Add straight line
Add straight line command lets you enter a new tie line that represents a straight horizontal or vertical line in an image captured by a side-looking camera. Typically, this tie line type is used in calibration to solve Zero radius function lens distortion in images of mobile ground-based systems.
The best Straight lines are located close to image boundaries and run over a large distance within an image. An image should show an object with long straight lines, for example, a building with long edges. A few horizontal and vertical straight lines close to all four edges of images provide a good base for solving the lens distortion.
See also Section Mobile camera calibration for more information about using Straight lines in calibration.
To enter a new straight line:
1. Select an image in the upper list of the Tie points window.
The software updates all views in order to show the active image.
2. Select Add straight line command from the Point pulldown menu.
3. Digitize the straight line as you see it in the image in the Active detail view. Due to the lens distortion, the line is not straight but curved in the image.
A straight tie line is defined by at least three vertices. The longer the line can be drawn, the better for solving the lens distortion. Place the vertices of the line by data clicks and finish it with a reset click.
The application adds the tie line to the list in the Tie points window. The values in the list show how much each vertex of the tie line differs from a straight line. The left column shows these differences after lens distortion correction, the right column before the correction. The values are given in pixels.
4. Continue with step 3 until there are enough Straight lines for solving the lens distortion.
Place Straight lines in different images and for each camera of the mission. You can check the distribution of the tie lines by using the Draw pixel distribution command from the File pulldown menu. For each camera, there should be lines along all four edges of the grid model.