The Réseau Measurement in ELCOVISION 10The réseau measurement is the first step in the photogrammetric measurement process. Basically it transforms a 'normal' picture into a photogram. Primarily a cartesian coordinate system is put into the picture so that the later measurements have a clear reference:
Photo system of coordinates after the réseau measurement
Photos of Digital CamerasIn the case of pictures of digital cameras this process is very simple. When JPG images which normally contain so called EXIF data or TIFF images with camera tags are used, the réseaumesaurement is done fully automatically when an image is inserted into a project.
If there is no such information the user must simply click on a thumbnail picture in order to turn the picture correctly:
Réseau measurement of digital Images
Photos of Analogous CamerasIn the case of pictures of analogous metric cameras the réseau measurement is a little more complicated. In this case the réseau measurement compensates a set of falsifying influences, e.g. the incorrect position of the film in the camera, the distortions of the film by the development process, and the nonlinearity of the image scanner. These errors are typically very small and not recognizable with a bare eye. However they prevent precise measurements.
Therefore there is a so called réseau plate built in in the analogous metric cameras. The highly precise etched in réseau crosses are along-exposed automatically with the picture.
Réseau plate of a Leica R5 ELCOVISION
Since we know the positions of the réseau crosses with an extreme accuracy a mathematical compensation of the errors specified above can be found by the measurement of the actual positions of the réseau crosses in the picture. As always also this method of réseau measurement is mostly done fully automatic by ELCOVISION 10:
Full automatic measured réseau crosses (with magnifier)
The réseau crosses are measured with an high sub pixel accuracy.
ELCOVISION 10 supports the following methods for the réseau transformations:
As a special case there is the réseau measurement of pictures of non-metric cameras e.g. pictures of monitoring cameras, pictures of normal amateur cameras or historical photographs. In this case we use the picture corners are measured as rée, seau crosses. The more exactly we know the negative size of the image the better the image will fit into the orientation process.