/1 PANORAMIC PHOTOGRAPHY AS A SURVEY TOOL

Author: Hugh Anderson 1st August 1978

Abstract

The paper describes the development of an optical device for use in detail

surveying, which enables panoramic photographs to be taken using conventional

cameras and lens systems, and suggests how such a device can be used as a tool

in some survey disciplines.

Introduction

Large scale detail surveys require the fixing of points to which subsequent

measurements can be tied to in order to enable the detail to be plotted. These

points are usually fixed to a control network by one of two methods; line and

offset, or radials. Both methods require the drawing of a field diagram to show

the relationship of the points, and to which point is referred. The latter is the

more commonly used method today, and involves angular measurements at the

Control Stations and distance measurements to the points. The production of a

diagram, or diagrams, for points fixed from the Control Stations is often a time

consuming operation as it is necessary for the diagrams to be clear. Rough

sketches often prove more confusing at the plotting stage than helpful. In

surveying building interiors, a diagram is usually required for each room

resulting in many more diagrams for a given area than when working in the

open, which consumes more time.

If diagrams could be produced photographically, considerable savings in time

would result, reducing the cost of surveys, especially building interior surveys.

The problem was to record the relevant points of detail without extraneous

coverage. This could be done by setting up a camera and photographing a full

360° by successive exposures at different pointings. For example: a 35mm

camera fitted with a 35mm lens would require 6 exposures 60° apart

/2 horizontally for each Control Station. A 28mm lens would reduce the number of

exposures required to 5 spaced at 72° intervals, but more extraneous subject

matter would be included (sky and foreground). This method would require

considerable care to ensure each exposure overlapped sufficiently to achieve full

coverage, and it is probable an experienced Surveyor would be able to draw a

diagram as quickly. Saving in time would thus be small, if any, whilst increasing

the cost of equipment and materials required. The number of exposures should

therefore be reduced to one, or two at the most, to make the saving in time

effective. To achieve this I considered the use of a fish eye lens which produces a

circular image covering a field of view of 180°. If such a lens is fitted to a camera

placed with the film plane horizontal, the resulting image would have the relevant

detail round the periphery. The disadvantages are that the relevant information

is restricted to a narrow band at the edge of the circular format, and that only

detail above an horizontal plane through the camera will be recorded. The

remainder of the photograph will be sky with the sun possibly included. The use

of a fish eye lens has however two considerable advantages; an horizon of 360° is

recorded in a single exposure, and the angles (directions) from the centre of the

photograph to points of detail in the image are preserved as the angles

(directions) from the camera station to the points of detail on the ground. The

preservation of angles is important, because not only can time be saved with

respect to the production of diagrams, but if the necessity to observe angles to

detail points were eliminated a further considerable saving in the Surveyors time

would result. Two methods for plotting the detail points fixed radialy are in

common use; computing co-ordinates from the observed angles and distance and

plotting these co-ordinates, and plotting the radials directly using a protractor

and scale. Considering the latter, a photograph with preserved angular

relationships can replace the protractor eliminating the need to record observed

angles and plot them later, thus eliminating two sources of error. The problem of

producing a photographic diagram now crystallised into producing an optical

device that would enable a camera to record an horizon encompassing a full

360°, include detail above and below the horizon and still preserve the angular

relationships to the detail without including large areas of extraneous subject

matter.

/3 The Panoramic Optic

The requirements listed at the end of the last paragraph can be achieved by

placing the panoramic optic in front of a conventional camera lens system. The

optic is essentially a section through a prism, rotated through 360°. Figure 1

shows a section through the panoramic optic, the axis A A’ being the axis of

rotation. Figure 2 shows a section through an optic mounted on a 35mm reflex

camera with the limiting rays above and below the horizon (acceptance angles).

This arrangement produces a specialised image. Figure 4 illustrates the image

produced when the detail shown by the plan in figure 3 is photographed from the

point marked by a cross. The circle in figure 4 represents the sensible horizon of

the camera/optic combination and will not appear in a photograph. The

specialised image in figure 4 s equivalent to a photograph made using a 35mm

camera with a 35mm lens system and the angle oc = 38°.

An inverted conical mirror occupying the same space as the totally reflecting

surface would produce the same specialised image, but presents mounting

problems as any supports would appear in the image. This could be overcome by

supporting the mirror with a glass cylinder of suitable optical quality.

The acceptance angles of the optic are dependent on three factors; the film

format, focal length of the lens system, and the angle oc in figure 1. The upper

angle is dependent on all three factors, whilst the focal length has little influence

on the lower angle. The acceptance angles can be altered by curving one, two, or

all three of the surfaces of rotation as shown in figure 5. Curving one or more of

these surfaces uses the refractive properties of the material used to construct the

optic and could introduce colour aberrations unless care is taken in the design.

With reference to figure 1, the refraction of the rays by the two surfaces through

which the light passes will be the same if angles oc and B are made equal.

With these angles equal the image produced by the optic will be the same as that

produced by an inverted conical mirror with the apex angle of the cone

= 180° - 2 oc.

/4 Application

As mentioned in the Introduction, the development of the panoramic optic was

for use in detail surveying where the preservation of the angles is of great

advantage. This angular preservation can be utilised in other survey fields. The

following examples illustrate three more possible uses for the optic. Other uses

and development potential exist.

STATION REFERENCE DIAGRAMS: In the establishing of control (e.g. primary

triangulation beacons) it is often necessary to produce panoramic diagrams from

the station to enable subsequent users to identify the locations of other stations

visible from the established stations. The same disadvantages to diagram

drawing apply as in detail surveying; it is time consuming, and the resulting

diagrams are often inaccurate or confusing. Two forms of photographic diagrams

can be produced by employing the panoramic optic. The first, using one optic

mounted in front of the camera lens as in figure 2 to produce the specialised

image as in figure 4. However, it is possible to produce a coherent panoramic

photograph covering an horizon of 360°. Figure 6 shows a camera using two

optics to produce the coherent photograph with a single exposure, and without

the use of either a moving lens system or moving film, during the period of

exposure. The acceptance angles can be selected to exclude large areas of sky

and foreground. Lenses of different focal lengths can be used to produce

different acceptance angles for any single or pair of optics. The strip image

(figure 7) produced by such a panoramic camera can also be produced by making

the specialised image (figure 4) with the combination as in figure 2, which is then

printed using a conventional enlarger, but with an optic mounted in front of the

lens focused onto a cylindrical piece of printing paper whose axis of rotation is

coincident with the axes of the lens system and optic. This system could also be

used to record and project coherent panoramic images with slide or cine-film.

The images being recorded as the specialised images, then projected through an

optic onto a cylindrical screen, the axes again being coincident. The advantage of

photographs over diagrams is that all visible objects from the point are recorded,

and changes to the landscape are more readily discernable.

/5 HYDROGRAPHIC SURVEY FIXES: Commonly used procedure to obtain a fix for

a boat carrying out inshore hydrographic work is to use sextants to record the

boats position. This is done by simultaneously reading two horizontal sextant

angles to shore based markers of known position. The method requires two

sextant observers, and errors can result from a number of sources;

misidentification of one or more shore beacons, readings not exactly

simultaneous, errors in observed angle recording, poor bisection of targets,

sextant observations not taken at the same point, etc. I propose that all the

mentioned error sources would be eliminated if the sextant observations were

replaced by a single photograph for each fix made with a gimbal mounted camera

fitted with an optic, their axes being vertical. To plot the fixes, the specialised

images are projected vertically onto an horizontal surface (e.g. with a multiplex

projector). The projected image can then be used in the same manner as station

pointers. This does not allow for the observations to be computed. This however,

can be done by using an electronic angle measuring device (as with coded

elements) [1], or a protractor, built into the horizontal surface. The cost of an

exposure per fix may initially appear expensive, but 15m of 8mm film can

accommodate 4,000 exposures.

TRIANGULATION: The property of angle preservation with the optic allows for

graphic triangulation using the specialised images formed. Mathematical

triangulation is also possible if electronic angle measurements, or protractor

angle measurements, are made from the images. The ease and precision of

theodolite use on land however makes this application a non-starter in such an

environment, but in unstable environments, such as on water, photographic

angle measurement may well prove invaluable. There is another environment

where mapping is still in its infancy, and photographic angle measurement may

open up new survey methods to replace the cumbersome grid system now used.

Underwater. Photographs taken of the sea bed and wrecks published in

magazines such as National Geographic clearly illustrate the potential of

photography as a mapping tool in this environment. I feel there is scope for

photogrammetric techniques, especially with stereometric cameras, underwater.

Photographic triangulation of pre-marks and or detail would considerably reduce

the number of taped measurements required whilst greatly improving accuracy.

/6 A Nikonos fitted with a panoramic optic would be the diving surveyors equivalent

of the land surveyors theodolite.

Conclusion

Survey techniques develop and evolve with the instrument changes brought

about by technology, and many techniques fade in popularity only to reassert

themselves later. For example, three hair theodolite tacheometry was largely

replaced by self reducing tacheometers, which in turn, have been effected by

advances in E.D.M. and Total Station instruments. However, with the

introduction of programmable pocket calculators, three hair tacheometry is once

more an economic and rapid means of fixing detail and spot heights.

In attempting to speed up detail surveying by finding a means to produce rapid

reliable diagrams, new survey techniques have been made possible using the

panoramic optic devised. The optic is patented but research and development is

needed.

References

[1] Gorham, B. J., 1976. Electronic Angle Measurement Using Coded Elements

With Special Reference To The Zeiss Reg Elta 14. Survey Review, XXIII(180):

271-279.

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/11 Related material produced 1st March 1979

/12 Related material produced 10th April 1979