Filter Tie Points

In the results of image alignment, some points may be incorrect or low-precision due to feature point mismatches or other factors. You can use the filtering tool to filter out these points and update the alignment with the filtered results to improve the accuracy of the image alignment.

The tie point filtering tool provides two filtering methods:

• Selection Tool Filtering
• Attribute-Based Filtering

Selection Tool Filtering

Use selection tools such as polygon and rectangle to select tie points or images in the 3D view. Click the Image Selection/Tie Points Selection tool, and choose Image Selection or Tie Points Selection to activate the selection tool. Click Graphics Tool to choose the selection tool you need, and draw a shape in the 3D view to make a selection. The Subtract tool can remove newly selected images or tie points from the already selected objects. The Invert tool can invert the selection across the entire image project. The Clear tool can clear all current selections.

Attribute-Based Filtering

• Current Project: The image project where the tie points need to be filtered, which must be added to the 3D view.
• Filter Attribute: The attribute used for filtering tie points, with the maximum and minimum values displayed at the ends of the slider.
• Filter Threshold: Specifies the threshold for the attribute used for filtering.
• Filtered Point Count: The number of tie points selected based on the current attribute and threshold.

Filter tie points that meet specified conditions based on their attributes. The attributes of tie points include:

• Reprojection Error: The average reprojection error of the tie point in all its observable images. If the average reprojection error is greater than the specified threshold, the tie point is selected.
• Triangle Angle: The intersection angle formed by the projection centers of the two images to which the tie point belongs and the tie point itself, measured in degrees. If the intersection angle for any two images of the tie point is smaller than the specified threshold, the tie point is selected.

  The intersection angle is the angle formed by the projection centers of two images and the tie point. It is defined as $\alpha$, and the minimum of $\alpha$ and 180-$\alpha$ is taken as the intersection angle. This can be used to filter out tie points with too large or too small intersection angles.

• Image Count: The number of images that can observe the tie point. If the number of images is less than the specified threshold, the tie point is selected.
• Image Observations: The number of observations corresponding to the images that can observe the tie point. If the number of observations in all observable images is less than the specified threshold, the tie point is selected.
• Distance: The distance from the tie point to the image projection center. If the distance from the tie point to any of its observable image projection centers is greater or less than the specified threshold, the tie point is selected.
  • Filter Points with Distance Less Than: When using the Distance attribute, selects tie points with distances less than the threshold.
  • Filter Points with Distance Greater Than: When using the Distance attribute, selects tie points with distances greater than the threshold.

Deleting Tie Points

After selecting the tie points to be deleted, click the delete tool to remove the selected tie points.

Undo and Redo

Click the Undo Tool to restore the last deleted tie points; click the Redo Tool to delete the last restored tie points.