Data Management Plan

These data are a derived product of the NOAA VDatum tool and they extend the tool's Mean Higher High Water (MHHW) tidal datum conversion inland beyond its original extent.

VDatum was designed to vertically transform geospatial data among a variety of tidal, orthometric and ellipsoidal vertical datums - allowing users to convert their data from different horizontal/vertical references into a common system and enabling the fusion of diverse geospatial data in desired reference levels (http://vdatum.noaa.gov/). However, VDatum's conversion extent does not completely cover tidally-influenced areas along the coast. For more information on why VDatum does not provide tidal datums inland, see http://vdatum.noaa.gov/docs/faqs.html.

Because of the extent limitation and since most inundation mapping activities use a tidal datum as the reference zero (i.e., 1 meter of sea level rise on top of Mean Higher High Water), the NOAA Office for Coastal Management created this dataset for the purpose of extending the MHHW tidal datum beyond the areas covered by VDatum. The data do not replace VDatum, nor do they supersede the valid datum transformations VDatum provides. However, the data are based on VDatum's underlying transformation data and do provide an approximation of MHHW where VDatum does not provide one. In addition, the data are in a GIS-friendly format and represent MHHW in NAVD88, which is the vertical datum by which most topographic data are referenced.

Data are in the UTM NAD83 projection. Horizontal resolution varies by VDatum region, but is either 50m or 100m. Data are vertically referenced to NAVD88 meters.

Process Steps:

• 2016-01-01 00:00:00 - Buffered Census Counties offshore by 6km. Source VDatum .gtx files were shifted from positive east longitude values to negative west values. The tss grid was subtracted from the MHHW tidal grid. This step resulted in the MHHW tidal datum referenced to an orthometric datum, specifically NAVD88. Each region's orthometric-referenced tidal grid was converted to a point file, where each cell's centroid is a point containing the value of the cell. The point data were then clipped to county boundaries that extend ~6 km offshore to limit the number of points in the interpolation below. 10 and 12-digit hydrologic unit code (HUC) watershed boundaries that intersect the point data were selected. Where VDatum regions overlapped for an area, the VDatum bounding polygons were used to remove points outside of these areas. Where there was still overlap, points from either the higher resolution region or regions covering bays and estuaries were used. Using the Inverse Distance Weighted interpolation method, the point data were used to interpolate the NAVD88-based tidal surface inland, using the selected HUC boundaries as barriers to interpolation. This approach uses only points within the watershed and limits the interpolation to areas only within the watershed. This process was done for both 10 and 12-digit HUC watersheds. The resulting interpolated surfaces were then nested or mosaicked, where the HUC 12 interpolated surface was the primary raster and the HUC 10 surface the secondary. A Euclidean Allocation approach was then used to extend the conversion further inland to areas where the HUC 10 watersheds did not cover the study area. County boundaries were used as the inland limit instead of watersheds since most topographic data are collected and/or available on a county basis.