Issue 62

Gridding and Displaying Lake Bathymetry Data Using Surfer Part 1

There are many ways to show bathymetry data for lakes using Surfer. You can simply show the lake Bathymetric data, you can show the local terrain with a flat lake, or you can overlay this with the terrain data. This article covers the steps in Surfer to grid and display lake bathymetric data. For more options on how to combine the lake data with terrain data, please see Part 2 of this article.

The bathymetric data used in this article was downloaded from the USGS Lake Tahoe Data Clearinghouse. The lake shoreline boundary was downloaded from the GIS Data Depot. You can download the exact files used in this article here.

Gridding and displaying bathymetry data for lakes usually looks the best when you use the shoreline for the lake as breakline file when gridding, and then blank the grid outside the shoreline. The steps are:

1. Create the BLN file
2. Grid the data
3. Blank and display the grid

The first step is to obtain or create a boundary file in BLN format defining the boundary of the lake (the lake edge) as a polygon. The most popular and easiest method for creating a BLN file of a lake edge is to obtain the boundary in another format (i.e. SHP, E00 or DXF) and convert it to a BLN file.

For example, if you have a SHP file of the lake edge as a polygon, you can:

1. Go to Map | New | Base Map.
2. Select the vector file (i.e. SHP) and click Open. The SHP imports into Surfer as a base map.
3. If the SHP contains other objects in it besides the lake boundary:
   1. Right click over Base in the Object Manager and go to Enter Group.
   2. Delete all the other objects from the base map so that only the boundary of the lake edge remains.
   3. Right click over Base again in the Object Manager and go to Exit Group.
4. Turn off the four axes by unchecking them in the Object Manager.
5. Go to File | Export.
6. Give the file a name, choose to export it as a Golden Software Blanking (*.bln) file, and click Save.
7. Accept the defaults in the Export Options dialog and click Save.

Other methods for creating a BLN file are outlined online in our Knowledge Base.

Once you have the BLN file, you also need to include a third column of Z values for the boundary (for a lake, this could be 0 for the lake edge) so you can use it as a breakline file. To do this:

1. Go to File | Open, select the BLN file and click Open. The BLN file will open in the Surfer worksheet. Column A is the X variable, Column B in the Y variable.
2. Make sure the value in cell B1 is a 0. This indicates to Surfer to blank the data outside the boundary. This will be important later when we blank the grid file.
3. Add a 0 (or whatever Z value you want for the lake edge) into Column C for each row of data. If you have many rows, you can do this automatically.
   1. Highlight column C by clicking on the column letter designation at the top of the window.
   2. Go to Data | Transform.
   3. Enter the Transform equation of C=0 (where 0 is the Z value you want for the lake edge).

       

      
      Enter the function C equals the number you want specified
      as the lake edge Z value.

       

   4. Click OK. Column C is now populated with the lake edge value you specified.

       

      
      Enter the Z value you want for the lake
      edge into Column C.
4. Go to File | Save to save the changes.
5. Go to File | Close to close the BLN file.

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Once you have the BLN file, you can grid the bathymetric data with a breakline following these steps:

1. Go to Grid | Data, select your data file and click Open.
2. In the Grid Data dialog, select the Kriging gridding method. A number of gridding methods support breakline files, but Kriging generally produces a grid that creates a nice, smooth map.
3. Click the Advanced Options button.
4. Go to the Breaklines tab.
5. Specify the BLN file as a breakline file.

    

   
   Specify the BLN file as a breakline file when gridding the data.

    

6. Click OK.
7. In the Grid Data dialog, specify or change the Grid Line Geometry to anything you like and click OK to create the grid file.

    

   
   Grid the lake data with the Kriging gridding method and any geometry you choose.

    

8. If you wish, you could create a contour or 3D surface map of this grid file to view the results by going to Map | New | Contour Map or 3D Surface, then select the grid file and click Open.

Grid the lake bathymetry data using the BLN file of the lake edge as a
breakline file, and display the results as a contour or 3D surface map.

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If you have gridded the lake data as above, or downloaded bathymetric data, you can blank the grid outside the lake edge. To do this:

1. Go to Grid | Blank.
2. Select the grid file and click Open.
3. Select the BLN file and click Open.
4. Enter a name to save the new grid file as and click Save.
5. Use the Map | New commands (such as Map | New | Contour Map or Map | New | 3D Surface) to create the map from this new blanked grid file.
6. Double click on the map to edit the map properties, such as line or fill color.

    

   
   Blank the grid to remove the extraneous data outside the lake edge, and display the grid as either a contour or 3D surface map. Double click the maps to edit their properties.
7. If desired, you can enhance the contour map by using the Map | Add | Base Layer command to add the BLN file of the lake edge to the contour map.

Add the lake edge as a base layer to the contour map.

For more options on displaying lake bathymetry data, please see Part 2: Displaying Lake Bathymetry with Local Terrain Data.