The Data menu provides options for bathymetry interpolation as well as accessing online bathymetry data. Being able to view bathymetry underneath the grid helps determine the resolution required in the deep and main channel regions compared to shallow regions, which can often have lower resolutions. The Grid Bathymetry Interpolation and Download Online Bathymetry options are shown in Figure 1 and their use is described below.
Figure 1 Data menu options.
Grid Bathymetry Interpolation
In cases where cross-section data is available, it can be viewed in Grid+ with files in ldb format. For example, a file with name XS.ldb contains cross-sections with 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 11), 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. The Import Cross-Sections form will be displayed. Browse to the cross-section file, then click the Open button (Figure 4).
Figure 3 Import cross-sections option.
Figure 4 Import cross-section file.
The Coordinate System form will pop up. We need to select proper projection for the imported cross sections as shown in Figure 5 then click OK button, as the result, the cross-sections will be displayed as shown in Figure 6.
Figure 5 Define projection.
Figure 6 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. Enter the Min and Max values for the elevation range (this range is based on the Z column value in the cross-section file). In this case, enter 0, -15 (Figure 7). Then click the OK button, and the cross-section will be displayed as shown in Figure 8.
Figure 7 Cross-sections loaded.
Figure 8 Cross-sections displayed in color ramp.
Suppose that a gird that covers the cross-sections is available. Now load the grid, then we are going to interpolate its bathymetry.
Go to the Data menu, then select Grid Bathymetry Interpolation as shown in Figure 9. The Topographic Interpolation form will pop up as shown in Figure 10.
- Grid Layer : is the name of the selected grid layer that we will interpolate its bathymetry.
- Using Visible Data: 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 Inverse Distance Weighting (IDW) is used. The IDW Options frame allows changing its properties.
Click the Interpolate button to proceed interpolation process, when it is done, click the OK button to close the form.
After the interpolation process is done, RMC on the grid layer and 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 11.
Note
The bottom elevation values for the grid are for grid nodes, not for the centroid of grid cells.
Figure 9 Grid Bathymetry Interpolation.
Figure 10 Topographic Interpolation form.
Figure 11 Grid with bottom elevation.
An alternative approach to interpolating the bottom elevation of the selected grid is to use a polygon.
Select the Add a new polyline button from the main toolbar, then start drawing a polygon that covers the grid as shown in Figure 12. Change the cursor to select object mode (press the S key or select the Select Object in the main toolbar). Select the overlay layer (the layer that contains the polygon), then LMC on the polygon (it will be highlighted by changing color to red), then RMC to display options, and select the Interpolate Grid Elevations option as shown in Figure 13. A message will be shown as in Figure 14 when the interpolation process is complete. Click the OK button to close the message.
RMC on the grid layer and 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 15.
Figure 12 Draw a polygon that covers the grid.
Figure 13 Interpolation by using a polygon.
Figure 14 Grid Bathymetry Interpolation message.
Figure 15 Grid with bottom elevation(2).
In the other case, there are few cross-sections, and they are far from each other. Apply the Inverse Distance Weighting (IDW) interpolation method as the previous introduction is not proper. The following section describes other interpolation methods.
Suppose that there are two cross-sections loaded as shown in Figure 16.. Next, generate a grid that covers the two cross-sections. Note that the grid should have high resolution (grid cells).
The grid is displayed as shown in Figure 17. We can see that only some grid cells in two head ends of the grid cover the cross sections. We should interpolate for those cell's bathymetry first.
Draw a polygon to cover the grid cells near the cross-sections.
Figure 16 Cross-sections loaded and located in the river segment.
Figure 17 High-resolution grid covers the cross-sections.
Figure 18 Draw two polygons covering grid cells and cross-section.
Download Online Bathymetry
After selecting the Download Online Bathymetry option, the Download Online Data form will be displayed as shown in Data Menu#Figure 15. Users are able to select data set 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.