Why are Targets Useful?

There are a number of factors that affect accuracy in a photogrammetry project. These are:

• how well the camera’s internals are known or can be computed,
• the positions and angles of the camera stations, and
• the precision of placing the points in the photographs.

In this user tip we will discuss the last item – improving the precision of point placement on photos.

One method of improving precision is to use PhotoModeler’s sub-pixel target marker (sub-pixel means smaller than one pixel). Marking of standard natural features, such as corners or other objects that have strong visual definition, is usually precise to within 1 to 10 pixels. In contrast, PhotoModeler’s sub-pixel target marking tool can be much more precise, usually achieving precision between 0.01 and 0.8 of a pixel. A target is a man-made, high-contrast and flat shape attached to the object. PhotoModeler has had circular target marking for a number of years and as a result its circular sub-pixel marker is very precise, repeatable and robust.

The accuracy of projects using only natural features is dependent on how well you can mark the same feature in two or more photographs. This relies upon PhotoModeler’s image zoom function and your own estimation ability to mark the exact same point in each photo.

Higher accuracy marking is accomplished with the sub-pixel target marker that automatically analyzes the shape of the target and calculates a very precise center. To perform sub-pixel marking, all that is required of the user is to roughly outline the region of the target. An alternative to manually outlining the target regions is to have PhotoModeler automatically search out and mark all targets visible in one or more photos.

Targets can be used in many types of projects including industrial measurement, engineering, architecture, object modeling and forensics. If you think that targets might assist with your projects please contact our sales department at sales@photomodeler.com for more information. We would be happy to answer any questions you might have.