The accuracy of your project can be checked in a number of ways. True accuracy checks can be done with reference to an external standard only but here we will use internal precision and accuracy interchangeably to show the typical steps.
Accuracy checks are normally done first in PhotoModeler to ensure the project is strong and without significant error. External checks are then done to verify true accuracy. The checks are described below:
- Total Error: A total error number appears after processing and can be found in the Processing Report. This value can be described as "goodness of fit". An analogy can be used to further explain total error. If you are a door manufacturer and you are checking accuracy, you would check how well the door fits into the frame. If the door fits into the frame with only a small consistent gap around the edge, this is a good measure of quality. The better the fit, the smaller the error. This gap error is similar to the value in the total error dialog. As a general guideline, a project should have a total error under a value of 1.0.
- PhotoModeler's Internal Precision Values: PhotoModeler provides a number of internal precision values, such as Point Precision and Marking Residuals, that can be used to study accuracy. For example, Point Precision values, found in the Point Table under the Quality configuration, represent what PhotoModeler expects for the accuracy of the point coordinates. These numbers should be very low in a strong project. Note that the following assumptions are made when calculating these values: a) there are no "gross errors", meaning there are no mismarked or misreferenced points and the camera calibration is good , and b) the input marking precisions reflect the true precision (in the Preferences / Process tab, the default Point Standard Deviation Weight values for normal marks should be "1").
- Compare with External Measurements: Measure your results in PhotoModeler and compare to external measurements taken on the actual object or at the scene using a measuring device such as a total station, laser tracker, or steel measurement tape. This is a true assessment of the project's accuracy.
More details about accuracy can be found in the PhotoModeler manual and help file.
