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Caveats & Frequently Asked Questions



This data should not be used for navigational purposes

Only the most recent NOAA Nautical Charts should be used for navigation. These data sets were created from many different hydrographic surveys which used different sounding techniques and were conducted over different time periods. Hydrographic surveys are conducted for varying purposes, so soundings are collected at different spatial intervals and depth resolutions. Therefore, these data sets (DEM's) may contain temporal and spatial discontinuities, and should not be used for navigational purposes.

Reliability of Contours < 2m Generated from NOS' Estuarine Bathymetry Data

For those users who intend to generate vector contours from the DEM data provided by this site, the following information may be useful.

While it is possible to generate 0 and 1 meter depth contours using only observed soundings within estuaries, most of the 0 and many 1 m contours will dangle, in some GIS programs, without interpolations to the high water shoreline. The 2 meter contour is usually consistently closed and is thus the shallowest depth contour which is reliable throughout most estuaries.

Data were generated for this effort using both NOS soundings and a shoreline which is assumed to have an elevation equal to mean high water using triangulated irregular network (TIN) procedures. A portion of this TIN is based on a data model which is known to be incorrect. The TIN model assumes that each triangle surface is represented by a plane and that each edge and interior point can be estimated by linear interpolation from the corner point elevations. The profile of a beach or shore transect in a sedimentary estuary is typically non-linear with steep inter-tidal beach landward of a tidal flat each of which can be approximated by different but constant slopes. It is rare that the break between these two features is sampled with real soundings for a variety of operational reasons. The net result is that depths interpolated from triangles which include a shore vertex have a relatively large uncertainty for the horizontal location of any interpolated contour.

In addition to these uncertainties, there are three other types of errors which affect interpolated contours and may be present in some estuary's bathymetric data set. These are 1) surveys which were adjusted to different datums, 2) real temporal changes related to erosion and deposition, and 3) the dearth of very shallow soundings related to operational constraints on the surveys.

Often, estuaries have points which come from multiple surveys collected over intervals of several decades. Thus the soundings were not taken at the same tidal epoch throughout, and there will be some unknown variability in the datum related to real changes in sea level caused by long term sea level changes or earthquakes. Typically these errors are less than 0.1 m and they will be more noticeable in shallow areas.

Second, there is also a good possibility of higher errors near shore, caused by erosional and depositional changes in the near shore area over the years between the surveys. These changes are relatively less important as one looks at the deeper areas of an estuary simply because the relative change is less. A compounding source of error is that the location of the shoreline which existed at the time of the survey does not necessarily coincide with the shoreline used as the boundary for the TIN. For our computations we used a shoreline best representing its present day location (circa 1980 for the 1:250,000 NOS digital shoreline). In the extreme, accreting shores require that some soundings be discarded because they are landward of the high water shoreline. Conversely, eroding shores are fringed with an artificially expanding tidal flat.

Third, soundings less than 0.5 meters depth are systematically not collected because of operational constraints (small boats used to collect soundings run aground). Prior to acoustic soundings when poles and lead lines were used to measure depths and when sail or oars were used for the boat propulsion, it was not uncommon for the inter-tidal region to be reasonably sampled. However since the propeller it is rare to find observations shallower that 0.3 to 0.6 meters below the datum. In regions with extensive tidal flats, this translates to a wide region where interpolation based on a marginal depth profile model must be used to compute depths.


Frequently Asked Questions (FAQ's)

Q: What is the source data for the bathymetry data?

A: NOS Hydrographic Surveys found on the NGDC GEODAS CD-ROM.

Q: How does one view the data in GIS applications?

A: ESRI’s ArcGIS does not allow the user to view the data natively – it must be imported first. To import the .DEM files to a format compatible with ArcGIS use the “DEM to Raster” tool available in through “Conversion Tools” section of ArcToolbox. The data are floating point, so verify that the data type is set to “FLOAT.” Other GIS applications may read the USGS DEM format natively, or they require that the data is imported prior to use – check the application’s online help for guidance on utilizing USGS DEM formatted data.

Q: Why do sections of some estuaries appear terraced?

A: In some cases the soundings were recorded to the nearest fathom. A fathom is an interval of 6 feet. While most of an area was recorded in feet or meters, a section of that area that was recorded in fathoms will look much more coarse when it has been triangulated because all of the information between the full fathom intervals has been omitted in the original data set. When the grids have been given values by the triangulated file, these areas will appear evenly spaced and colored and will stand out from the rest of the gridded data.

Q: Why do sections of some estuaries appear ramped or have a constant slope?

A: The constant slope may be real or an artifact of sampling density. Very few estuaries have a constant sounding density over the entire area of the estuary. Some estuaries have larger gaps in data than others, and the density of the original soundings often change from one section of the estuary to another. A triangulation process is used to produce the gridded data. Areas where there are dense concentrations of soundings will produce more accurate grid values than areas where the soundings are sparse. Some areas have gaps of many hundreds of meters. These areas will show interpolation across large distances which will appear evenly spaced. In order to determine where these areas are, the user can download the Sounding Coverage Images from the individual estuary's data page. These images allow the user to get a general idea of the distance between the original soundings data so that they may assess for themselves whether or not the original point coverages meet their project's resolution needs.




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