Tip 29: Sphere Targets in PhotoModeler
Precise 2d marking of points in photos is key in a photogrammetric project to obtain the highest 3d accuracy. There a number of ways to get this marking precision. The highest precision is obtained when PhotoModeler can use a known target shape for sub-pixel marking.
Sub-pixel marking means that the program can detect the position of the target center in the image to less than one pixel. Human marking of even very-well defined shapes is usually no better than two pixels. You can see how sub-pixel marking can improve accuracy quite a bit.
PhotoModeler has had the ability to mark flat high-contrast dot targets for some time. A recent release added sphere target marking as well. A sphere target is a small contrasting-color sphere that is placed in the scene before photos are taken. Example spheres might be table tennis balls or billard balls.
A few advantages of sphere targets are: a) they can be viewed from more angles than flat dot targets, b) they handle being partially obscured better, and c) a number of laser scanners use them for alignment targets, making laser scan /photogrammetry integration easier and more accurate. Two disadvantages of sphere targets are: a) they tend to be more expensive, and b) their marking is not as automated as dot targets in PhotoModeler.