TECHNICAL FIELD

This invention relates to a photogrammetric apparatus for use in determining three-dimensional measurements from a stereoscopic image which is derived from viewing a stereo-pair of photographs.

Photogrammetry is the science of determining dimensions of objects revealed by photographic images. Relative dimensions of the objects are derived from the photographs and are scaled, usually as a photogrammetric machine function, with reference to actual measurements which are obtained, separately from the photographs, from one or more, points in the fields of the photographs.

BACKGROUND ART Apparatus which currently is being employed for photogrammetric measurements may be regarded as falling in one of two general groups.

The first group includes Mechanical Analogue Machines which are operated by setting a stereo-pair of photographs in their correct relative orientation and by arranging a viewing system such that mechanical "observing rays" are coupled to a drawing mechanism. The second group includes so-called Analytical Machines which measure the co-ordinates of selected corresponding points on each photograph of a stereo-pair and compute from the derived observations the true position of each point. These machines may be categorised as falling within one of two sub-groups, consisting of Simple Systems and Precision Analytical Machines. In the Simple Systems an operator manually positions a reference mark on a selected point on each photograph of a stereo-pair, obtains a read-out of co-ordinates with respect to the photo-centre and computes from this data the actual geographic location of a point. The Precision

Analytical Machines are highly complex machines which incorporate built-in computers and elaborate servo-mechanisms for the purpose of providing an opto-electrical function which is analogous to the Mechanical Analogue Machines.

Of the above mentioned known types of apparatus, the Mechanical Analogue Machines are expensive, are difficult to set-up and require a highly trained operator. Moreover, with the Mechanical Analogue Machines, the focal point is fixed mechanically in the plotter and the machines can only be used with photographs which have been obtained from compatible cameras. Additionally, corrections for problems such as lens distortion in the camera have to be made by use of elaborately constructed optical or mechanical correcting devices. The known

Analytical Machines avoid most of these problems, but the Simple Systems are slow to use and are relatively inaccurate and the Precision Machines are very expensive. The above mentioned types of (prior art) apparatus are suitable for use by mapping organisations that can justify heavy capital expenditure and specialised staff employment. However, there are situations, such as in the mining or forestry industries or in military survey applications, which require relatively inexpensive photogrammetry machines which can be set up quickly and simply by an operator with limited photogrammetric training.

The present invention seeks to provide an apparatus which has an operational flexibility approaching that of the more expensive types of known Analytical Machines, but at a significantly lower cost.

DISCLOSURE OF THE INVENTION

Broadly defined, the present invention provides an apparatus for use in photogrammetry and which comprises a viewing system which incorporates a binocular microscope

having an optical convergence angle α . The apparatus further comprises two carriages located in spaced-apart relationship, one at each side of the viewing system, and means for mounting one photograph of a stereo-pair of photographs to respective ones of the carriages. Drive mechanisms are provided for driving each of the carriages independently in X and Y orthogonal directions, and each carriage is inclined at an angle α/2 with respect to the centre axis of the viewing system, whereby the carriages are separated by an angle which is equal to the convergence angle of the microscope. Optical means are disposed in an image projection path between the viewing system and each of the carriages to permit projection into the viewing system of images from the stereo-pair of photographs when mounted to the carriages, and means are provided for projecting the image of a reference mark into the viewing system in a manner such that the reference mark is perceived on the viewed image of the stereo-pair of photographs. The optical means which permit projection of the photographic image to the viewing system preferably comprise a pair of beam splitting cubes. Also, the reference mark is preferably projected into the viewing system from a single object source, and the means which provide for projection of the single reference mark into the viewing system preferably comprise a pair of beam diverting elements which are aligned with the beam splitting cubes.

Both of the carriages are carried on a supporting structure in a manner such that each carriage may be moved in the Y-direction. Also, each carriage includes a photo-mount portion which is independently moveable in the X-direction. The photo-mount portion of each carriage incorporates the above mentioned means for mounting the respective photographs, and the mounting

means preferably provide for manual rotational (kappa) adjustment of each of the photographs.

Drive for the carriages is provided by motors which operate under the control of a micrprocessor system. The microprocessor system also samples hand controls and function switches and it preferably is incorporated within a housing for the apparatus. The motors, which may comprise stepping or d.c. servo motors, are coupled to re-circulating ball screws which drive the carriages and the photo-mounts, and the carriages and photo-mounts are carried by linear races which are moveable along supporting precision ground shafts.

The invention will be more fully understood from the following description of a preferred embodiment of the photogrammetric apparatus which is illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, Figure 1 shows an overall perspective view of the apparatus,

Figure 2 shows a diagrammatic sectional view of the principal elements in the apparatus, the view being taken in the direction of arrow 'A' as shown in Figure 1, Figure 3 shows a more detailed view of beam splitting cubes and beam diverting elements which are illustrated in Figure 2,

Figure 4 shows an elevation view of one carriage of the apparatus, as seen in the direction of arrow 'B' in Figure 2, and

Figure 5 shows a block diagram representation of a control system for use in conjunction with the illustrated apparatus.

MODES FOR CARRYING OUT THE INVENTION

As illustrated, the apparatus comprises a supporting structure or base 10 on which and in which are mounted the various elements of the apparatus. The elements include left and right-hand carriages 11 and 12, which are intended to carry so-called left and right-hand photographs of a stereo-pair of photographs, and a binocular viewing system 13. The viewing system includes a binocular microscope 14 which has a convergence angle ά, for example equal to 10°, and the carriages 11 and 12 are inclined at an angle α/2 with respect to the centre axis 15 of the viewing system. Thus, the carriages 11 and 12 are separated by an angle which is equal to the convergence angle of the microscope. As best seen from Figure 4, both of the carriages 11 and 12 are carried by way of the linear races (not shown) which travel along precision ground guide rails 16. The carriages 11 and 12 are driven in the Y-direction by a re-circulating ball screw 17 which is itself driven by a stepping motor 18.

A photo mount 19 is supported by each of the carriages 11 and 12 and is moveable in the X-direction. Here again, the photo frame is carried along precision-ground shafts 20 by linear races (not shown) and the photo-mount 19 is driven in the X-direction

(upwardly and downwardly) by a re-circulating ball screw 21 which is powered by a stepping motor 22.

The photo-mount moves in a direction orthogonal to the carriage itself but in a plane parallel to that in which the carriage moves.

A transparent retainer 23 is positioned on each of the photo-mounts 19 and is rotatable with respect to the photo-mount. Thus, the retainer 23 is formed with V-shaped edge portions 24 which are located within grooved guide mounts 25, and the retainer 23 may be removed from the photo-mount 19 by rotating the retainer

through 90°, such that the edge portions 24 move out from engagement with the guide mounts 25. Also, by being rotatably mounted to the photo-mount, the retainers provide for rotational positioning of the photographs with respect to the carriages 11 and 12.

In operation of the apparatus and as best seen from Figure 2, a stereo-pair of photographs 30,31 is mounted to the instrument, one photograph of the pair being mounted to the left-hand carriage 11 and the other to the right-hand carriage 12. The photographs would normally be diapositive prints, and they are mounted so that they are disposed in approximately confronting relationship but separated by the included angle α. Light from two light sources 26 and 27 is reflected by means of plane mirrors 28 and 29 to illuminate the surface area of the respective photographs 30 and 31, and the light passes into beam splitting cubes 32 and 33 which function to reflect the light up into the binocular viewing system 13, 14. Each of the beam splitting cubes 32 and 33 effectively comprise two prisms with a semi-silvered mirror 34 constituting the interface of the prisms.

When the photographs 30 and 31 are correctly positioned relative to one another, a three-dimensional image will be seen to exist by a person who views the image through the viewing system 13, 14.

A reference mark is superimposed on the perceived three-dimensional image by way of a secondary optical system which is located below the beam splitting cubes 32 and 33 and which is shown in greatest detail in Figure 3 of the drawings.

The secondary optical system comprises a light source 35 which is employed to illuminate a reference mark 36 on a glass plate 37, the light being transferred to the reference mark by way of an optical fibre system

38. Images of the reference mark are transmitted through

beam diverter elements 39 and 40 and into the underside of the beam splitting cubes 32 and 33 where each image iε combined with one of the images from the photographs 30 and 31. In use of the apparatus, the right and left-hand photographs of the stereo-pair are mounted to the respective carriages as above described, and the photographs are first orientated in the required manner by a manual positioning procedure. Thereafter, a more precise adjustment is affected by employing computer aided techniques that are similar to those which are employed in the context of Analytical Photogrammetric Machines.

The preliminary orientation is effected by manually setting the approximate orientation of the two photographs with respect to each other by rotating the transport retainers 23. This provides an approximate Kappa adjustment and corresponds to the concept of "base lining" a stereo-pair of photographs before viewing them with a stereoscope.

Interior orientation involves measuring reference . points on the photographs which have a known relationship to the camera geometry. Co-ordinates of these points (know as fiducial marks) are stored in a file in a host computer which communicates with the apparatus as above described. The interior orientation programme is used to drive the right and left hand carriages near to each of the fiducial marks in turn, and waits for an operator to position the (floating) reference mark exactly on the fiducial mark and record the position. Although not shown in the drawings, control elements and switches which are mounted to the apparatus are employed for this operation.

At the end of the observing procedure, the relationships between the stereo camera axis systems and the apparatus measurement systems are computed, stored in

a file in a host computer, and sent to a micro-computer in the apparatus. The apparatus can then record information in terms of the axis systems of the stereo-camera. Absolute orientation involves measuring sufficient reference points to determine the relative orientation in space of the stereo-pair of photographs at the time of exposure, and if control information is available, the absolute position in space of each photograph at time of exposure. Details of control data (the X, Y and Z co-ordinates) are stored in a file in the host computer, and the absolute orientation computer programme drives the carriages to a preselected series of points and waits for the operator to position the floating mark and record the point. If the selected point is also a control point its name is also recorded. At the end of this observing procedure, the orientations of the two photographs and their positions in space are calculated. If sufficient control points have been recorded, these results will be based on the control co-ordinate system. Otherwise, they will be on a local system which is frequently referred to as "model" co-ordinates.

Transformation parameters to convert from true or model co-ordinates to the camera co-ordinate system are then calculated, stored on file on the host computer and sent to the micro-computer in the apparatus. The apparatus can then function in terms of an X Y Z co-ordinate system, with one set of controls directing motion in the X-Y direction and the other set of control directing motion in the Z direction.

A system which is suitable for use in controlling the previously described apparatus is illustrated in Figure 5 of the drawings.