Lat/Lon Bounds (degrees, minutes): These are the limits of the Grid. If you chose Design-a-Grid you may enter values directly or use the spin buttons provided. Do not use negative values as the hemisphere is entered separately. Latitudes must be between 0 and 90. Longitudes must be between 0 and 180. For whole-world plots use longitudes 0 deg 0 min to 0 deg 0 min or 180 deg 0 min to 180 deg 0 min.
Grid Cell Size: A drop-down list gives the allowed cell size in arc-seconds. This is the resolution of the output grid. When you choose a Grid Cell Size greater than 3 arc-seconds, your output grid will be a sub-sampling (rather than a re-calculating) of the input grid. i.e. a 6 second output grid will simply contain every other grid cell value from the input 3 second grid.
Number of Cells (Lat X Lon): These values are automatically changed for you according to the Lat/Lon Bounds and the GRID CELL SIZE.
Grid-Radius: (At this time grid radius is only relevant to
NGDC Coastal Relief Model grids.) This option can be used to discard grid data
depending on the proximity of the actual hydrographic data values. i.e. NGDC has used
NOS Hydrographic Survey Data to create grid data values for all the
grid cells in each grid (except on the
far periphery of the NOS data extent, see below). If you do not wish to use
the grid values where actual data was not in close proximity, you may apply the Grid-Radius algorithm.
If Grid-Radius is set to zero or greater and the nearest actual data was
more than Grid-Radius grid cells away, the output cell will be given the Empty Grid Cell Value
instead of the value contained in the NGDC grid files. Note that Grid-Radius values are based on the
GEODAS database grid cell size (3 arc-seconds), not the output cell size. Thus if output cells size is greater
than GEODAS cell size (sub-sampling), the Grid-Radius value should at least equal the number of GEODAS cells per
output cell. This will insure that you are not missing information about where the actual data values contribute
to the grid.
Note: Grids on the far periphery of the
NOS hydrographic data have already had a Grid-Radius value of 110 applied to them, since grid cells any
further from the data would be totally speculative.
Note: Since there is no density data for land areas, some lakes, and sea areas where
contours were used, Grid-Radius cannot be applied to these areas. So Grid-Radius values
which you set for your grid will be ignored in these areas. Land areas will plot (unless
Sea Cells Only is selected). Contoured sea areas will not plot.
Minimum, Maximum: These read-only values show the minimum and maximum values of the opened grid. They will be transferred to your output grid header. If you are custom designing a grid, min and max values will be taken from the input grid(s) and will be inserted into your output custom grid header. Note that min and max values are for actual grid cell values, e.g. if the precision of a grid is tenths of meters, and the deepest depth is 1234.5 meters, the minimum would be 12345.
Number Type: You may create output grids in several number types (1-byte integers, 2-byte integers, 4-byte integers and 4-byte floats). The NGDC grid header contains this field. Minimum and Maximum values along with precision can be used to determine if your chosen number type will be sufficient.
Precision: This is the precision (whole meters or tenths of meters) of your output grid data. The Coastal Relief Model CD/DVD bathymetry data has a precision of tenths of meters, while the land data has a precision of whole meters.
Empty Grid-Cell Value: This value will be used where grid cells do not have a data value. This will be the case for output grid cells where the original data points are further away than a user-chosen Grid-Radius The Empty Grid-Cell Value will also be used when custom designing grids, if part of the custom area is not covered by the NGDC grids on the CD. Note: Except for grids on the outer periphery of the data set, NGDC grids were created without using the Grid-Radius algorithm (set to -1), thus the grid cells will be filled with data values.
Water Datum: The vertical datum which non-land values are tied to. For ocean grids this will always be Mean Sea Level (MSL). Certain areas such as the Great Lakes will also have grids available for which the water depths are based on the Local Vertical Datum. Thus for these Great Lakes areas the MSL grids can have "depths" which are many meters in the positive, while the local-datum grids will contain water depths which are negative through zero and a "cliff" where land starts, since the land cells will always be based on MSL. If you choose the Local Vertical Datum option GEODAS will use local datums whenever they are available i.e. for inland lake data.