Another form of error is introduced during measurement by marking imprecision. Nobody can mark a point perfectly, and occasionally the point you wish to identify is fuzzy or hard to position exactly in the photograph. If PhotoModeler has good Camera Station positions, but imprecise point locations in the photographs, the projected 3D point will be inaccurate. To reduce this problem, we would mark the desired point in three or more photographs. That way, if the point was positioned incorrectly on one of the photographs, the other two photographs could compensate for it. If it is marked on only two photographs, marking errors cannot be found and will cause an inaccurate 3D point to be created.
Guideline 2: Take at least three photographs of the object or scene. Try to get all important points to show on at least three photographs. For highest accuracy, try to image all points on four or more photographs. It is usually a good idea taking more photos than you think you might need so that when it comes to starting your project you have a collection to choose the best ones from. You may realize that not all detail was captured in some photos, so having a stock of photos to choose from can save you a trip back to the scene. Also try to be aware of objects in your scene that are obstructing key features, and try to take photos of these features so they are clearly visible, which in some cases may require taking multiple photos of a specific feature within the scene.
The figure below shows an interesting case of trying to get each point imaged on three photographs. In this example we are measuring a hole, a dent in some object or an inset doorway/alcove in a building. When the hole has straight edges, it is difficult to get points A and B imaged on three photographs if only three stations are used. To get around this problem we would take two more photographs at positions C and D. Some of the outside points would be imaged on five photographs and points A and B would be imaged on three photographs.
Camera Station Positions for an Alcove
Note: The photography requirements for a SmartPoints project are different than those for a regular PhotoModeler project. See DSM Photo Requirements and SmartPoints Photo Requirements for more information on photography for PhotoModeler SmartMatch projects.