Georeference (assign coordinates to) an image in Surfer

Have you ever imported an image into Surfer only to have your whole map disappear? Or imported the image only to see coordinates that don't make sense (0,0 origin for example)? These situations are caused by the image being unreferenced. When an unreferenced image, such as a Google Earth screenshot, is used to create a Surfer base map the coordinates are in pixel units.  In order to use the image with your geographic map it must be georeferenced. 

Golden Nugget : You can download high resolution, georeferenced imagery directly into your Surfer map from Google and Open Street Maps. Check out Add High Resolution Imagery to your Maps with XYZ Tiles servers for more information!

What is Georeferencing?

Georeferencing is the process of assigning real-world map coordinates to a raster image that does not have spatial information, such as a scanned paper map, an aerial photograph, or a site drawing from a PDF.

In Surfer, this is done by identifying several known locations (calibration points) on the image and entering their corresponding real-world coordinates (e.g., in UTM, State Plane, or Latitude/Longitude). Surfer then uses these points to stretch, scale, and rotate the image so it is positioned correctly. The georeferenced image can be accurately overlaid with other map layers and data files.

How to Georeference an Image in Surfer

1. Click Home | New Map | Base | Base.
2. In the Import dialog, select your image file and click Open.
3. In the Contents window, click the Base(raster) layer to select it.
4. On the General page in the Properties window, click the Georeference Image button.
5. In the Georeference Image window choose one of the following methods to georeference:
   
   
   1. Method 1: Internal calibration points in the image (most common)
      
      1. Zoom in to your first known location (calibration point).
      2. Click the Add Mode button () in the toolbar.
      3. Click on your calibration point. The pixel coordinates for that point are entered into the Source X and Source Y columns in the table below the image.
      4. Enter the appropriate real-world coordinates into the yellow-highlighted Target X and Target Y columns in the table. If using lat/lon coordinates, ensure Target X = Longitude and Target Y = Latitude.
      5. Click View Image () in the toolbar and then zoom into the next calibration point location.
      6. Click on the calibration point and enter the real-world coordinates in the Target X and Target Y columns.
      7. Repeat steps v. - vi. until all desired points are defined.
         Note: You must have at least 3 calibration points to georeference an image.
         
      8. Continue to step 6.
         
         
   2. Method 2: Corner Coordinates
      1. Click the Add Corner Points button () in the toolbar to add those four calibration points to the corners of the image. The pixel coordinates for that point are entered into the Source X and Source Y columns in the table below the image.
      2. Enter the appropriate real-world coordinates into the yellow-highlighted Target X and Target Y columns in the table. If using lat/lon coordinates, ensure Target X = Longitude and Target Y = Latitude.
      3. Continue to step 6.
         
         
   3. Method 3: Calibration points in a file
      If you have calibration points saved in a format like the attached RegistrationFile.georef file, click the Load Points button () in the toolbar. This will add the calibration points to the image with the correct Target X and Target Y real-world coordinates. Continue to step 6.
      
      
6. Click the Update Map () button to calibrate the image.
   1. You may see a Surfer Warning telling you the new or modified layer exceeds the current map limits, and asking if you want to adjust the map limits. Click Yes.
   2. Click OK if you see another Surfer Warning telling you the scale was adjusted to be viewable. 
7.  Click the X in the upper righthand corner of the Georeference Image window to close it if you're done using it.

The Spatial Extents in the Properties window will update to match the real-world limits.

Now that the image has been assigned real world coordinates, assign the source coordinate system to the Map. Assigning the source coordinate system allows Surfer to convert from the existing coordinate system to any other coordinate system or overlay with another map layer that uses a different coordinate system. To assign the source coordinate system:

1. Select the Base layer in the Contents window.
2. On the Coordinate System page in the Properties window, click the Set button in the Coordinate System field.
3. Select the coordinate system for the Target X and Target Y coordinates you entered into the Georeference Image window, and click OK to apply that coordinate system to the map layer.
   Note: this system will not necessarily match the coordinate system used to display the map. See Difference between setting the coordinate system for a layer, and changing it for a Map in Surfer for more information.

Additional Resources:

• Surfer Help: Georeference Image
• Create Dynamic Models from Scanned Maps with Surfer (webinar)
• Load a scanned map, digitize the data and create a vector base map in Surfer
• Add High Resolution Imagery to your Maps with XYZ Tiles servers