Measurement Accuracy

Measurement Accuracy

In geo-referenced flights, the accuracy of measurements made on the 3D model by DatuBIM depends on the following parameters:

  1. The camera resolution and the distance between the camera and the measured object. The accuracy of measurements made by DatuBIM is a factor of the camera’s resolution and the proximity to the measured object. Typically, a 20-megapixel camera provides a 1 to 2-centimeter (3 to 6 hundredth of a foot) measurement accuracy at a distance of about 50 meters (150 feet).
  1. The precision with which measurements in the survey area are done. This refers to the coordinates of the reference control points. These should be as exact as possible as they set an accuracy baseline for all the measurement and drafting related to the images. When you measure ground control points, make sure you have several of them on a surface showing a very clear feature, such as marks on the ground or marks on walls. Better yet, the surveyors can paint their own marks, such as a cross of 60cm by 60cm (2 feet by 2 feet). This will assure that a very precise measurement is taken. It is very strongly recommended to make two sessions of measurement, and take the average.  If the difference of measurements of a specific point is more that 2-3 cm, consider disposing the point or retaking the measurement.  
  1. The scattering of the control points and the accuracy of digitizing them in the images. For best measurement accuracy, control points for image geo-referencing should be chosen on the perimeter of the surveyed area so that measurements made on the 3D model by DatuBIM are interpolated, rather than extrapolated. Moreover, control points should be well distributed in 3D space, i.e., relative to both depth and height, and digitized as accurately as possible in the geo-referenced images. For more details on measuring control points in the field.

Various quality control measures in DatuBIM provide the user with detailed information about the accuracy of measurements. Firstly, after the images have been geo-referenced, DatuBIM verifies the accuracy of the image geo-referencing by comparing the relative orientation of the control points to the relative orientation of the images. The Geo-Referencing Quality Indicator gives an indication of the accuracy of the referencing of the photogrammetric model. Furthermore, after calculating the coordinates of the new points measured from the images, DatuBIM provides the accuracy of each measurement, i.e., the (x,y,h) residuals for every point.



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