Neah Bay to Cape Alava, Northwest Coast, Washington State - Topographic Survey Data

Downloadable Data

Date that the source FGDC record was last modified.

Converted from FGDC Content Standards for Digital Geospatial Metadata (version FGDC-STD-001-1998) using 'fgdc_to_inport_xml.pl' script. Contact Tyler Christensen (NOS) for details.

Partial upload of Spatial Info section only.

Replaced entire Lineage section to populate new Source Contribution field.

Partial upload of Positional Accuracy fields only.

FUGRO Pelagos Inc. | Type of Source Media: CD-ROM

The SHOALS airborne system acquires a tremendous volume of raw data during a single mission. The lidar data are unique and require a specialized Ground Control System (GCS) for survey planning and post-processing. The GCS main functions1) import airborne data stored on removable hard drives;2) perform quality control checks on initial depths and horizontal positions;3) provide display and edit capabilities;4) calculate depth and position (XYZ) values for each sounding; and5) output final positions and depths for each sounding. The GCS possesses an automated capability to post-process the data to obtain corrected depth and horizontal positions within the specified system accuracies. This is accomplished by accurately identifying the surface and bottom returns from the airborne data. Depths are determined by computing the time difference between the arrival of the surface and bottom returns. Corrections are computed and applied for depth biases associated with light propagation, water level fluctuations, and various inherent system characteristics. Surface waves are modeled and removed so that depths can be referenced to a common mean water surface. Applying tidal corrections and/or KGPS corrections then produces a depth referenced to a known datum. A manual processing capability also allows evaluation of anomalous data by providing display and edit functions of sounding data and system parameters. Digital photos collected once per second (and easily viewed during editing) permit visual scrutiny of the area to aid the hydrographer in deciding whether to exclude suspect data from further processing. Output from the GCS is a digital data set of XYZ(positions/depths) for each laser sounding that is compatible with most GIS and other contouring and mapping systems.

Both positional information and project control for the LIDAR survey were supplied by the KGPS in the ellipsoidal datum of NAD83.For ease of merging the data set with existing multibeam sounding data, survey instructions required the data to be projected during post-processing and delivered in the UTM Zone 10 projection. The Geoid99 height model was used to convert the vertical datum from the ellipsoidal 3-D datum of NAD83 to the orthometric vertical datum NAVD88. But to accurately merge the LIDAR data with existing multibeam sounding data, the data sets must obviously be in the same vertical reference frame (Milbert 2002).Since the multibeam sounding data were referenced to an averaged tidally-derived vertical datum (MLLW) and the LIDAR data were referenced to an orthometric vertical datum (NAVD88) based on Mean Sea Level (MSL), a VDatum model (Spargo et. al 2006)was used to vertically transform the LIDAR data to MLLW for compatibility and comparison with the multibeam sounding data. The VDatum model relates the NAVD88 to MLLW by using a grid, or zone, of tide model comparisons with known leveled tide benchmark stations to better account for the spatial variability of tidal dynamics over a given area (Milbert 2002;Spargo et. al 2006). The VDatum model that was used is available for download athttp://chartmaker.ncd.noaa.gov/csdl/vdatum_projectsWA.htm.