When an unreferenced image is loaded into Surfer as a base map, the coordinates are in pixel units. These coordinates can be reset to real-world coordinates by georeferencing the image. You can georeference the image if you know some real-world coordinates for internal points on the image or if you know the real-world coordinates of the image's corners. Note, in order to georeference the image, it must be in a base layer and at least 2x2 pixels.
To create a georeferenced base map from an image:
- Click Home | New Map | Base | Base.
- In the Import dialog, select your image file and click Open.
- In the Contents window, click the + next to Base to expand the base layer.
- Select the Image.
- On the General page in the Properties window, click the Georeference Image button.
- In the Georeference Image window choose one of the following methods to georeference:
- Using saved 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 7.
- Using the corners of an image for your calibration points: click the Add Corner Points button () in the toolbar to add those four calibration points to the corners of the image. This will not set the real-world coordinates for those calibration points, so you will need to enter that information into the Target X and Target Y columns for each point in the table below the image. Continue to step 7.
- Using internal points in the image for calibration points:
- Zoom in to your first calibration point using the mouse wheel or the Zoom In button () in the toolbar.
- Click the Add Mode button () in the toolbar.
- 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.
- Enter the appropriate real-world coordinates into the yellow-highlighted Target X and Target Y columns in the table.
- Either pan over to the next calibration point location using the Pan button () in the toolbar, or zoom out using the Zoom Out button () or the View Image () button in the toolbar and then zoom into the next calibration point location.
- Repeat steps 6.a.i.2.-6.a.i.4 to set the next calibration point and enter the target coordinates for it.
- If needed, you can move or remove a point. To move a point, select that row in the table or click the Select Mode () button in the toolbar and click that calibration point on the map to select it, then click the Zoom Selected () button in the toolbar to zoom into that calibration point. You can zoom out if needed and then click and drag the point to move it. To delete a point, click the Delete Mode () button in the toolbar and click on the point.
- Repeat steps 6.a.i.5 and 6.a.i.6 for any other calibration points you have. You must have at least 3 calibration points to georeference an image.
- If desired, change the warp method by clicking the Edit | Set Warp Method and then clicking the desired method.
- After finalizing your calibration points, if desired, you can use the Save Points button () to save the calibration points to a file for future use.
- Click the Update Map () button to calibrate the image.
- 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.
- Click OK if you see another Surfer Warning telling you the scale was adjusted to be viewable. Click the X in the upper righthand corner of the Georeference Image window to close it if you're done using it (this dialog is modal though, so you don't have to close it in order to keep working in Surfer).
You will see the Spatial Extents in the Properties window update accordingly.
Now that the image has been assigned real world coordinates, assign the coordinate system to the Map. Assigning the coordinate system allows Surfer to convert from the existing coordinate system to any other coordinate system or overlay another map layer that uses a different coordinate system. See the steps below:
- Select the Base layer in the Contents window.
- On the Coordinate System page in the Properties window, click the Set button in the Coordinate System field.
- 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.
Updated August, 2020