To make a georeferenced map or access general settings for Grid+, go to the Tools menu as shown in 2128019457. The function of each option is described in the table 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. In many cases, bathymetry data needs to be interpolated to cover the whole domain. This section describes features unique to Grid+ whereby a user may interpolate between two cross-sections along even over a sinusoidal domain. The Grid Bathymetry Interpolation and Download Online Bathymetry options are shown in Figure 1 and their use is described below.
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Figure 1Tools . Data menu options.
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This option allows the user to make a background georeferenced map (described in detail below).
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General settings for Grid+
Image Georeferencing
Although Grid+ has online maps, sometimes a user may want to use a proprietary background map they possess or use Grid+ offline. In that case, a georeferenced background map can be created as described here.
Define the region of interest in Google Earth with two points (P1 and P2), as shown in 2128019457. We will save the image to a file (with *.jpg extension format).
Go to the Tools menu from Grid+ GUI, select Image Georeference and the Image GeoReference Tool form appears, click the Add Images button and browse to the image, which was generated by Google Earth.
By default, the coordinates of the four corner points of the image are updated in the Control Points frame, as shown in 2128019457. LMC on the row numbering from the table to select a point of the Control Points, then we can find that point by clicking the Find Point button, or remove that point by clicking the Delete Point button. Clicking the Clear Point button will delete all points in the table. Click Add Point button then LMC on a point in the image to add a new point to the table (a new row is added).
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Figure 2 Make an image file with at least two points from Google Earth.
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Figure 3 Add an image to the Image georeference tool.
Now we will assign World X and World Y coordinates to defined points. Click the Clear Points to empty the Control Points frame and then usie the vertical and horizontal bars as shown in 2128019457 to move the image to find the defined points (P1 and P2).
- Assigning coordinates for P1: After finding the P1 on the image, LMC on that point, its Image X and Image Y are updated in the Control Points frame. Now enter the coordinates for World X and World Y (UTM coordinates) then press the Enter key, and it will jump to the second row.
- Assigning coordinates for P2: Finding the P2 on the image, then LMC on that point, its Image X and Image Y are updated in the Control Points frame. Now enter the coordinates for World X and World Y (UTM coordinates). The UTM coordinates of the P1 and P2 have been defined previously, as shown in 2128019457.
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Figure 4 Define and update coordinates for the control points.
Next, click the Save button, the Save As form will be displayed. Enter a file name. Select extension *.geo for file type then click the Save button (2128019457).
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Figure 5 Save the georeferenced file.
The georeferenced map can be made from multiple images by using the Add Images button and then assigning coordinates as described above. In case we want to remove the image from the Image Files frame, select that image then RMC to display the options. Select Remove Selected Image as shown in 2128019457. Selecting Remove All Images will delete all images in the Image Files frame.
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Figure 6 RMC options in the Image Files frame.
Settings
Selecting the Settings from the Tools menu will show the Global Settings form as shown in 2128019457. The values in this form are the default settings.
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Simple Interpolation of Grid Bathymetry
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 contains the number of data points (e.g 11), and the number of columns (e.g 3) for each data point. This includes coordinates (X, Y) and Z (elevation)
- Data points start from the third row to end.
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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 (Data Menu#Data Menu#Figure 4).
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Figure 3. Import cross-sections option.
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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 Data Menu#Data Menu#Figure 5. Click the OK button and the cross-sections will be displayed as shown in Data Menu#Data Menu#Figure 6.
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Figure 5. Define projection.
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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 (Data Menu#Data Menu#Figure 7). Then click the OK button, and the cross-section will be displayed as shown in Data Menu#Data Menu#Figure 8.
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Figure 7. Cross-sections loaded.
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Figure 8. Cross-sections displayed in color ramp.
In cases where a grid that covers the cross-sections is available we can load the grid to interpolate bathymetry onto it.
Go to the Data menu, then select Grid Bathymetry Interpolation as shown in Data Menu#Data Menu#Figure 9. The Topographic Interpolation form will pop up as shown in Data Menu#Data Menu#Figure 10.
- Grid Layer : is the name of the selected grid layer for which we will interpolate the bathymetry.
- Using Visible Data: is the number of data points. In this case, there are 2234 points in 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: Inverse Distance Weighting (IDW) is used. The IDW Options frame allows changing of the interpolation properties.
Click the Interpolate button to proceed with the interpolation process. When it is complete, click the OK button to close the form.
After the interpolation process is complete, 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 Data Menu#Data Menu#Figure 11.
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The bottom elevation values for the grid are for grid nodes, not for the centroid of grid cells. |
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Figure 9. Grid Bathymetry Interpolation.
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Figure 10. Topographic Interpolation form.
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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 Data Menu#Data Menu#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 Data Menu#Data Menu#Figure 13. A message will be shown as in Data Menu#Data Menu#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 Data Menu#Data Menu#Figure 15.
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Figure 12. Draw a polygon that covers the grid.
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Figure 13. Interpolation by using a polygon.
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Figure 14. Grid Bathymetry Interpolation message.
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Figure 15. Grid with bottom elevation(2).
Other Interpolation Methods
In the second case, we will look at there are few cross-sections, and they are far apart from each other. The Inverse Distance Weighting (IDW) interpolation method used in the previous introduction is not an appropriate approach. Instead, interpolation using an orthogonal curvilinear grid will be used in this case.
Suppose that there are two cross-sections loaded as shown in Data Menu#Data Menu#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 Data Menu#Data Menu#Figure 17. We can see that only some grid cells in two ends of the grid cover the cross sections. We should interpolate for the bathymetry of those cells first.
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Figure 16. Cross-sections loaded and located in the river segment.
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Figure 17. High-resolution grid covers the cross-sections.
Click Add a polyline button from the Main Toolbar to draw a polygon to cover the grid cells near the cross-sections, as shown in Data Menu#Data Menu#Figure 18. Then, change the cursor to select object mode (press the S key). Select the overlay layer (the layer that contains the new polygon), then LMC on the polygon to select it (it will be highlighted by changing its color to red), then RMC to display options, and select the Interpolate Grid Elevations option as shown in Data Menu#Data Menu#Figure 19. A message will be shown as in Data Menu#Data Menu#Figure 20 when the interpolation process is complete. Click the OK button to close the message. Apply similar steps to interpolate the bottom elevation of grid cells near the second cross-section.
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Figure 18. Draw two polygons covering grid cells and cross-section.
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Figure 19. Interpolate grid cells near the cross-section.
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Figure 20. Grid Bathymetry Interpolation message.
To view the bottom elevation of cells that are interpolated, RMC on the grid layer and select Show Properties / Bottom Elevation as shown in Data Menu#Data Menu#Figure 21. The grid bottom elevation will be displayed as shown in Data Menu#Data Menu#Figure 22.
Next, we will interpolate the bottom elevation for the entire grid based on the cells with the existing bottom elevation in two ends of the grid. Select the grid layer in the Control Panel and set the cursor in selection mode. Select the first grid node and hold the Shift key to select the second node (select two grid nodes to define the grid block we want to interpolate). In this case, we select two grid nodes to cover the entire grid. The grid block will be highlighted by changing the color to red, then RMC to display options and select the Interpolate Elevations in I Direction option as shown in Data Menu#Data Menu#Figure 23. After the interpolation process is done, the grid bottom elevation is displayed as shown in Data Menu#Data Menu#Figure 24.
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Figure 21. Show the bottom elevation of grid cells.
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Figure 22. Grids with bottom elevation at two cross-sections.
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Figure 23. Interpolate the grid block in I direction.
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Figure 24. Grid with bottom elevation.
Extrapolation of Bathymetry
Besides this interpolation feature, Grid+ has an extrapolation feature. In this case, some cells in the two ends of the grid are missing the bottom elevation. To fill in their bottom elevation, select the grid layer in the Control Panel and set the cursor in selection mode. Select a grid block that covers both cells missing bottom elevation and cells having bottom elevation. The grid block will be highlighted by changing the color to red. Then RMC to the grid cells to display options and select the Extend Elevations in I Direction option as shown in Data Menu#Data Menu#Figure 25. The bottom elevation values will be extended to the cells that are missing values. As a result, the bottom elevation for those cells is assigned as shown in Data Menu#Data Menu#Figure 26.
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Figure 25. Extend elevation in I-direction.
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Figure 26. Grid cells after extension bottom elevation.
Exporting to a Scatter Data File
After interpolating the bathymetry of the grid cells, Grid+ allows exporting the bathymetry to a scatter data file. To do this, RMC on grid layer, then select Create Scatter Data from Elevation option as shown in Data Menu#Data Menu#Figure 27. This will extract the grid node’s elevation and then create an XYZ scatter layer in the Layer Control. It is named “XYZ from Grid”. RMC on XYZ from Grid layer, select Export Data Point ( as shown in Data Menu#Data Menu#Figure 28) and browse to the location to save the file, enter a file name, then click OK button to export the layer to a file.
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Figure 27. Create scatter data from elevation.
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Figure 28. Export bottom elevation to data file.
Download Online Bathymetry
After selecting the Download Online Bathymetry option, the Download Online Data form will be displayed as shown in Figure 29. 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.
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Figure 29. Download online data for bathymetry.