertcon Conversion -Vertcon Conversion - bin/VERTCON/vert_con.prlhttp:// bin/VERTCON/vert_con.prl MOTF SOPs – SOPs – US Interagency Elevation Invnetory - Interagency Elevation Invnetory -