If you've converted a very large image into a grid file with the same number of grid nodes as the image had pixels, it's likely your resulting image map will look lower-resolution than the original image. This is because the default maximum image size for an shaded relief map is 3000x3000. You can easily adjust this though in the Surfer set file. 

1. Close Surfer.
2. Open a Windows File Explorer window.
3. Navigate to C:\Users\<your username>\AppData\Roaming\Golden Software\Surfer
4. Right click on the surfer.set file, click Open with, choose Notepad, and click OK.
5. Scroll all the way down to the bottom of the file. 
6. In the $Hdr(Miscellaneous) section, the 13th entry down is MaxImageMapRes. After the equal sign, type the bigger of the two pixel dimensions of your image (so if it was 20000x15000, type 20000).
   
7. Click File | Save to save the file.
8. Open Surfer and create your image map. Now the image should be the appropriate resolution.

Updated February 2020