GB1603936A - Method of photogrammetry - Google Patents

Description

(54) A METHOD OF PHOTOGRAMMETRY (71) We, THE VICTORIA UNIVERSITY OF MANCHESTER, an institution constituted according to Royal Charter, of Oxford Road, Manchester, M13 9PL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the follow ing statement:- THIS INVENTION relates to a method of photogrammetry.

Photogrammetry is the science of making geometric measurements of objects by calculatlons or by optical or mechanical analogies based on the measurement of photographs of the objects.

The basis for accepting dimensions in a photograph for measurement of an object is that e projective law which relates each point of a photographed object to a corresponding point of the image in the photograph may be replicated, either mathematically or by optical or mechanical analogy, in the machine in which the photograph is measured. The measurements made in this machine are not necessarily in dimensioned units such as inches or millimetres but may be made by matching elements of the mechanism to the lengths measured on the photograph and then making drawings which become maps or plans of the object photographed. Such machines must be specially designed and constructed for the purpose, and will be referred to as a plotter.

Ibe correspondence between the projective law on which the plotter operates and the projective law which relates parts of the object to corresponding parts of the image in the photographs must be sufficiently close for measurements of the photograph to be used to produce measurements of the object, i.e., the photographs must be of so-called photogrammetric quality. In addition, the photographs used in the plotter must have fiducial marks recorded on them so that the principal point, which is that point on the photograph which lies on the axis of the lens, can be correctly placed with respect to the measuring devices of the plotter.

This method of measurement when compared with direct measurement of the object enables accurate measurements to be made of objects which move or change their shape and which can therefore be measured at some defined instant when the photograph was taken, as well as objects which are too soft or too tenuous for direct measurement, or indeed objects in situations where the introduction of measuring apparatus would cause disturbance to the objects or where the objects themselves might be harmful to personnel or apparatus used to carry out the measurement. In photogrammetry the only disturbance applied to the object is that caused by whatever radiation produces the photograph.

Measuring techniques other than ohoto- grammetry usually call for the establishment of rigid axes of reference connected to the measured object by the measuring apparatus. In photogrammetry these reference axes may be provided by the plotter and by the camera which takes the photographs.

Possible applications of photogrammetry for measurement purposes are manifold.

These include biophysical considerations such as measurements of the human skull, or of the palate orthodontic treatment.

Furthermore, conditions prevailing in such inaccessible areas such as nuclear reactors and underwater structures may be recorded for consideration using a photogrammetric technique.

A major factor preventing the wider use that at present of photogrammetric techniques in such instances as have been mentioned above is the vast cost involved in manufacturing cameras which will take photographs of photogrammetric quality in such conditions and at such scales as would be required. The projective law connecting object and image in cameras of lesser quality which can take photographs in conditions and at scales which would be required tor the application of the photogrammetric method in the instances quoted above, is not replicated with sufficient accuracy in the plotters normally used for measurement.An alternative method for applying photogrammetry in the instances quoted would be to design and construct new plotters using mathematical, optical or mechanical analogies of the projective laws obtaining in the cameras used, but such a course would also entail very great expense.

An object of the present invention is to provide a method of photogrammetry which enables photographs of photogrammetric quality to be produced which can be used in standard photogrammetric plotters, using camera lenses which are of sound quality but which do not satisfy normal photogrammetric demands in having a projective law which can be replicated in the plotters.

According to the present invention there is provided a method of photogrammetry, comprising the steps of initially projecting an image of an object through a lens on to a recording medium such as a photographic film, subsequently re-projecting the recorded image through the same lens or through a lens providing the same optical path to produce a positive image of the said object such as a diapositive, the reprojection being effected through the same optical path as was used for the initial recording but with the direction of light being reversed, fiducial marks being imprinted on the positive image in the reprojection step, and finally applying the positive image to photogrammetric plotting or measuring apparatus thus to measure, or to produce a map or plan of, the said object, the physical dispositions of the lens and the positive image in the re-projection step being selected so as to match the requirements of the said plotting or measuring apparatus in so far as they relate to the positive image so applied.

An example of the execution of the method according to the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic illustration of a twinlens camera designed and constructed to carry out the first stage of the process; Fig. 2 is a similar view of the camera modified to serve as a projector used to carry out the second stage of the process; Fig. 3 is a diagram to illustrate how the Scheimpflugge condition is met in this pro cess; Fig. 4 is a diagram illustrating how a sin gle lens camera may be used in the process; Fig. 5 is a diagram illustrating the re projection step using the camera lens of Fig.

4; and Fig. 6 is a diagram illustrating a single-lens camera attached to a framework on which there is more than one possible position of attachment for the camera.

Referring now to the drawings, for the purpose of this description it will be assumed that the plotter selected for application of the diapositive produced by this method, requires that the distance from the lens to the diapositive should be a distance X.

The negative planes in the camera 10 are denoted by the reference numeral 13. Each negative I3 and its associated lens 12 is established by mounting in a frame 14.

Referring now to Fig. 2, after the photographs of the object P have been taken, the negatives, after processing, are replaced in the frames 14 and, as a simple example, a lamp housing S is attached to the frames, one at a time, so that the image on the negative can be re-projected through the lens 12 on to a photosensitive surface 16 thus to produce a diapositive which can be applied subsequently to a plotter. In re-projection, it is necessary to ensure that the distance of the lens from the surface 16 is exactly the same as that required by the parameters ot the plotter, which distance we have assumed is X. It is necessary also to produce fiducial marks to indicate the positions where lines perpendicular to the surface 16 pass through the centre of the lenses 12.These positions are the principal points, and the marks may be produced by the use of an overlay indicated at 17, preferably attached to the camera body.

By re-projecting the image in the manner just described the optical inaccuracies of the lens used for taking each photograph and the re-projection thereof are obviated by the act of retracing the optical rays through the same lens and thus through the same optical path, this produces in the diapositive as accurate a match ot the original object as is required for photogrammetric purposes.

The diagram of Fig. 3 illustrates how the inclined angle of the negative plane 13 is selected to fulfill the Scheimpflugge condition. In the diagram the perpendicular distance from the plane of optimum focus to the lens in each camera/projector is X.

In the re-projection step it is necessary that the negative should be placed ready for re-projection in exactly the same position with respect to its associated lens as it was before processing when the photographs were taken. For this purpose, each film negative is accurately placed in a mount and the mounts are fitted precisely with reter- ence to the camera whether during photography or during re-projection.

Some parts of the object photographed will not be at the distance X from the lens of the camera. Possibly no part of the object photographed will be at this distance from the camera lens, in which case there will be a loss of definition both in the negative and in the re-projected diapositive. It is necessary, however, that the definition be adequate for the purposes of measurement in the plotter.

Referring now to Figs. 4 and 5, it will be seen that the method may be carried out using a single wide angle lens 18 placed on a line between a pair of parallel plane mirrors 19. Thus, three images 20, 21 and 22 are produced on the negative 13'. Fig. 5 shows how image 22 on the negative, when reprojected vla one of the mirrors ly, can produce a diapositive on surface 16 which is ree from distortion owing to the optical path being retraced through the lens and mirror. By repeating this re-projection with the other outer image 20 a pair of diapositives of photogrammetric quality suitable for use in the plotter can be produced.

Referring to Fig. 6 it will be seen that the method may be carried out using a single camera in a framework F to which it may be attached in two different positions. Pairs of holes, 23 and 24 are provided for this purpose. The same single lens camera is used for photography and re-projection sequentially in both positions in the framework.

It will be appreciated that certain difficulties exist when using photographic techniques for underwater measurements owing to the fact that the refractive index of the lens glass with respect to the water outside is different from that with respect to air. A further complication occurs when external pressure on the lens causes it to distort.

Attempts to isolate the lens from the water make modifications of the plotting apparatus necessary to correct the refraction which occurs when light rays enter or leave the water. By using the method according to the invention where photographs are reprojected, accurate diapositives of photogrammetric quality may be produced. Reprojection need not be carried out at the site of the photography. Provided the water is sampled or the refractive index is measured in some way, re-projection may be carried out through water or some other suitable medium in the laboratory. The chamber in which the reprojection is carried out can be sealed and pressurised to reproduce any pressure distortions of the lens and any other defects arising under the condtions of photography.

WHAT WE CLAIM IS: 1. A method of photogrammetry, comprising the steps of initially projecting an image of an object through a lens on to a recording medium such as a photographic film, subsequently re-projecting the recorded image through the same lens or through a lens providing the same optical path, to produce a positive image of the said object such as a diapositive, the reprojection being effected through the same optical path as was used in the initial recording but with the direction of light being reversed, fiducial marks being imprinted on the positive image in the re-projection step, and finally applying the positive image to photogrammetric plotting or measuring apparatus thus to measure, or to produce a map or plan of, the said object, the physical dispositions of the lens and the positive image in the re-projection step being selected so as to match the requirements of the said plotting or measuring apparatus in so far as they relate to the positive image to be so applied.

2. A method according to claim 1, wherein said initial projection is carried out using a twin-lens camera positioned relative to the object so that the photographic film, after processing and re-projecting produces an image of the said object of sufficiently clear definition to match the requirements of the said plotting or measuring apparatus in so far as they relate to the image of the object to be measured.

3. A method according to claim 1, wherein the initial projection is carried out through a single-lens camera capable of being mounted in a framework in a plurality of positions with respect to the object to be measured.

4. A method according to any one of claims 1 to 3, wherein a camera capable of reprojection is used and wherein, the reprojection step, the distance of the lens from the plane of the positive image to be produced is selected to be in precise accordance with the requirements of the said plotting or measuring apparatus.

5. A method according to claim 2 or claim 3, wherein the plane of the negative is inclined to comply with the Scheimpflugge condition thus to ensure sharp images in a common plane of focus.

6. A method according to claim 1, wherein a camera containing a single lens is used in combination with a pair of mirrors so disposed as to enable a pair of images of the object to be recorded and subsequently re-projected.

7. A method of photogrammetry according to claim 1 substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. the purposes of measurement in the plotter. Referring now to Figs. 4 and 5, it will be seen that the method may be carried out using a single wide angle lens 18 placed on a line between a pair of parallel plane mirrors 19. Thus, three images 20, 21 and 22 are produced on the negative 13'. Fig. 5 shows how image 22 on the negative, when reprojected vla one of the mirrors ly, can produce a diapositive on surface 16 which is ree from distortion owing to the optical path being retraced through the lens and mirror. By repeating this re-projection with the other outer image 20 a pair of diapositives of photogrammetric quality suitable for use in the plotter can be produced. Referring to Fig. 6 it will be seen that the method may be carried out using a single camera in a framework F to which it may be attached in two different positions. Pairs of holes, 23 and 24 are provided for this purpose. The same single lens camera is used for photography and re-projection sequentially in both positions in the framework. It will be appreciated that certain difficulties exist when using photographic techniques for underwater measurements owing to the fact that the refractive index of the lens glass with respect to the water outside is different from that with respect to air. A further complication occurs when external pressure on the lens causes it to distort. Attempts to isolate the lens from the water make modifications of the plotting apparatus necessary to correct the refraction which occurs when light rays enter or leave the water. By using the method according to the invention where photographs are reprojected, accurate diapositives of photogrammetric quality may be produced. Reprojection need not be carried out at the site of the photography. Provided the water is sampled or the refractive index is measured in some way, re-projection may be carried out through water or some other suitable medium in the laboratory. The chamber in which the reprojection is carried out can be sealed and pressurised to reproduce any pressure distortions of the lens and any other defects arising under the condtions of photography. WHAT WE CLAIM IS:

1. A method of photogrammetry, comprising the steps of initially projecting an image of an object through a lens on to a recording medium such as a photographic film, subsequently re-projecting the recorded image through the same lens or through a lens providing the same optical path, to produce a positive image of the said object such as a diapositive, the reprojection being effected through the same optical path as was used in the initial recording but with the direction of light being reversed, fiducial marks being imprinted on the positive image in the re-projection step, and finally applying the positive image to photogrammetric plotting or measuring apparatus thus to measure, or to produce a map or plan of, the said object, the physical dispositions of the lens and the positive image in the re-projection step being selected so as to match the requirements of the said plotting or measuring apparatus in so far as they relate to the positive image to be so applied.

2. A method according to claim 1, wherein said initial projection is carried out using a twin-lens camera positioned relative to the object so that the photographic film, after processing and re-projecting produces an image of the said object of sufficiently clear definition to match the requirements of the said plotting or measuring apparatus in so far as they relate to the image of the object to be measured.

3. A method according to claim 1, wherein the initial projection is carried out through a single-lens camera capable of being mounted in a framework in a plurality of positions with respect to the object to be measured.

4. A method according to any one of claims 1 to 3, wherein a camera capable of reprojection is used and wherein, the reprojection step, the distance of the lens from the plane of the positive image to be produced is selected to be in precise accordance with the requirements of the said plotting or measuring apparatus.

5. A method according to claim 2 or claim 3, wherein the plane of the negative is inclined to comply with the Scheimpflugge condition thus to ensure sharp images in a common plane of focus.

6. A method according to claim 1, wherein a camera containing a single lens is used in combination with a pair of mirrors so disposed as to enable a pair of images of the object to be recorded and subsequently re-projected.

7. A method of photogrammetry according to claim 1 substantially as hereinbefore described.