An HTML file on the CD/DVD allows the images of the grids to be searched for and retrieved using any common web browser. Opening this document yields the regional map, which is embedded in the html document. At this point, the user has four options, which are presented at the top of the regional image. The first option is to view a high-resolution image of an individual elevation grid. When this option is selected, the user clicks on a 1° x 1° square in the regional image opening a separate window displaying the JPEG image of the corresponding elevation grid. Because of the size of the JPEG images, only part of the image is displayed on the screen. Some web browsers have the capacity to zoom out and show the whole image. However, in most cases, the user will be forced to scroll around the image to see it in its entirety. This limitation is removed when the image is viewed with other graphics software, which, in fact, can be launched automatically from the web browser, if it is properly set up.
A second option available to the user is to view a high-resolution image of a data-density grid. When this option is selected and the user clicks on a 1° x 1° square in the regional image, a GIF image of the data-density grid for that area is displayed in another window. As shown in Figure 3, images of both the elevation grid and the data-density grid for the same 1° x 1° area can be displayed simultaneously. This allows the user to quickly see the data density in the area and its affect on the quality of the elevation grid, due to the sounding data distribution.
The third option available is to copy a 1° x 1° grid from the CD/DVD directly to the users computer. By clicking on a 1° x 1° square in the regional image, the web browser (when properly set up) launches Grid Translator (Fig.4). With this application, the user can either select and save a grid or the Grid Translator dialog window, shown in Figure 4, can be used to modify attributes of the grid before copying it. These attributes include the grid cell resolution (from 3 arc seconds to 1 minute), the grid format (a binary raster grid, an ASCII grid, and an NGDC binary grid), the precision of the grid elevations (meters or tenths of meters), the elevation storage type (e.g., short or long integer or floating point number), and the not-a-number value (e.g., 999999 or -32,767).
The fourth option offered the user is the ability to create a custom sized grid, or create their own custom-sized grid; one that can be smaller or larger than the standard 1° x 1° area, and span two or more of the grids on one or more CD/DVDs. When this option is chosen, the Grid Translator dialog window is again opened. However, in this case, additional attributes of the grid can now be modified, the most important of which is the grid area. The user defines the minimum and maximum latitudes and longitudes (i.e. custom-sized grids) of the grid they want to create. If this custom grid extends beyond the geographic extent of the grids on CD/DVD, then the empty regions in the grid are filled with the no-data flag selected by the user. If the custom grid requires elevations in grids contained on another CD/DVD, Grid Translator locates these grids using a table of contents that lists not only the grids on the CD/DVD currently loaded, but also all the grids on other CD/DVDs that have been released up to that point. The user is then prompted to load the next CD/DVD(s) on which the remainder of the elevation data are stored in order to finish building the custom grid.
An additional feature offered when building a custom grid is the ability to specify the radius of elevations to retain within the grid beyond a sounding location. An example of applying a raduis filter to an elevation grid is shown in Figure 5. Figure 5A is an image of an elevation grid for a 1° x 1° area where the default value (i.e. all cells are used) for the grid radius option in Grid Translator is used. Figure 5B is an image of this same area after a radius of 10 grid cells has been applied. In this second grid, only those grid cells within a radius of 10 grid cells to a sounding location have elevation values, the remaining grid cells have been set to a null elevation value. The second grid clearly shows the ship tracklines along which the soundings were collected. The width of the soundings about these tracklines can be expanded or reduced by increasing or decreasing the radius when constructing the grid. This gives the user the flexibility to make elevation grids that simultaneously reveal where soundings have and have not been collected. (Figure 5 was created using the GEODAS Hydro-Plot MS Windows/Linux-x86 software application which is on the CD/DVD set.
GridTranslator runs on MS Windows, Macintosh and UNIX Xwindows. The latter two versions are not on the earlier CD/DVD sets, but are available from the NGDC Marine Geology and Geophysics bathymetry web page www.ngdc.noaa.gov/mgg/coastal/