A grid is composed of a number of rows and columns of nodes. The Grids | Calculate | Math or

**Grids | Calculate | Volume**(when calculating the volume between two grids) command does its operations on a node-by-node basis and it requires that the two grids have*exactly*the same number of rows and columns and the exact same XY extents. The Grids must be the same size error occurs when the two grids do not have the same spacing in either the X or Y direction, or the same minimum/maximum values in either the X or Y directions.

While in the Grid Math or **Grid Volume **dialog, you can select a grid and click on the Grid Info button to get information about the grid, including the grid size (the number of rows and columns) and the X and Y extents.

You can correct this problem by changing one grid to match the XY extents and grid size of the other grid. You can do this a couple different ways:

**Method 1:** If you have the original XYZ data, you can regrid one (or both) of the grids by:

- Clicking Grids | New Grid | Data, selecting the data file for one of the grids and clicking Open.
- In the
**Grid Data**dialog, specify the # of Nodes, the*Minimum*, and the*Maximum*in the X and Y directions (X is columns, Y is rows) so it matches that of the other grid. - Click OK to create the grid.

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**Method 2:** You can resample one or both grids using the Grids | Resize | Mosaic command:

- Click
**Grids | Resize | Mosaic**, select one of the grid files and click Open. - In the Grid Mosaic dialog, specify the # of Nodes, the
*Minimum*, and the*Maximum*in the X and Y directions (X is columns, Y is rows) so it matches that of the other grid. - Click OK to create the grid

Once the two grid files match in XY extents and grid size, then you can use them together in **Grid Math** or **Grid Volume**.

*Updated October 31**, 2017*

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