Georeference an Image Tutorial

The orthoimage used for this tutorial is of an area of Lafayette, Indiana, US courtesy of the Indiana Spatial Data Portal. The image has no georeferencing, however for the purpose of this tutorial, it has six green arrows indicating the positions of where you will place control points while you georeference it.

1.In Adobe Photoshop, browse to the tutorial folder and open Lafayette.jpg from the Tutorial Data folder.

On the Geographic Imager panel, the Reference file field is not specified because no corresponding reference file is present in the image folder. Similarly, there is no coordinate system specified.

2.On the Geographic Imager panel, click the Georeference button.

The image was originally in a NAD83 / UTM Zone 16N projected coordinate system. You can enter world coordinates as projected coordinates, but to make it simpler you can enter world coordinates as latitude and longitude in the WGS 84 geodetic system.

3.To the left of the image preview, use the navigation controls to pan and zoom to green arrow 1 located at the upper-left corner of the image.

4.Click the Add Point button and click precisely at the tip of the green arrow on the image preview (zoom-in more if necessary).

This adds the first control point named Point 1 at a pixel location of 352,489. A new row is added in the control point table. You'll add a world coordinate to Point 1 to create a control pair. Good locations to place control points are at identifiable locations like road intersections, building corners or property boundaries.

5.In the control point table, double-click the Coordinate System, WX, or WY cell.

6.Choose the WGS 84 coordinate system, and enter:

• Lat: 40.417492
• Long: -86.860779

7.Click OK.

The world coordinates are added to the point and the table is updated.

8.Using the navigation controls, go to green arrow number 2 at the upper-right corner of the image.

9.Click the Add Point button and click precisely at the tip of second pushpin.

10. In the control point table, enter the world coordinate for Point 2 as follow:

• Lat: 40.420841
• Long: -86.839035

To add the control points for the third and fourth points, you'll import a CSV file.

11. Click the Import Additional Control Points button. Navigate to the Tutorial Data folder, select Lafayette_point3and4.csv and click Open.

The control points — Point 3 and Point 4 — are added to the table and the image view. The CSV file includes both pixel and world coordinates. You can move the image view to see the green arrows for the third and fourth control points. You'll continue to add the fifth and sixth control points.

12. Add a control point for green arrow 5.

13. Click the Assign World Coordinates Online button.

The Assign World Coordinates Online dialog box opens a web map. The purpose of this feature is to find the matching location on the web map to assign it to the fifth control point.

14. Make sure Point 5 (UNASSIGNED) is chosen from the Matching Point Location drop-down.

15. Click the SATELLITE button to switch to a satellite web map view.

16. Using the other points as a proximity reference, find the location of the fifth control point. (It's at the intersection of Sagamore Pkwy and Kossuth St). When you have found this location, click the exact location you want to use as a world coordinate (in the middle of the intersection). Click Yes to assign the world coordinate to Point 5.

You can navigate and interact with both the Georeference dialog box and Assign World Coordinates Online dialog box at the same time.

17. Repeat the same procedure for the sixth (and final) control point.

The reference Method is set by default to Affine which requires a minimum of three points. Affine is used for referencing images and higher methods are generally used for rectifying images. It is recommended to use as many control points as necessary to calculate a good reference (six or more control points is good practice).

18. Click the Solve button.

The Solve feature calculates the optimal coordinate system to use based on the control points you specified (that's why six control points were used). These coordinate systems are very similar with similar combined errors. While anyone of these could be used, it's best to use the coordinate system that matches data in your own project. In this case, remember that the image was originally in a NAD83 / UTM Zone 16N projected coordinate system.

19. In the coordinate system list, select NAD83 / UTM zone 16N and click OK.

The control point table contains several columns on the positioning error of each control point (in pixel or world units).

To reduce the error values, you could navigate and adjust the position of control points. Look at the error columns in the control point table to see if this improves the positioning accuracy. In addition, use the Validate tool and View Image Extents Online to check how close your georeferencing is.

20. Use the Validate tool to check the accuracy of your referencing.

When using the Validate tool, you can still interact with the Georeference dialog box at the same time.

21. Click OK to complete the georeference. If a message appears, click Continue to reference the image.

The image is now georeferenced. The Geographic Imager panel indicates a reference file named Lafayette.tfw and the coordinate system (NAD83 / UTM zone 16N). The World (tfw) reference file format is the default reference format. It can be changed in Geographic Imager Preferences.

22. To save the reference file and image file (although the image itself has not been modified), choose File > Save in the Adobe Photoshop main menu. The reference file, Lafayette.tfw, is saved in the same folder as the image file. Alternatively, use Save As to save as a different format, including TIFF (which will save the image as a GeoTIFF).