At the same time that the grids are converted to an NGDC format, two other types of data are generated as well. The first of these is a shaded-relief JPEG image of each elevation grid (Fig. 2A). An equivalent size GIF image is made of each sounding-density grid; one in which the number of soundings in each grid cell is color-coded (Fig. 2B). Both types of images provide ready-made figures of the grids in commonly-used computer graphic formats. Consequently, the images can be easily viewed using a range of UNIX and personal computer software. A number of these graphic applications can also be used to quickly convert the images into other formats that can be directly inserted into text documents.
The second type of data generated during this stage of the processing are radius grids. These grids are equivalent in size and resolution to the elevation and data-density grids, and indicate the distance to the nearest cell in the data-density grids that includes a sounding. These distances are stored in terms of grid cells. For example, if a cell in the data-density grid encompasses an area in which one or more bathymetric soundings are located, the same cell in the corresponding radius grid has a value of zero, which means its distance from a sounding in terms of grid cells is zero. If a cell in the data-density grid does not encompass any soundings, then the corresponding cell in the radius grid has a value equivalent to the number of grid cells between it and the closest sounding location. The maximum distance to a sounding that is stored in the radius grids is 110 cells, the same maximum distance to which bathymetry is extrapolated beyond a sounding in the elevation grids. A radius of -1 identifies those cells that encompass land areas. The radius grid can be used to modify the distances over which depths are projected beyond sounding locations in the elevation grids, the benefit of which is explained more below. (To see how Grid-Radius is applied to user-generated output grids see Figure 5A and 5B below.)
