The Grid Data dialog in Surfer may not always have the same minimum and maximum for the grid file in the Grid Line Geometry section as the original data. The most common cause of this behavior is that the data extents in the X and Y directions are very different from each other. The default Grid Line Geometry is calculated so that the grid Spacing in both directions is the same and that each direction have a minimum of 4 grid nodes, and to ensure this sometimes the minimum or maximum values for the grid must differ from the data.
In the Grid Data dialog, this is how the default grid geometry is calculated:
- Surfer takes the min/max of the longest direction and divides it by 99 so that there are 100 grid nodes in that direction.
- The grid Spacing in that direction is calculated.
- Surfer then applies the same grid Spacing to the other direction. This is to ensure that the grid cells are square (they don't have to be square, this is just the default).
- If the other direction data min/max values are not equally divisible by this Spacing value, with a minimum of 4 grid nodes, then Surfer adjusts the grid min/max values in that direction so that it is equally divisible by the Spacing with a minimum of 4 grid nodes.
If you do have very different data extents in the X and Y directions, and you want the grid file to have the same XY extents as the data, you will need to manually enter in the minimum and maximum values for the shorter direction. You can click the Statistics button on the first page in the Grid Data wizard to get the exact data min/max values to enter under the Grid Line Geometry section. You may also need to decrease the Spacing value so there are at least 4 grid nodes in both directions.
Updated October, 2021