Data Specifications

BlueTopo™ Specifications

BlueTopo is available for download as three layer GeoTIFFs with elevation, uncertainty, and raster attribute table (RAT) layers. Both the elevation and uncertainty layers are presented as floating point values.

BlueTopo is also available for download as two layer bathymetric attributed grid (BAG) files, format specified by the Open Navigation Surface Working Group.

The current naming convention for the files is made up of a tile name corresponding to the reschemed chart tile and the date the file was created. We expect to update the tessellation scheme and the associated naming convention in the future. Use the "Electronic Charts (ENC)" tab on NOAA's Chart Locator to identify the ENC cell name for the corresponding BlueTopo tile that you may be interested in downloading. Alternatively, there is BlueTopo_Tile_Scheme geopackage located in the Big Data bucket. Depending on region, you can expect to see new BlueTopo files with the latest data, weekly to monthly. In the future, the nowCOAST map viewer will make available map services and data visualization.

GeoTIFF Image Description Tag: The TIFF format allows for tags that provide additional metadata. Within the tag "TIFFTAG_IMAGEDESCRIPTION" we have the following text.

These data are not for navigation; please see the NOAA's Electronic Navigational Charts site to access navigational products.

This dataset is a multi-layered raster and part of NOAA Office of Coast Survey BlueTopo bathymetry. This product may include preliminary and unqualified information and as such may contain position and depth errors and/or other quality issues. Products generated with these data are not to be used for measurements or navigation. Furthermore, BlueTopo is not on a navigational vertical datum.

The user assumes the entire risk related to the use of these products. NOAA Office of Coast Survey (OCS) is providing these products "as-is" and Office of Coast Survey disclaims any and all warranties, whether express or implied, including any implied warranties of merchantability or fitness for a particular purpose. In no event will Office of Coast Survey be liable to you or any third party for any direct, indirect, incidental, consequential, special, or exemplary damages or lost profit resulting from any use or misuse of these data.

The bathymetry is compiled from multiple sources with varying quality and includes forms of interpolation. The bathymetric quality and interpolation is described through the vertical uncertainty layer and the associated data quality raster attribute table.

When using this National Bathymetric Source (NBS) product in publications and presentations please acknowledge that data used in this project/study was derived from NOAA Office of Coast Survey BlueTopo product.

The vertical coordinate reference system for this product is NAVD88 and is further described in the private GDAL_METADATA TIFF tag as VERTICALDATUMWKT.

For further information please contact ocs.nbs@noaa.gov

Elevation layer description: Elevation values are increasingly negative with increasing water depth, values landward of the specified vertical datum are positive. The vertical datum is North American Vertical Datum 1988 (NAVD88). Elevation values present in BlueTopo come from several different sources, identified in the RAT (third layer of the geotiff, see Table 2). The elevation layer consists of both measured bathymetry data and interpolated values. Some of the interpolated data is supported by other survey methods, such as side scan, and some is strictly a best guess interpolation. If bathy_coverage is true, the value is sourced from a measured depth. If bathy_coverage is false the value is sourced from an interpolated depth. The bathymetry is compiled from multiple sources with varying quality and includes forms of interpolation. Areas of generalization have not been limited by the shoreline, so erroneous interpolation may occur where there is land if no other data is present.

Uncertainty layer description: Uncertainty values are derived from source data according to the equipment that was used and water depth. Uncertainty for interpolated data is derived from the source.

Contributors layer description: The contributors layer contains an index that corresponds to an entry in the associated Raster Attribute Table.

Raster Attribute Table (RAT): The RAT defines where the data came from, the license for use, and some quality metrics that are associated with the International Hydrographic Organization S-101 Quality of Bathymetric Data. The RAT will be updated to suit S-102 once it is finalized. Table 2 describes the metadata included in the raster attribute tables in more detail. The no data value is 1000000.

Acronym Name Description Value
value Value A list of each unique cell value in the raster dataset. Float
count Count The number of cells in the raster dataset with the cell value in the VALUE column. Float
data_assessment Data Assessment Provides an overall indicative level of assessment of bathymetric data from which further attribution is derived. Unsigned Integer
1 = assessed
3 = unassessed
significant_features Significant Features Detected

A binary indication that a systematic method of exploring the seafloor was undertaken to detect significant features. If false, feature_size and feature_least_depth attributes are both not applicable.

In the context of bathymetry, a feature is any object, whether manmade or not, projecting above the seafloor, which may be considered a danger to surface navigation.

Boolean
1 = True
0 = False
feature_least_depth Least Depth of Detected Features Measured A binary expression of the ability of the survey to detect the least depth of features. Boolean
1 = True
0 = False
feature_size Size of Features Detected The size of the smallest feature that the survey was capable of detecting with a high probability - unit is cubic meters. Float
coverage Full Seafloor Coverage Achieved A binary statement expressing if seafloor coverage has been achieved in the area covered by hydrographic surveys. If false, the bathy_coverage attribute must also be false. If true, bathy_coverage may either be true (i.e. indicating a source measurement) or false (i.e. indicating an interpolated measurement derived from associated side scan sonar coverage). Boolean
1 = True
0 = False
bathy_coverage Full Bathymetric Coverage Achieved A binary expression stating if full bathymetric coverage has been achieved in the area covered by hydrographic surveys. If true, this indicates the value is sourced from a measured depth, not an interpolated depth. Boolean
1 = True
0 = False
horizontal_uncert_fixed Horizontal Uncertainty Fixed The best estimate of the fixed accuracy of a position. Reported at a 95% Confidence Interval. Float
horizontal_uncert_var Horizontal Uncertainty Variable The best estimate of the variable accuracy of a position as a multiplier of depth. Reported at a 95% Confidence Interval. Reported at a 95% Confidence Interval. Float
vertical_uncert_fixed Vertical Uncertainty Fixed The best estimate of the accuracy of depths, heights, vertical distances and vertical clearances. Reported at a 95% Confidence Interval. Float
vertical_uncert_var Vertical Uncertainty Variable The best estimate of the variable accuracy of depths, heights, vertical distances and vertical clearances. Reported at a 95% Confidence Interval. Float
license_name License Name The license information regarding restrictions on data redistribution, usage, and source attribution. String
license_url License URL or DOI The URL or DOI where the license is available. String
source_survey_id Source Survey ID The survey filename. String
source_institution Source Institution The institution responsible for the survey. String
survey_date_start Survey Start Date The start date of the survey. String ISO 8601:2004
survey_date_end Survey End Date The end date of the survey. String ISO 8601:2004