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Photographic Intersection - the technicalities …
To extract data from conventional stereo images three orientations are required. Inner Orientation is the reconstruction of the camera geometry and the correction of lens distortion. This is followed by the Relative Orientation to obtain the relationship between the two images forming the stereo model utilising common points in the two images. Control points in the model are then used to perform the Absolute Orientation to bring it into the required co-ordinate system, which is in effect a resection for the location and orientation of the camera positions. With Photographic Intersection the Inner Orientation is still required as the camera geometry needs to be known and the Relative Orientation is now the relationship between the camera and the Theodolite or Total Station on which the camera is mounted. The Absolute Orientation is derived from the known location and orientation of the Theodolite or Total Station and the other two orientations, giving a more precise and accurate result than that used in conventional stereo photogrammetry.
The relationship between the camera and the theodolite on which it is mounted can be considered as the difference in location of the front nodal point of the lens and the intersection of the theodolite trunion axis with that of the telescope. The computation of the co-ordinates for the front nodal point of the lens is from the location and orientation of the theodolite together with the three components representing the linear, lateral and offset of the nodal point. Even if the theodolite telescope was replaced by the camera there would still be a linear separation between the 'centre' of the theodolite and the front nodal point of the lens. Such a device was prototyped by Leica Geosystems in 2001 and would have been ideal for Photographic Intersection.
In theory the
orientation of the camera principal ray will be parallel to the axis
of the theodolite telescope. However, this is unlikely to be
the case for the precision required, but correction for misalignment
can easily be accommodated as part of the operating procedure and
computation of the data. For Photographic
There is another source of error that needs to be considered. If the camera is not level then the rotation of the camera will have to be 'corrected' for before any further computation is made. It is probably easier to ensure that the camera is correctly mounted on the theodolite, as is the case with the Wild P30 Photo Theodolite and Wild P32 Camera mounted on a theodolite, than apply a rotation correction to the image. The rotation of the camera can be easily be examined by photographing an horizontal line, such as the horizon of the sea, or by photographing a 'target' point with the point at the centre for one image, extreme left for another and extreme right for a third. The three images can then be complied into a single image, but in three different layers in Photoshop or Paint Shop Pro, and the erase tool used to remove the relevant parts of the layers so that all three position of the point can be seen together. To enable the correct origin and orientation in space of the rays used to compute the intersection it is important that the Nodal Point of the lens is determined as accurately as possible. The front Nodal Point of the lens can be considered as the point at which the rays entering the lens converge and is the ideal point to rotate your lens around when taking images to 'stitch' together to produce panoramic images. There is also a rear nodal point, and in a simple lens the two nodes converge to a single point. If you are interested in Photographic Intersection, or wish to comment on the subject, please contact me on PhotoIntersect@AOL.com.
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Stereo
Photogrammetry benefits from the use of wide angle lenses
to get the best coverage and to ensure that the rays to the discrete
points intersect with the best geometry. For a single stereo
pair with 60% overlap, 40% of each image is not useable. This
can be overcome by using convergent images, but viewing and
extracting data from these becomes troublesome, which leads on to
the idea of photographic intersection.
been
considered as unsuitable for photogrammetric data extraction because
the lens characteristics change with the changes in focal length,
but as this can now be easily accommodated with software such as DxO
and PTLens, zoom lenses are ideal for Photographic Intersection as
the focal length best suited to cover the subject can be selected.
Intersection
it is necessary to orientate the theodolite to a known point.
Preferably orientation
is
established by observing a known point and checked by observing
another known point after taking the images. The target used
at these points consists of two targets separated by the difference
between the theodolite telescope axis and the camera principal ray.
The lower target is bisected with the theodolite crosshairs and a
photograph taken. The upper target is digitised in the image
and its pixel co-ordinates used as the reference for the principal
ray of the camera rather than using the centre of the image.
This gives a point in the image for which the origin and orientation
in space is know. Any other points in the image can now be
digitised and its origin and orientation in space calculated using
the Inner Orientation information and the pixel co-ordinates.
When the same point of detail is digitised in the second image the
two rays, which both now have known origin and orientation can be
used to calculate the 3D co-ordinates for the point of detail by
intersection. If the same point can be digitised in a third or
subsequent image, the calculations will give an indication of the
precision of the 3D co-ordinate value and an improved solution.
The
rapidly increasing concentric circles target was developed by David
Stevens and myself in 1980, when we we working for BKS Surveys, as
the ideal target for close range photogrammetry as it could be
accurately bisected by the theodolite crosshairs, a floating point
or cursor, even if the target was acutely oblique to the observer.
The red and white colours were chosen as they provided excellent
contrast, especially when we used black and white film, and did not
blank out the theodolite crosshairs as black would have done.
If
film, or a digital camera, for which there is no lens information is
being used to record the image, calibration of the lens at a
particular focal length can be determined by photographing a
‘calibration chart’ and calculating the values for correction in the
different parts of the image and these ‘calibration values’ can be
applied to the digitised pixel co-ordinates prior to calculating the
origin and orientation of ray in space.
