Digital elevation model (DEM) of south San Francisco Bay, California, created using bathymetry data collected between 2005 and 2020 (MLLW)

Using ArcGIS, the source DEMs were displayed and areas that would need further interpolation were identified. The lineage, vertical datum, and resolution of each DEM surface were noted for order of precedence when constructing the combined DEM and need for resampling or datum conversions. Using the ArcGIS Mosaic to New Raster tool, the surveys that were from the same sources, and were not continuous surfaces, were assembled into separate DEMs. Elevation contours were digitized across the DEMs to constrain the interpolation across data gaps in intervals of 20 centimeters across broader shallower areas (generally in water depths less than 4 m), and in intervals of 1 meter in deeper areas. The sections were converted from raster to points to use as inputs in the interpolation process. Using the ArcGIS Topo to Raster tool (ESRI 2015), the point data were used, along with the newly drawn contours, to interpolate continuous DEMs at a resolution of 1 meter, using the contours as the primary interpolation constraint. There were areas of multibeam data with varying resolutions, and in a different vertical datum from the rest of the data used in this project. The data were resampled to 1 meter using the resample tool in ArcGIS, and then converted to NAVD88 using NOAA’s VDATUM tool (https://vdatum.noaa.gov/). There were a few areas with data gaps that were filled using the Topo to Raster method from above. Once all data were in the same horizontal (UTM NAD83) and vertical (NAVD88) datums, the different DEMs were fitted together into a seamless DEM using the ArcGIS Mosaic to new Raster tool. In areas with obvious discrepancies between DEMs, edges were smoothed by clipping out a narrow gap between surveys using the pixel editor tool (ESRI 2020) to set the areas to no data, then filling in elevation holes. The final pieces were all put together using the Mosaic to new Raster tool, and VDATUM was again used to create versions in both Mean Lower Low Water (MLLW) and NAVD88. In 2004 NOAA created a specific NAVD88 to MLLW zones for the area of the 2005 USGS South Bay Survey. These zones have yet to be fully incorporated in the VDATUM grids. The NOAA change zones were used for the MLLW conversion in the regions of the 2005 USGS Survey. Vdatum was used for parts no included in the NOAA grid. The VDATUM tool did not have the full extents of the MLLW version available for conversion, so the pixel editor tool was used to interpolate based on the edges of the available data.