A Sphere Target is a small sphere (table-tennis ball, white billiard ball, bowling ball, laser scanner alignment tool, etc.) used as a target point in PhotoModeler – similar to a Sub-pixel Dot Target. Sphere targets are also marked in PhotoModeler to sub-pixel precision.
Sphere target marking is often used in the integration of point clouds generated by laser and white-light scanners into a PhotoModeler project. Laser scanners commonly use sphere targets as multiple-scan alignment tools. These same spheres can be used by PhotoModeler in control point or multi-point coordinate system definition to tie a PhotoModeler project to the same coordinate system as a laser scan. An external xyz data file of the sphere centers from the laser scanner software can be imported into PhotoModeler as control or a multi-point transform definition and then associated with the sphere marks. See Importing and Setting Up Coordinate Systems, and Import or Add Coordinate Systems Dialog.
While often used for laser scan data integration, sphere targets can be used in standard PhotoModeler projects as well. Sphere targets are subpixel marks that are useful for tying photographs together. PhotoModeler’s dot and RAD targets are more precise than sphere targets but spheres have one advantage: they can be seen from any angle and do not degrade at shallow angles like the flat dot targets do. For highest accuracy though, we still recommend the use of dot and/or RAD targets with appropriate photography for flat target detection.
The spheres used as targets should have a solid color that is high contrast relative to the background. They can be any color (solid white, solid black, solid red, etc.) as long as they differ from most of the background/scene color. Sphere targets are not coded so sphere target marks need to be manually referenced across photos. Sphere targets can be mixed in a project with dot and RAD targets, and manual marked points.