What is Field Calibration and When Is It Used?
Field calibration is a method of calibrating or fine tuning a camera at the same time a project is being processed. How does this work, what is it good for, and when should it be used?
Note that there is also a tutorial video on the topic of Field Calibration here.
How Does Field Calibration Work?
Every project goes through an optimization stage where a number of things are solved or improved. The obvious quantities that are solved are point X,Y,Z positions and camera station positions and angles. Programs with advanced optimization algorithms can also fine tune camera parameters (such as focal length, principal point, and lens distortion coefficients) during this processing. This fine tuning is called Field Calibration in PhotoModeler.
What is Field Calibration Good For?
For a project to be accurate, the camera parameters have to be well known. While a separate calibration is usually good enough to meet this requirement, in some circumstances it helps to calibrate or fine tune the camera parameters even further for every project being carried out.
A camera is an instrument that can change. It can change with time or temperature and it can change because various settings are manually modified (such as focus distance of the lens). A stored calibration for a camera may not exactly match the camera at the time of project photography. Being able to re-calibrate the camera from the actual project images at the same time the project is being processed will remove this factor and can improve accuracy.
When should Field Calibration be used?
When high accuracy results are needed, a project should be set up with procedures to achieve those results. One of these procedures is field calibration. For field calibration to work well there are some requirements for the project: a) the camera should be consistent during the shoot (no changes in zoom or focus for example), b) the camera positions should cover a wide range of angles, c) there should be good redundancy (points appearing on many photos from different angles), d) the images should be consistently covered, and e) there should be some rolled camera positions (camera used in both landscape and portrait positions). The point marking accuracy should also be good (targets are preferred but not mandatory).
Field Calibration in PhotoModeler
Field calibration is easy to use in PhotoModeler. When the Processing Dialog opens, select the Options tab and request "Full Field Calibration". It is as easy as that! You can study the post-processing report to see how the camera parameters changed and to see if any warnings are shown, such as high correlations or high deviations on the camera parameters (which indicates that either the project is not strong enough for field calibration or that too many camera parameters are being solved). Field calibration is only available in one-camera projects and only when the requirement for the minimum number of points per photo is met. We also recommend that the project be processed with low residuals first (to ensure there are no marking or referencing errors and that the project is high quality), before trying the field calibration.
There is another option in PhotoModeler called self-calibration. This is similar to field calibration but treats the camera for each photograph independently. This option can be used when the project has more than one camera but is suited mostly for instances where the camera being used changes significantly between photographs (significant changes in focus distance for example). Field calibration is the preferred choice for high accuracy work.
Give field calibration a try in your full or demo copy of PhotoModeler. If you need further clarification of the results please do not hesitate to contact our technical support team. They are there to help you before and after your PhotoModeler purchase.
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