There is a very useful feature in Surfer that is often overlooked. When working with grid files, it is not uncommon to want to resize a grid so that the grid line geometry can match other grids that you are using in the same project.
Grid | Mosaic can be used on several unequally sized grids so that they can be set with the same grid spacing. This feature is useful when you need to perform calculations on grid files which were created from different sources.
Follow these steps to use Grid | Mosaic to resize a grid file:
1. Click Grid | Mosaic,
2. In the Open Grids dialog (shown below), select the grid file you wish to resize.
3. Click Open.
4. To Grid Mosaic dialog will open with the Input Grid File you selected listed.
5. Specify the location where you would like to save the Output Grid File.
6. Adjust the Output Grid Geometry to reflect the new grid geometry. Adjust the Spacing or # of Nodes to increase or decrease the grid resolution. When the spacing or number of nodes is changed, the other will update to reflect the appropriate spacing or nodes for that change.
The Grid Extents section will graphically show the general extent of the original grid, and the Output Grid Geometry section will display the original grid geometry. To get a better idea of the original grid used in this example, the grid file is plotted in XY space, below.
Original input grid file
For this example, I will reduce the extent of the grid by one grid cell on each side. To do this, I have increased the minimum X and Y values by 1, and decreased the maximum X and Y values by 1. The Grid Extents section of the dialog displays an example of the new grid geometry relative to the original grid extent.
In this example, I have also increased the number of nodes in the X direction from 50 to 100, and the number of nodes in the Y direction from 39 to 100. This will increase the density of the grid in the resulting output grid. This can be useful when you want to achieve a finer or coarser grid in your output file.
7. Click OK to create the Output Grid File.
The image below shows the example output grid file displayed (in red) over the input grid (in black). You can see the original grid has been “cropped” by one unit on each side, and has a much finer grid spacing.
Input and Output Grid Files using Grid | Mosaic.
Below I have provided a better “real-world example of using Grid | Mosaic to resize a grid file. In the Input Grid File (Helens2.grd) the grid is elongated in the Y direction. The Grid | Mosaic command is used to resize the grid file so that the spacing in the X and Y direction is the same and the resulting grid is square.
Original “elongated” input grid file
The red dashed line in the image above represent the location of the clipping point used to resize the grid so that the output is a square grid file. The output grid file, shown below, has been resized so that the resulting grid file is square.
Square output grid file
The Grid | Mosaic command is a useful tool when working with grid files of different XY extents. This is just one example of the use of the Grid | Mosaic command, thought its application can be useful in other situations as well.
Additional information about using Grid | Mosaic can be found in the Help. Click Help | Contents to open the Help. In the Contents tab, navigate to Surfer 11 | Gridding | Grid Operations | Grid Mosaic.
Thank you for your question. The filter/interpolation used depends on the Resample method selected in the Grid Mosaic dialog. You have the option of choosing Bilinear Interpolation, Nearest Neighbor, or Cubic Convolution. Each resample method is explained in Surfer's Help on the Resample Methods page.
When resizing to a coarser grid (larger cell size), does Surfer apply any filter or averaging to reduce aliasing before resampling, or is it a simple interpolation using the nearest grid points?