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Expanded and Adapted from Columbia University Libraries (2010) "Georeferencing an image in ArcGIS," Accessed August 2, 2010 from http://www.columbia.edu/cu/lweb/indiv/dssc/eds/georef.html  All information indicated by an asterisk (*) is taken directly from the Columbia University tutorial without change.  Other items may have been changed or adapted as necessary.

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The Add Control Points tool is the tool that allows you to georeference the image. This is the process of matching points on the image layer to corresponding points on the shape layers.

Add Control Points Using Latitude and Longitude

You can enter x (longitude) and y (latitude) coordinates as Control Points rather than use the method mentioned above. To do this,

1. Left click on a known point on the image. This will place a cross mark on that location.
2. Right click on the point you just created. A menu will appear.
3. Choose Input X and Y. The Enter Coordinates dialog box will appear.
4. Enter the coordinates in the appropriate boxes.

Add Control Points Using Visual Cues

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Note: it is a good idea to zoom in on your image when adding control points for better accuracy.*

1. In the Georeferencing pull down menu, make sure that Auto Adjust is turned off (no check mark).
2. Add a Georeferenced Basemap to your new Map Project.  We recommend adding the ESRI Imagery World 2D Map or ESRI Street Map World 2D from the ESRI servers.
3. In the Table of Contents, drag the Basemap below the image you are georeferencing. That will allow you to see the image superimposed on the Basemap.
4. Select the  Add Control Points button
   Add Control Points Button
   
   
   
5. Left click on a known point on the image. This will place a cross mark on that location.
6. Left click on the matching control point in your Basemap. This will 'move' the image and better align the control points.Create a companion point on the Basemap.  The two points will be connected by a bright, blue line.
7. Repeat steps 5 & 6 Repeat this step with each control point.

Add Control Points Using Latitude and Longitude

You can enter x (longitude) and y (latitude) coordinates as Control Points rather than use the method mentioned above. To do this,

1. Left click on a known point on the image. This will place a cross mark on that location.
2. Right click on the point you just created. A menu will appear.
3. Choose Input X and Y. The Enter Coordinates dialog box will appear.
4. Enter the coordinates in the appropriate boxes.

For every set of control points you create, an entry is created in a table that records the original coordinates, the control point coordinates, and the residual error. Access the table by choosing the View Link Table icon from the Georeferencing toolbar

Entries in this table can be deleted one at a time (highlight the entry in the table and click the delete icon) thus making corrections is easy.

Adjusting the Final ProductResidual error is the measure of the fit between the true locations and the transformed locations of the output control points. If there is a link with a high level of error, consider deleting it by highlighting the point in the link table and clicking the delete icon.

1. When you have finished adding Control Points, select Transformation from the Georeferencing pull down menu. 

*Depending on the number of control points you have, from the Georeferencing table you can perform either a 1st, 2nd, or 3rd order transformation. The transformations compare the coordinates of the source image with the control points creating two least-square-fit equations to translate the image coordinates into map coordinates.coordinate

• A 1st order transformation shifts the image up, down, right, or left, stretches the image larger or smaller, or rotates the entire image
• The 2nd and 3rd order transformations fit higher order polynomial equations to the data, allowing points to be shifted in a non-uniform manner
• Most of the time either a 1st or 2nd order will suffice, try all three and choose the one that works best

For every set of control points you create, an entry is created in a table that records the original coordinates, the control point coordinates, and the residual error. Access the table by choosing the View Link Table icon from the Georeferencing toolbar

Entries in this table can be deleted one at a time (highlight the entry in the table and click the delete icon) thus making corrections is easy.

Saving changes

When you are satisfied with the georeferencing process, three options for saving changes are offered:

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1. Choose Rectify from the Georeferencing pull down menu.  The Save As dialog box will appear.
   
   Georeferencing Save As dialog box for saving georeferenced files
   
   
2. For each of the options in the dialog box, fill in the following values:
   • Cell Size: Leave the default filled in by ArcMap
   • NoData as: Leave the default filled in by ArcMap
   • Resample Type: Nearest Neighbor (for discrete data)
   • Output Location: ArcMap defaults the output location - that is, the place where the program will save your newly georeferenced file - to the location from where you added it to the map. If you want to save your georeferenced image file to a different location, you will need to click on the browse button and add the new location.  We recommend putting the new file in the same location as the other map files.
   • Name: Give your file a name that will allow you to remember what it is
   • Format: TIFF (changing this setting will change the file extension in the Name field - this is perfectly normal).  This will, however, create an ugly, black background that will partially obscure any elements you might want to show underneath the georeferenced image.  Choose TIFF as your Format option only if you will be using the image as a stand-alone element or as the basemap itself, and not if you are going to put it on top of another basemap.

     Note
     Choose TIFF if you want to use your georeferenced files in ArcPad with the GPS units for mobile GIS

   • Format: GRID (changing this setting will change the file extension in the Name field - this is perfectly normal).  Use this Format option if you want to put the georeferenced image over the top of another basemap.

     Note
     GRID formats are not supported in ArcPad.  Do not choose this format if you want to use the georeferenced image on the GPS unit for mobile GIS applications.

   • Compression Type: None
3. When you've filled in all of the values, click OK.

You should now be able to add this georeferenced file to any Map Project you create.

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