During making a grid for water bodies, viewing the bathymetry underneath the gird is meaningful as it gives an idea about the high resolution of cells in the deep and main channel regions and the low resolution of cells in shallow regions. The Data menu supports do this. It includes Grid Bathymetry Interpolation and Download Online Bathymetry as shown in Figure 1. The function of each option is described below.
Figure 1 Data menu options.
Grid Bathymetry Interpolation
Suppose that we have a cross-section file (filename XS.ldb), the file contains cross-sections. Each cross-section has the following information (Figure 2).
- The first row is the name of the cross-section
- The second row has a number of data points (e.g 111), and a number of columns (e.g 3) of each data point. It includes coordinates (X, Y) and Z (elevation)
- Data points start from the third row to end
Figure 2 Cross-section data format.
From the Import button in the main toolbar, select Import Cross-Sections option, as shown in Figure 3. Then the Import Cross-Sections form appears, browse to the cross-section file then click the Open button (Figure 4), as the result, cross-sections are displayed as shown in Figure 5.
Figure 3 Import cross-sections option.
Figure 4 Import cross-section file.
Figure 5 Cross-sections loaded.
To display the elevation of the cross-sections by color ramp, press Alt+C from the keyboard, and the Specify Elevation will pop up, and enter Min and Max values for elevation range (this range is based on the Z column value in the cross-section file). In this case, enter 110, 120 (Figure 6). Then click OK button, the cross-section will be displayed as shown in Figure 7.
Figure 6 Cross-sections loaded.
Figure 7 Cross-sections displayed in color ramp.
After loading the cross-sections, now we are going to interpolate bathymetry for the grid. Selecting the grid layer in the Layer Control panel, next select Grid Bathymetry Interpolation option from the Data menu as shown in Figure 8. The Topographic Interpolation form will be displayed as shown in Figure 9.
Grid Layer: That is the name of the selected grid layer that we are going to interpolate its bathymetry.
Using Visible Data: That is the number of data points. In this case, there are 2234 points from the cross-section file.
Interpolation Options: There are two options including Update Missing Only (update elevation values for the missing grid nodes only, in case other nodes already have elevation values) and Overwrite All Data (create new elevation values for all grid nodes and overwrite the nodes that have existing elevation values).
Interpolation Method: The Grid+ is using one interpolation method currently. It is Inverse Distance Weighting (IDW). The IDW Options frame allows changing its properties.
Clicking the Interpolate button to start interpolation. Then click the OK button to close the form.
After the interpolation process is done, RMC on the grid layer then select Show Properties / Bottom Elevation to display the bottom elevation of the grid after its grid nodes are assigned elevation values as shown in Figure 10.
Note
The bottom elevation values for the grid are for grid nodes, not for the centroid of grid cells.
Figure 8 Grid Bathymetry Interpolation.
Figure 9 Topographic Interpolation form.
Figure 10 Grid with bottom elevation.
An alternative approach to interpolating the bottom elevation of the selected grid is using a polygon.
Selecting the Add a new polyline button from the main toolbar then start drawing a polygon that covers the grid as shown in Figure 11. Change the cursor to select object mode (press the S key or select the Select Object in the main toolbar). Selecting the overlay layer (layer contains the polygon), then LMC on the polygon (it will be highlighted by changing color to red), then RMC to display options, select the Interpolate Grid Elevations option as shown in Figure 12. The message will be shown up as shown in Figure 13 after the interpolation process is done. Click the OK button to close the message.
RMC on the grid layer then select Show Properties / Bottom Elevation to display the bottom elevation of the grid after its grid nodes are assigned elevation values as shown in Figure 14.
Figure 11 Draw a polygon that covers the grid.
Figure 12 Interpolation by using a polygon.
Figure 13 Grid Bathymetry Interpolation message.
Figure 14 Grid with bottom elevation(2).
Download Online Bathymetry
Selecting the Download Online Bathymetry option, the Download Online Data form will be displayed as shown in Figure 15. Users are able to select data set from using the drop-down Data Set. The currently available open-source sites are:
- GEBCO 2021: This is 2021 GEBCO’s gridded bathymetric data set, the GEBCO_2021 Grid, is a global terrain model for ocean and land, providing elevation data, in meters, on a 15 arc-second interval grid. The data information can be found on GEBCO's website GEBCO 2021.
- GEBCO 2014: This is 2014 GEBCO’s gridded bathymetric data set. The data information can be found on GEBCO's website GEBCO 2014.
- GEBCO 08: a continuous terrain model for ocean and land with a spatial resolution of 30 arc-seconds released by the General Bathymetric Chart of the Oceans (GEBCO) in 2009. Bathymetric data is available to download and the user manual can be found on the website GEBCO 08.
Depending on the data set selected, different data information is provided in the Spatial Information section. The Data Extraction Limits is auto-set to the whole model domain, but the user can manually select the specific area by checking to Using Polygon File and browser to the closed polygon that contains the area inside.
Click on Download button and Grid+ will download and apply the bathymetry data of the selected Data Set to the specific defined area.
Figure 15 Download online data for bathymetry.