- Date: 2004 (process 1 of 7)
-
Terrapoint LIDAR was delivered gridded at 6-foot resolution, Washington State Plane North. However, it still included water-surface data. We manually traced the marine waterline and extracted the topography. I also filled several thin (1-2 pixel wide) linear artifact gaps in the dataset by using the EUCALLOCATION function to smooth over the gaps. This dataset was then resampled to 30-foot resolution and formed the basis of the "Master DEM".
Person who carried out this activity:
Elizabeth Cassel
University of Washington
Contract Employee
Data sources produced in this process:
- Date: Dec-2004 (process 2 of 7)
-
The SHOALS Bathymetric LIDAR datasets were delivered in NAD83, NAVD88 by Ralph Haugerud. However, the LADS data off Whidbey and the SHOALS data off Camano Island were still in MLLW. I used VDatum to convert the LADS data to NAVD88. However, Camano Island is north of where VDatum works (48 degrees 10 minutes). I resorted to looking up the NAVD88 value of MLLW at Crescent harbor (-73cm) and Glendale on Whidbey Island (-67cm) and split the difference (-70cm) for the Camano Island dataset. I then interpolated the two datasets into a raster in Washington State Plane North feet (NAD83).
Finally, I merged all of these datasets into the Master DEM by allowing the bathy LIDAR to fill NODATA gaps in the Master DEM.
Person who carried out this activity:
Data sources used in this process:
- SHOALS LIDAR
- Tenix LADS
- SHOALS LIDAR (Camano Island)
- Master DEM
- Date: Dec-2004 (process 3 of 7)
-
The Lake Washington Bathymetric Contours were compared with NOS Chart 18477. Contour depths were determined to be in the vertical datum "Low Water of the Lake" which is 20 feet above MLLW and 18.75 feet above NAVD88 at the entrance to the Locks). A raster DEM was created in NAVD88 by TINing the contours and then trimming the results to a manually drawn polygon outlining the two lakes and the canal. This dataset was merged into the Master DEM by filling the NODATA areas associated with Lake Washington, Lake Union and the ship canal above the locks.
Person who carried out this activity:
Data sources used in this process:
- GeoMap Lake Washington
- Master DEM
- Date: Dec-2004 (process 4 of 7)
-
The NOS soundings were downloaded from the internet and projected into Washington State Plane North using CORPSCON 5.11.08. The data was then split into two parts at 48 degrees 10 minutes North. Soundings south of this line were converted from MLLW to NAVD88 using VDatum (48 10 is the northern limit of VDatum). Soundings north of this line were gridded in MLLW and then adjusted to NAVD88 using a correction surface interpolated from NOS tidal benchmarks. This later processing step introduces 0.75 cm error (established by cross-validation of the NAVD88 surface). Otherwise the two point sets were treated the same.
First the data was divided into 10 tiles of 0.5 x 0.5 degree extent (this was necessary to save memory while interpolating at 30-foot resolution). All of the points within the tile plus a 1 km overlap were gridded using a TIN interpolator. This TIN included a thin ribbon of points taken from the shoreline of the MASTER DEM to ensure that the TIN merged perfectly at the shoreline. The TIN was converted to a raster and a shaded relief map was produced from the raster. Obvious errors in the data were edited out of the point file. This process was repeated until all obvious errors in the bathymetry were eliminated.
Person who carried out this activity:
Data sources used in this process:
Data sources produced in this process:
- NOS Sounding Raster Tiles
- Date: Dec-2004 (process 5 of 7)
-
The USGS Swath Bathymetry, UW Swath Bathymetry and LDWG Swath Bathymetry were treated similarly. Each dataset was downloaded from the internet (except the UW data) as point files. They were projected to Washington State Plane North in CORPSCON 5.11.08 and imported to ArcGIS as points. The points were gridded using a TIN interpolator and converted to a raster. The raster was trimmed using a manually drawn polygon that outlined the data area. Finally, the raster was converted from MLLW to NAVD88 by applying a single correction value determined in VDatum for the centroid of the dataset.
The resulting raster was subtracted from the appropriate NOS hydrographic tile(s) and a mean difference was established between the two elevation datasets. The correction was applied to the Swath Bathymetry (since these data were not vertically controlled to survey grade) so that the two datasets approximately matched. The corrections applied were: -4.16 ft (Duwamish Delta), -7.3 ft (Puyallup), -5.14 ft (Nisqually Delta), +6 ft (Possession Sound), +5.6 (Admiralty Inlet).
The low-resolution NOS bathymetry was trimmed around the swath bathymetry with a 300 ft overlap. Then these datasets were merged together using ArcGIS's "Mosaic to New Raster" with the "Blend" option, which is a proprietary algorithm that feathers the two data sets into one another reducing the margin artifacts.
The Admiralty Inlet swath bathymetry from the UW overlapped three tiles. Since the low-resolution NOS bathymetry was mathematically identical in each of the three tiles (where they overlapped) only a single correction was necessary (+5.6 ft).
Person who carried out this activity:
Data sources used in this process:
- USGS Swath Bathymetry
- LDWG Swath Bathymetry
- UW Swath Bathymetry
- NOS Sounding Raster Tiles
- (process 6 of 7)
-
A manually drawn polygon was used to outline gaps in the Master DEM that were to be filled with the lower quality data from Harvey Greenberg. This prevented the low-quality data from spoiling areas where there was good LIDAR data (but the two datasets did not agree on how it should look) and allowed the low-quality data to fill areas in the Master DEM where there were real gaps in the LIDAR.
I used ArcGIS's proprietary "Mosaic to New Raster" with the "Blend" option to smooth the transition between the low-quality elevation data and the high-quality LIDAR-derived data. The overlap between the two datasets was hand-drawn and varied from a few hundred feet in the eastern Puget Lowlands, to many hundreds of feet over the Olympics.
Person who carried out this activity:
Data sources used in this process:
- Date: Jan-2005 (process 7 of 7)
-
For each of the 10 tiles there was a bathymetry raster and a portion of the Master DEM. The two portions were merged using the bathymetry raster to fill NODATA areas in the Master DEM. Next, all 10 tiles were merged together into a single DEM.
A few NODATA gaps remained in River Channels. And I set back to NODATA a few small areas that had obvious errors (like the seam between the Lidar Bathymetry and Multibeam bathymetry off Alki Point), a mountain in Elliott Bay that doesn't exist, etc. I then interpolated over the gaps using a TIN made from the pixels surrounding each data gap. This formed the final DEM.
Person who carried out this activity:
Data sources used in this process:
- Master DEM
- NOS Sounding Raster Tiles
Data sources produced in this process:
