Using Georeference

Requirements

Index colour mode is not supported. Any referenced or unreferenced image can be used with the Georeference dialog box—verify a referenced image or reference an unreferenced image.

Georeference Workflow

In general, the steps for georeferencing an image are as follows:

1. Add control points. Through the image view or import into the Control Point List.
2. Assign world coordinates (WX and WY) to each control point (PX and PY) to create a control pair. Enter manually or assign world coordinates using an online map service.
3. By default, the Georeference dialog box is set to reference an image without rectification. If you want to rectify the image, choose an appropriate transformation method (rectify is generally used to scale, stretch, rotate, and distort an image).
4. Choose an appropriate image coordinate system. Specify one or use Solve/Find Best and choose one from a list of estimated coordinate systems.
5. Check total world error of control points and adjust as necessary.
6. Validate the accuracy of the georeference.
7. Reference or rectify the image when you’re satisfied with the registration.

Georeference Dialog Box

With an opened image (referenced or unreferenced), click the Georeference button in the Geographic Imager panel to open the Georeference dialog box. The image view shows the current image and any existing control points. These control points and their coordinate and error values (if available) are shown in the Control Point List.

Georeference button

Add a Control Point Using Image View

To add control points in the image view, first, click the Add Point button to enable it. The cursor in the image view becomes a crosshair. Click within the extent of the current image to add a control point. Pixel and coordinate locations of the cross-hair are displayed along the bottom of the image view. Points are added to the Control Point List below the image view and are automatically assigned a name and pixel coordinates (PX and PY). If you are assigning world coordinates online you can add points later with the Assign World Coordinates Online dialog box opens. This way you can ensure you can identify the location online before creating the control point.

Add Control Points from a File

To import a set of defined control points, click the Import Additional Control Points button. The only supported reference point file format is Comma Delimited files (.csv). Imported control points are added to the existing list. Exported CSV files created with Geographic Imager contain the control point name, pixel location, world coordinates, and the associated coordinate system the world coordinates are entered in.

When a CSV file contains control points that don't have coordinate system information, the imported points will be assigned to the image's coordinate system. If no image coordinate system is specified, it will be assigned "Image CS" until one is specified.

Enter World Coordinates Manually

For each control point, enter the corresponding real-world coordinates (WX and WY) in the Control Point List to create a control pair. Double-click the Coordinate System, WX, or WY cell of a control point to enter or edit its world coordinates manually in the Edit Location dialog box.

You can specify a control point coordinate system using the Coordinate System hyperlink. Any coordinate system can be used to specify coordinates. If it is unknown, entering X and Y values will use an Image Coordinate System. It is common for lat/long values in WGS 84 coordinates are used. See the Coordinate Formats section to learn about the types of coordinate formats that can be entered.

World coordinates can be sourced from the information displayed on the image itself (such as grids or graticules), from external sources (e.g. survey, corresponding maps, etc), or from online map sources.

 The display format of world coordinates in the table is set in Georeference Preferences.

Assign World Coordinates Online

A control point must be created prior to assigning it a world coordinate online. Click the Assign World Coordinates Online button to assign real-world WGS 84 coordinates using an online map service (an Internet connection is required). In the Add World Coordinates Online dialog box, choose an unassigned control point location in the Matching Point Location drop-down list, then click the corresponding location on the online map to assign it a world coordinate. It is best practice to zoom in to the image to ensure the point is in the correct location. You can create/move/edit or delete a control point at any time with the Assign World Coordinate Online dialog box open.

Both the online map window and image view in the Georeference dialog box can be navigated at the same time, often side by side. To navigate the online map, pan and zoom with the mouse (and mouse wheel) or use the navigation control slider. Optionally, enter street or city names into the Place search bar to find places quickly. To navigate the image view of the Georeference dialog, use Ctrl (or Command) and the mouse wheel to move left or right, use shift and the mouse wheel to move up or down, use Alt (or Option) and the mouse wheel to zoom in or out, and use the keyboard shortcuts to quickly switch between tools. (Keyboard shortcuts for tools can be seen by hovering over each button.)

 

World coordinates in the Control Point List are updated after assigning matching points in the Assign World Coordinates Online dialog box. When a control point has both pixel coordinate and world coordinate entered, it is considered a control pair.

When at least four control pairs are valid and an image coordinate system is chosen, an image extents border is drawn in the Assign World Coordinates Online dialog box.

Edit a Control Point

When adding control points, some inaccuracy can occur and may result in larger than desired error values. Editing the location of control points can greatly improve the error statistics to fine-tune the georeferencing. Modifying control points in the Control Point List or directly in the image view. To edit the world coordinates, double-click the corresponding world coordinate cells or click the Assign World Coordinate Online button.

Delete or Disable a Control Point

To delete a control point in the image view, right-click one to open the context menu and choose Delete Selected Point. Optionally, choose Disable Selected Point to disable the control point and prevent it from being used in the calculation for georeferencing

To delete a control point in the Control Point List, click a control point row to select it and click the Delete Selected Control Points button. To disable a control point in the Control Point List, under the Use column, click the corresponding check box to uncheck it.

Reference an Image

By default, the Georeference dialog box is set to reference an image without rectification. Georeference requires a minimum of three valid control points to exactly locate an image to a spatial location. When more than three control points are used, residual errors are introduced. These errors should not be seen in a negative light as they can provide feedback on the accuracy of the reference. It is common to use more than three control points because if one control point is in the wrong location, it can greatly affect the reference. Thus, even though the residual error may increase as you add more control points, the overall accuracy of the reference may increase as well.

Rectify Using a Transformation Method

To rectify an image, click the Rectify using the check box and choose a transformation method. The control points are used to build a polynomial transformation that will shift the image from its existing location to the correct spatial location. Depending on how many control points are valid, one method may be better than another. Depending on the method, a minimum number of control points are required:

 

Format	Control Points Required	Used to
Affine (3 pts)	minimum 3	Reference/Rectify
Quadratic Polynomial (6 pts)	minimum 6	Rectify
Cubic Polynomial (10 pts)	minimum 10	Rectify
Quartic Polynomial (15 pts)	minimum 15	Rectify
Quintic Polynomial (21 pts)	minimum 21	Rectify

 

An affine method preserves collinearity (i.e. preserves points, straight lines and planes) and ratios of distances (e.g. the midpoint of a line segment remains the midpoint after transformation). In this sense, typical maps you are working with are affinely referenced to their respective plane of projection and may only need a slight shift to improve referencing. Rectifying an image using the affine method will perform a transformation and create a north-up image without correcting any distortion.

Higher transformation methods than affine have the ability to correct complex image distortions, also known as rectifying. However, it is not common to use transformations higher than third-order. Higher-order methods require more control points and will require more inputs, processing, and adjusting to achieve an acceptable result. In general, if your image needs to be scaled, stretched, and rotated, use a lower-order method (quadratic). If the image must be distorted even more (i.e. bent or curved) it may be more appropriate to use a third-order method.

While an image with a large set of control points should provide the best results, using affine or a second-order method may not take advantage of the additional control points. In general, your control points should be accurate, have good coverage across the image, and are well spread out when attempting to rectify. Having a sufficient number of points does not guarantee that the highest degree polynomial available will result in a plausible solution; a trial-and-error approach may be appropriate to obtain the best results.

All control points will be maintained and can be edited until the document is closed, rectified, or transformed. Maintaining the control points will allow you to make adjustments until the desired referencing is achieved. Control points are not saved with the image itself. To save control points to a file, use Export Control Points.

 Control points are maintained in the coordinate system they are entered.

Choose an Image Coordinate System

With a sufficient number of control pairs and a chosen transformation method, the next step is to choose an image coordinate system. If you know the image coordinate system, click the Specify hyperlink and choose a coordinate system. If you do not know the image coordinate system, click the Solve button to see a list of suggested coordinate systems. The Solve feature is only available for reference. When rectifying and using a transformation method, click the Find best button to see a list of coordinate systems to rectify the image.

When you click the Solve or Find best button, a list of coordinate systems that are estimated based on calculations made with the selected transformation method and control pairs provided is shown. The table is initially sorted by descending rank. A rank of 0.00 or 1.00 means that it is a highly suitable coordinate system based on the control pairs you specified, however, it doesn't necessarily mean the reference is perfect. The Rotation column shows how much rotation the image is subject to for a selected coordinate system. The Shearing column shows how much the image will be skewed for a selected coordinate system. Selecting a coordinate system is based on additional information such as your knowledge of the image, the geography that the image represents, and your project requirements. It is recommended to view and compare combined errors before selecting a coordinate system to reference or rectify to.

To use a coordinate system as the reference solution, select it from the list and click OK. In the Control Point List, errors are populated in the error columns for each control point. Try to minimize the combined error by removing or adjusting control points with high error. Each time you remove or modify a control point's coordinates to minimize error, the Solve or Find best coordinate system list may be different and results could have improved. It is an iterative process to achieve the best results.

The following are the types of errors provided:

Format	Extension
PXY Error	Total pixel coordinate error
PX Error	X pixel coordinate error
PY Error	Y pixel coordinate error
WXY Error	Total world coordinate error
WX Error	X world coordinate error
WY Error	Y world coordinate error
Forward Residual	Error in pixel units Sum of Error in pixels (X and Y individual errors)
Inverse Residual	Error in the same units as the image coordinate system Sum of Error in units of the coordinate system (X and Y individual errors)

Interpret Residual Errors

When there are enough control pairs to compute a georeferencing solution, residual errors for each control point are calculated. A residual error is the computed difference between an observed source coordinate and a calculated source coordinate. It is the measure of the fit between the true locations and the transformed locations of the output control points.

A high residual error indicates possible error in either the observed source coordinates or the reference coordinates of the reference point in question. The forward residual is displayed in the same units as the spatial reference. The inverse residual is displayed in pixel units. All residuals closer to zero are considered more accurate.

When the error is particularly large, you may want to remove and add control points to adjust the error. As a general rule, apply several different transformation methods, select/deselect questionable points, and select the method and reference points that yield the minimum residual error, assuming that the defined reference points are correct.

Advanced Options

Click the Enable Advanced Options check box to enable the Remove Shearing and Square Pixels options. Enable the Remove Shearing check box to remove any shearing that may result (averages the X and Y rotation values and makes them the same). Enable the Square Pixels check box to ensure that the X and Y pixel dimension size is equal. The combination of these options will affect both pixel coordinate and world coordinate errors.

 

Validate (Show point online)

When a georeference solution has been applied to the image, you can use the Validate tool to visually check how close your georeferencing is. Click the Validate button and click a location on the image view. An online map service will open and a corresponding placemark is placed on the web map with its WGS 84 Lat/Long coordinates displayed. The extent (red outline) is also displayed on the web map to show the extents of the image. You can continue to click the preview image to validate locations and review them on the online map. Like the Assign World Coordinates dialog box, the zoom of both the image view and the online map can be independently changed.

 The Validate tool is disabled when the georeferencing solution is not complete.

View Image Extents Online

You can use View Image Extents Online to see the extents (red outline) of the image on a web map. It is a useful tool to help verify how close the extents are from image to real world.

 The View Image Extents Online tool is disabled when the georeferencing solution is not complete.

Additional Georeference Features

Import Control Points Formats

A CSV or TXT file can be formatted with the following syntax so that Geographic Imager will recognize the values when importing it into the Georeference dialog box.

Format	Syntax
CSV	pixel X value, pixel Y value, ground X value, ground Y value
TXT	pixel X value [TAB] pixel Y value [TAB] ground X value [TAB] ground Y value

 Importing a file with mixed delimiter formats is not supported.

 Log file format cannot be imported.

Export Control Points

Click the Export Control Points button and save the points using the available export reference formats: log files (.log) and comma-delimited (.csv).

The exported log file will contain all of the reference point pixel and world coordinates (including used and unused points), referencing method, residual error values, and points used. The exported comma-delimited file will include pixel name, reference point pixel, world coordinates, and coordinate system of the world coordinates, no error values or indication if the point is used and unused.

Context Menus

In the image view, for any control point, right-click to open a context menu that allows you to delete the selected point, disable the selected point, or copy the coordinate values.

When copying coordinate values, several formats are copied to the clipboard. The coordinates copied are based which ones are enabled in Georeference Preferences. For example, pasting the values into a text editor with Pixel, WGS 84, and Image's Coordinate System enabled will result in the following:

292        424        42.367407        -71.050393        237033.936174        901976.024778

 

220, 288 represents the pixel coordinates (PX/PY)

42.3697, -71.0520 represents the geodetic base coordinates (Lat/Long)

236898.936174, 902231.024778 represents the world coordinates (WX/WY)

 

In the Control Points List, right-click any control point entry to open its context menu. You can delete or disable the point as well as center at or zoom to the point.

Right-click any column heading in the Control Points List to open the Show/Hide Columns context menu. In the Show/Hide Columns dialog box, uncheck any of the columns to hide them in the Control Points List.

Unreference

Click the Unreference button to remove all control points and any reference information associated with the image within Adobe Photoshop. The unreference action can’t be undone within the same work session (i.e. you can’t step backward using Adobe Photoshop history). This does not delete any reference information in the actual file unless you overwrite it during a save or export.

View DEM/DTM Elevation Values

To view the elevation value of a DEM or DTM, it must be imported using a DEM schema. In the Georeference dialog box, actual elevation values are displayed below the image view. Move the mouse cursor to any position within the image view to see the values (updated in real time). Note that after applying Terrain Shader, the elevation value will not be available.