There are a few ways you can merge GRD or DEM files.

Method 1. Overlay separate maps.

The first is to create separate contour layers for each grid and overlay them in the same map. This works well for most files:

1. Click Home | New Map | Contour, select the first GRD or DEM file and click Open. That creates the map with the first contour layer.
2. Select the map and click Home | Add to Map | Layer | Contour, select the next GRD or DEM file and click Open. That creates a new contour layer and adds it to the map. Now the map has two contour layers.
3. Repeat for all GRD or DEM files you have so that all the contour layers are combined and displayed in the same map.

Method 2. Mosaic the grids.

The next method is to use the Grid Mosaic command to merge two (or more) grid or DEM files together into a single grid. You would create the map from this one combined grid file:

1. Click Grids | Resize | Mosaic, select the first grid file and click Open.
2. In the Grid Mosaic dialog click the Add button, select the second DEM and click Open.
3. You have a choice of the Resample Method and the Overlap Method (what to do when the grids overlay and there are multiple sets of data in a specific area).
4. Specify an Output Grid file name and location and click OK.

Method 3. Convert the grid to data, merge the data, and regrid.

If overlaying maps or mosaicking is not working as you expect, or the contact/seam just isn't looking right (perhaps due to NoData values), see the instructions here: When I merge my grids in Surfer and create a map, there is a line between them. How can I fix this?.

Updated October 25, 2018