View - Panes - Control Points

Use the Control Points pane to add new control points to a drawing, image or map. Control points are used to georegister an image or drawing to the same geographic location and projection used by a "known good" image or drawing.

 

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The Control Points pane always shows a list of control points defined for the active image or drawing together with their coordinates. When we click on a different window the Control Points pane will switch context to show any control points defined for the active window.

 

The Control Points pane is also used to rename control points or to change their location by changing their coordinates. We can click into the Name field to change the name from the defaults, if desired. To change the coordinates of a control point we click into their X / Longitude or Y / Latitude fields.

 

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New Control Point - Add a control point by clicking with the mouse in the component window. A new control point will be added in the Control Points pane.

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New Blank Control Point - Add a blank control point row in the Control Points pane.

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Delete Control Point - Delete the highlighted control point. More than one control point may be highlighted by clicking on a starting point and then SHIFT-clicking on the ending point to highlight the starting and ending point and all points in between. All the highlighted points may then be deleted.

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Register - Georegister this drawing or image using matching control points in a reference component.

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Load Points - Load control points from points that exist in the drawing (enabled for drawings only).

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Save Points - Save control points as points in the drawing (enabled for drawings only).

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Show Names - Show the names of control points in the component.

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Preview - Show control points in the component.

Name

Name of the control point. Control points are matched between two components by using their names.

X / Longitude

The X or Longitude position of the control point.

Y / Latitude

The Y or Latitude position of the control point.

 

 

Adding a Control Point to a Component using the Mouse

 

1. Open the component. Zoom into the location where the control point is to be located if greater accuracy is required.

2. Click on the New Control Point button in the Control Points pane's toolbar.

3. Click the location in the component where you want the control point. It will appear in the drawing and a new control point row with coordinates will appear in the Control Points pane.

4. If you don't like the name assigned to the control point by default double click onto the name to change it.

 

No matter what other tool modes are selected (for example, Zoom In or Center), after clicking New Control Point the mouse cursor will temporarily shift modes so that the next click locates the control point. If the mouse is clicked within the component window a new control point will be created. If the mouse is clicked anywhere else the New Control Point mode is abandoned and the mouse cursor will return to whatever state it was in before the New Control Point button was clicked. This is why we zoom in first to the region of interest where we wish to place the control point.

 

At times we will add new control points manually by specifying their coordinates.

 

Adding a Control Point to a Component Manually

 

1. Open the component to which the control point is to be added.

2. Click on the New Blank Control Point button in the Control Points pane's toolbar.

3. A new control point row appears with zero coordinates.

4. Double click into the X / Longitude and Y / Latitude coordinate fields to specify the desired coordinates.

 

Show Names

 

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A control point appears by default with a label showing its name. The screen shots above show how control points appear in drawing layers at left and in images at right.

 

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Clicking OFF the Show Names button removes the name labels from the display.

 

Georegistration Dialog

 

images\btn_register.gif To georegister the active image or drawing, click on the Register button in the Control Points pane.

 

Note: Some options will be enabled only for certain methods or when sufficient control points have been defined.

 

Reference

The name of the drawing or image to be used as a source of control points to be used as reference points.

Method

One of the following algorithmic methods:

Affine (triangulation) - Also called geometric transforms, affine methods can georegister a target component to a reference component using fewer control points than required by the Numeric method. However, affine registration requires care in placement of control points.

Affine (scale, shift, rotate) - A fast method that is algorithmically equivalent to using Numeric with an Order of 1. This is the default method since it works with any number of control points.

Simple (scale, shift) - Match components using XY translation and re-scaling only. Works with any number of control points.

Numeric (polynomial) - Numeric matching uses numerical computation methods to transform one component to match another. A large number of control points are required for good matching but the method is algorithmically simple and fast. Available only when eight or more control points have been defined, thus allowing an order of 2 or greater.

Note: Only those methods usable with the number of control points you have defined will be displayed. If you don't see a method, add more control points.

Order

The level of mathematical sophistication applied. Higher orders result in better matches but take more time and more control points. In numeric methods the highest order exponent used in the polynomial equations generated to transform the coordinate system of the target component. Enabled only for the Numeric method.

Modify coordinate system

Enabled when the Simple method is selected. If checked (default) the coordinate system (projection) of the component being georegistered is converted to the coordinate system of the guiding component. If not checked, the coordinate system is not changed.

Scale pixels equally in X and Y direction

When this option is off, the user can specify both the width and the height of the resulting image or surface component. When the option is turned on, the user can only specify the width of the resulting component and the system will automatically compute the height. By default the option is turned on.

Interpolate pixels

Enabled when georegistering non-palette images or surfaces using the Numeric method. Creates a much smoother image when transforming the image into the new projection. Very computationally expensive: requires approximately twice the processing time to georegister an image if enabled. Note: Methods other than Numeric always interpolate pixels for non-palette images and surfaces and never interpolate pixels for palette images.

Save error surface using

Create a surface containing the root mean square error value for each location in the georegistered component. Choose the data type for the error number saved in the surface at each location. If checked, this option doubles the time required for georegistration. Enabled for the Numeric method. See the Error Surfaces topic.

Size

The transformed size of the image or surface in pixels. By default this will be set to some value that attempts to approximately preserve the size of the image or surface before georegistration. Enabled for images or surfaces.

 

The Order chosen will have a great impact on the number of control points required, especially when using the numeric method. For numeric georegistration, there usually must be at least four times the order number in control points. Thus, for numeric registration of order 4, there must be at least 16 control points. This guideline is a minimum value. Certain arrangements of control points may require an even greater number of control points.

 

Affine registration works with fewer control points, as few as two or three in the case of Affine (scale, shift, rotate). However, greater care must be taken when assigning affine control points. In general, control points should be placed for affine registration so that control points are drawn along the outer border of an imaginary shape without control points in the middle of the shape. For example, placing control points in a rectangular or rhomboidal arrangement is OK. Placing control points in a circle with several additional control points in the center of the circle is not OK.

 

Affine registration is a good choice when registering scanned images of maps that have a graticule grid of latitude and longitude lines shown. One can choose four intersections of latitude and longitude graticule lines that are near the four corners of the image to use as control points. Because fewer control points are required affine registration is almost always a better choice if we can place control points in a rectangular or rhomboidal arrangement.

 

Numeric registration may be a better choice when registering images to control points that are scattered throughout the image. When using numeric registration, evenly distributed control points will yield better results. This will, however, usually require many more control points than affine registration.

 

Either affine or numeric registration is a good choice when registering scanned images of maps that have a graticule grid of latitude and longitude lines shown if we are able to place many control points. One can choose a number of intersections of latitude and longitude graticule lines to rapidly mark many control points.

 

In either case, if control points are restricted to only part of the image it is quite likely that other parts of the image will not be georegistered well. When used in maps they will appear to be out of alignment with overlying drawing objects. It is critically important to use control points that are dispersed throughout the entire image. With the numeric method, the more control points that are used, the better the ultimate georegistration.

 

Given the labor of marking many control points the appeal of Simple registration is obvious. If an image is already in a North up overhead view and need only be resized and moved to be registered to a given target drawing this is a good choice. If the image is in Orthographic (the default when importing from geographically-unaware formats like .jpg), re-project the drawing into Orthographic and note the central latitude and longitude. Use Edit - Assign Projection with the image to specify the same central latitude and longitude. Then use Simple registration to georegister the image to the drawing.

 

Tech Tips

 

Uncheck the Autoscroll window on edit or selection operations option in Tools - Options when working with control points, or use the Require Alt key so that no autoscroll happens unless the Alt key is pressed. Because using the New Control Point tool activates the mouse as soon as it leaves the Control Points pane, if this option is not deactivated the target window will begin to autoscroll as the mouse enters it.

 

Picking a control point within a component window sets the focus to that window.

 

Large images or surfaces can take a very long time to georegister when using Affine (triangulation) method or the Numeric method with higher Order. . If desired, the georegistration process can be cancelled and then re-launched using a different method or lower Order.

 

Occasionally we might make a mistake in the placement or naming of control points that results in a bizarre and obviously wrong georegistration. In such cases, it is nice to know that Undo will work.

 

Images must be resized during some types of georegistration. Palette images will always be resized using the nearest neighbor method (no interpolation of colors) to guarantee that the georegistration process does not introduce any new colors.

 

See Also

 

·      The File - Print dialog used to print a component includes a Control Points option that may be used to print the control points in a component.

·      See the Georegistration topic for details on how to use control points to georegister images and drawings.