The Options menu allows you to setup various options related to the rectification process, as described below:
Rectification Method - the Rectification Method submenu allows you to select which rectification method (hence the name) to apply to the entered control points. Different methods are available based on how many control points have been entered. The various methods are as follows:
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Automatic - the Automatic select automatically selects the "best" rectification method that is available based on the number of pairs of control points that have been entered.
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With 2 control points, it uses Linear, unless the difference in X and Y pixel dimensions of the image exceeds 30 percent; then it uses Helmert.
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With 3 control points, it uses Affine.
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With 4 control points, it uses Polynomial unless the Polynomial error is greater than the Affine error and also larger than about 0.2% of the image size (unless you uncheck the option to Use Affine for Polynomial if less error as seen lower in the menu.
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With 5 or more points, it uses Triangulation.
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Additionally, if all the control points are along a line, it will use the linear method because the math for all the others would fail.
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Linear - the Linear rectification method is used when only two control points are entered. This is the simplest of the rectification methods and is equivalent to supplying a world file for the image. This method does not allow for any distortion or rotation in the image, but often works well if the correct projection is selected for the image.
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Helmert - the Helmert, or similarity, rectification method requires at least two controls points to be entered. This rectification method is useful if you need to maintain the original shape of data after a transformation. This method will only be used if either explicitly selected from the list of rectification methods, or if the linear transform method results in significantly different X and Y pixel sizes.
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Affine - the Affine rectification method requires at least three controls points to be entered. This rectification method calculates a best fit to a simple equation of the control points entered. This method can account for some degree of rotation and distortion, but not with a high degree of accuracy. Because a best fit approximation is used, some of your ground points may move a little in order to minimize the error among all control points.
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Polynomial - the Polynomial rectification method requires at least four control points to be entered. This rectification method calculates a best fit to a more complex polynomial equation described the transformation from pixel space to ground coordinate space. Because a best fit approximation is used, some of your ground points may move a little in order to minimize the error among all control points. If you have 6 or more control points, a second order (N=2) polynomial will be used to transform the points (unless you uncheck the option Use 2nd Degree Polynomial for 6+ GCP in the menu).
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Triangulation - the Triangulation rectification method requires at least five control points to be entered. This rectification method performs a Delaunay triangulation for the control points and will exactly preserve the location of each entered control point. Occasionally the Triangulation method generates some odd results around the edges of the rectified image. If this happens, try to enter more control points around the edge of the image, or change your rectification method to Polynomial.
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Use 2nd Degree Polynomial for 6+ GCPs - Default setting is that this option will be enabled, thus if you have 6 or more control points, a second order (N=2) polynomial will be used to transform the points when you select Polynomial. You will need to manually select Polynomial (instead of Automatic) to use the 2nd Degree Polynomial, because Automatic setting always chooses Triangulation instead if you have 5 or more GCP. Unchecking “Use 2nd Degree” will switch to the 1st Degree Polynomial.
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Use Affine for Polynomial if Less Error at GCPs - Default setting is that this option will be enabled. Even if you manually select Polynomial, GM will override you and use Affine if Affine has lower error values. Unchecking this allows you to always use Polynomial (when selected), despite higher error values than Affine.
Resampling Method - the Resampling Method submenu allows you to select how the source image pixels will be resampled to create the rectified imagery. These settings simply control which resampling method is applied initially for rectified images. This can be changed later in the Control Center.
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Nearest Neighbor - simply pick the closest pixel from the source image for each pixel in the rectified image. This can result in some blockiness.
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Bilinear Interpolation - determines the value of a new pixel based on a weighted average of the 4 pixels in the nearest 2 x 2 neighborhood of the pixel in the original image. The averaging has an anti-aliasing effect and therefore produces relatively smooth edges with less stair-step effect.
Auto-Name GCPs - this setting controls whether or not you will be prompted to enter a name for each ground control point (GCP) entered. Disabling this allows you to specify a name for each GCP, but can interrupt workflow. Unless you have a specific need to name your GCPs, we would recommend turning this option on.
Display GCP Labels - this setting controls whether or not the names of each ground control point (GCP) will be displayed in the Zoomed and Reference Image views.
Remember Last Coordinate Reference System - this setting controls whether or not to initialize the coordinate system to the last used rectification system when rectifying new data. If this is not enabled, the coordinate system will always default to Geographic/Arc Degrees/WGS84.
Add Control Points at Corner Points - this option allows you to easily create a control point at each corner of the layer being rectified. If enough other control points have already been entered to do a rectification, the new corner control points will automatically be initialized with the best guess at the ground/projected location, otherwise they will be initialized with dummy values.
Display Transformation Equations - this option displays the actual transformation equations used for transforming this image based on the selected rectification method and the entered control points. Note that this is only available if enough control points have been entered for a transformation to be calculated and if the transformation method is based on a simple set of equations.
Show Crosshairs Across Entire Map - this option controls whether or not crosshair lines are displayed across the entire Zoomed View and Reference Image views.
Ctrl+Left Click Recenters and Zooms Other View - this option controls whether or not Ctrl+Left Clicking in the Zoomed or Reference Image view just recenters the other view if possible, or if it also zooms the other view to the same approximate extents.
Automatically Save GCP File When Done - this option will automatically prompt the user to save a GCP file when they select the OK or Apply button to complete a rectification operation.