There are a few things you can try to eliminate the bulls eye effect:
- You can open the grid in the Grid Node Editor by clicking File | Open, selecting the grid and clicking Open. Edit the Z values of the grid nodes themselves by clicking on a node (one of the + symbols) and editing the Z value for that node in the toolbar. So in the center of the bulls eye, change the Z value to another value. Then save the grid by clicking File | Save (or Save As to save it to a new name) and click the X on the view tab to close it. Then update the map with the File | Reload Map Data command.
- You can use Grids | Edit | Convert to convert the grid to an DAT XYZ data file, then simply regrid the data using the Kriging gridding method. The reinterpolation of the grid data may smooth out the bulls eye.
- You can use Grids | Edit | Filter to filter the grid using a low-pass filter. That may also smooth out the bulls eye.
- If you created the grid in Surfer, you could try other gridding options. The Inverse Distance to a Power gridding method is known for creating a bulls eye effect. You could grid the data using a different gridding method, such as Natural Neighbor or Kriging.
- If you created the grid in Surfer or gridded XYZ data in another program, you could try adding more data in the area of the bulls eye. More data points (even if they're just dummy data points) may help shape the contours how you want.
Updated January 9, 2017