Factors Affecting Accuracy in Photogrammetry
PhotoModeler is based on the science of Photogrammetry, which is defined as “measurement from photographs”. There are a number of factors that determine the accuracy of a photogrammetric project.
The main factors are:
Photo Resolution:The higher the resolution of the images, the better chance of achieving high accuracy because items can be more precisely located. Image resolution is defined by the capabilities of your digital camera or film scanner. Trying for the highest resolution you can or can afford is the best approach.
Camera Calibration: Calibration is the process of determining the camera’s focal length, format size, principal point, and lens distortion. There are a number of ways of obtaining this camera information and using a camera fully calibrated in PhotoModeler will give the best results.
Angles Between Photos: Points and objects that appear only on photographs with very low subtended angles (for example a point appears in only two photographs that were taken very close to each other) have much lower accuracy than objects on photos that are closer to 90 degrees apart. Making sure the camera positions have good spread will provide the best results.
Photo Orientation Quality: During processing, PhotoModeler computes the location and angle of the camera for each photo – this is called Orientation. One factor that contributes a lot to the accuracy of projects is an accurate orientation for every camera position. The orientation quality improves as the number of well-positioned points increases and also as the points cover a greater percentage of the photograph area.
Photo Redundancy: A point’s or object’s position is usually more accurately computed when it appears on many photographs – rather than the minimum two photographs.
Targets: The accuracy of a 3D point is tied to the precision of its locations in the images. This image positioning can be improved by using targets. PhotoModeler uses the image data to sub-pixel mark the point and this increases the precision of its placement and hence the overall accuracy of the point’s computed 3D location.
To see how the various combinations of these factors contribute to project accuracy, please review the table below.
Photogrammetry Accuracy Scale
Moving down each column one gets increasing accuracy given the other items remain constant.
So for example, the lowest accuracy with PhotoModeler is achieved with a low resolution uncalibrated camera, low angles between photos, low coverage of every photo, marks only on two photos, and no subpixel targets.
Conversely the highest accuracy is achieved with a high resolution field calibrated camera, with strong angles between photos, most photos have good coverage, most points appear on 8 or more photos, and all points are retro-reflective targets.
The accuracy figures “1 part in NNN” are the one sigma standard deviation accuracies. At 1 part in 30,000 on a 3m object, point positions would be accurate to 0.1mm at 68% probability (one sigma).
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