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Abstract of Marin NHD Workflow

Marin NHD Local

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California: Marin County workflow, processing from LiDAR-derived terrain to yield candidate polygon features for Watershed Boundary Dataset HUC-16 and HUC-14 and networked flow line edge features for National Hydrography Dataset Local Resolution.

This document summarizes workflow to produce model-derived surface flow lines and standards-compliant watershed boundary polygons, at nominal 1:2400 map accuracy when derived from a minimum 2-pulse/m2 LiDAR-derived terrain grid. The workflow was developed by GIS analysts at County of Marin, California between 2009--2013 and is being refined as candidate watershed areas and flow lines are reviewed by WBD and NHD stewards at state and federal levels.

The 2009 work built a county terrain model from LiDAR with photogrammetry data, later adding bathymetric coverage. The terrain product was first used to evaluate parcel slopes, and evolutions of the terrain since 2010 have provided a foundation for countywide creek mapping at parcel scales. From the start, improvements in county creek mapping have been intended for publication in National Hydrography Dataset. As terrain resolution and mapping accuracy have improved after ARRA-funded LiDAR arrived in 2011, we now target the NHD Local Resolution repository for flow lines that will be used in a county creek protection ordinance implementation.

With USGS integration of NHD and WBD mapping efforts, we now incorporate WBD HUC-16 definition to the workflow, and are adapting to production and QA details of Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD), Fourth Edition and working with our state WBD steward toward certification of HUC-16 from the nominal 1:2400 map accuracy process.

Figure 1 This diagram provides a schematic view of the production process for (1) contour lines derived from the terrain used to generate other products, (2) WBD HUC-16 polygon features that can be rolled up into HUC-14 summaries, and (3) NHD Local Resolution flow lines at nominal 1:2400 parcel-scale map accuracy.

Terrain inputs are mostly point features starting from return-classified LiDAR point clouds delivered in LAS format plus various bathymetric datasets obtained from NOAA and the California Seafloor Mapping Project. The bathy-topo terrain data are compiled in an ESRI Terrain Dataset, from which natural neighbor interpolation is used to estimate a 50cm grid bare-earth surface that feeds all subsequent branches of the work flow.

(1) The WBD review process requires that reviewers are provisioned with contour lines. Customary aesthetics for readable contours require strong smoothing of the surface to generate more readable contour lines. After mean filters over 7x7 circle (sharper) or 9x9 square (smoother), contours are generated from a 1-meter gridded surface on a minimum 1-meter vertical interval. A clip of countywide contours near the HUC-12 extent under review is provided to reviewers along with candidate HUC-16 polygon and NHD Local flow line features.

(2) WBD HUC-16 polygons are aggregated from smaller catchment areas that are a by-product of the ArcHydro process used as part of the NHD flow line creation. HUC-16 areas are adapted from legacy watershed designations in local use, the federal standards for drainage areas not otherwise split, and named by GNIS features related to them. Once aggregated, the boundaries of these polygons are smoothed to filter out the blocky appearance grid-edge textures along the model-generated boundaries.

(3) Model-derived flow lines suitable for NHD Local Resolution repository are the core product of this workflow. Key augmentations before and after standard ArcHydro procedures are defined in this workflow; these ensure parcel-scale accuracy and incorporate local knowledge into model-derived flow line geometry.
Before ArcHydro, hydrologic enforcement (HE) line features are compiled by hand as upstream-directed edges with no other attributes. These HE features summarize low-flow pathways through storm drain pipes, along drainage swales at road edges, or wherever flow paths need to diverge from the way that our bare-earth terrain would otherwise guide the ArcHydro process. Most local knowledge of geometry can be accommodated this way.
After ArcHydro, a subset of HE features that have been split and conflated with existing NHD High Resolution flow line ReachCode, FCode, and GNIS Name attributes is used to conflate these attributes onto appropriate model-derived flow line reaches from ArcHydro. The model-derived flow lines are smoothed to filter out grid-edge texture and ArcHydro is used to build a flow network for the HUC-12 extent being processed.