The GIS Facility is providing buffered, HUC12 based 2020 P class and 2010 P and S class elevation data for download. HUC12 watersheds that cross the Iowa border are not yet available in this format although the original DEM tiles can be downloaded from the USGS National Map viewer.
In order to download HUC12 DEM data, you will be asked for some information that we use to track use of our application. The application tools will NOT work with Microsoft Edge or Internet Explorer; we recommend using Chrome or Firefox.
The 2010 Iowa HUC12 2m DEM database was derived from the original LiDAR point cloud and processed through ArcGIS using the following steps:
- LiDAR point-cloud to points to TIN
- TIN to raster with pyramid thinning
- Conversion to integer cm Z values to address quantization error
- Pit filling to remove one-cell sinks
- Hole punching to remove small (<=10cm) depressions in the DEM
This process results in the P-Class of HUC12 elevation models. This collection has been augmented by performing a partial hydro enforcement using the IA DNR’s stream centerline to create the S-Class library. The process of preparing the stream centerlines for use in hydro modification is as follows;
- Stream features are extracted from the state-wide feature class for the HUC12 watershed
- Features are generalized using the SimplifyLine method using the Bend_SIMPLIFY algorithm and a 10m tolerance
- Resulting features are split at vertices to create individual features with no intermediate vertices
- Very short segments, <= 8.5m, are merged where possible
The resulting features are used as cut lines when an obstruction to flow is present in the DEM that should be "burned through", such as a culvert which routes flow beneath a road. The centerline data were developed from a 1m DEM so there are some minor flow compatibility issues, but the time savings that is realized when involved in the hydro conditioning phase of the toolbox is significant. Average size of the HUC12 2m DEM in a 7Zip archive is 75Mb.