One way to check this is to look at the values displayed on the bottom (X) and left (Y) axes before the maps are overlaid. If the X and Y coordinates are vastly different, then the points/map objects will either be on opposite ends of the map, or one or both maps may be so small that it looks like they disappear after they are overlaid.
If the units of the grids are in a geographical or projected coordinate system, you can specify the coordinate system for each of the layers and then overlay them. Please see the article Use map layers with different coordinate systems in the same Surfer map for additional information about how to define the proper coordinate systems for each map layer and the final map display.
If the units of the grids are not in a geographic or projected coordinate system, but the units are not equal, custom local coordinate systems can be used to define the units. Please see the article Change the XY units on a map in Surfer for additional information about how this can be done.
Alternatively, you can convert the units of one grid file into the units of the other grid file using the Grids | Calculate | Transform command with the Scale option. For example, to convert a grid from XYs in meters to XYs in feet, set the X Scale and Y Scale to 3.2808 to multiply the X and Y values by 3.2808. Click OK and the new grid in feet is created.
Updated December 2021
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