Photogrammetry 1. Introduction

Photogrammetry is the art science and technology of taking measurements off photographs. The techniques are based on the geometry of perspective scenes and on the principles of stereovision, and actually pre-date the invention of photography. There are two kinds of photographs used in photogrammetry, aerial and terrestrial. Aerial photographs are usually acquired from aircraft but can also come from satellites, hot air balloons or even kites. Terrestrial photographs come from cameras based on the ground, and generally are used in different applications from aerial. There are two main data extraction methods used for analysing these photographs:

a. Quantitative: that is size, length, shape, height, area, etc.

b. Qualitative: geology, vegetation, drainage, land use, etc.

This chapter is primarily concerned with the quantitative evaluation of survey photographs.

2. The Aerial Survey Camera

Photographs taken with a survey camera differ to those taken with amateur (or 'non-metric') cameras in that they have a rigid known geometry that allows measurements of a predictable accuracy to be made.

The rays of light pass through one central point in the lens of a survey camera, and the distance between this point and the film is calibrated to decimals of a millimetre. This distance is known as the focal length. The film plane is as flat as possible, often using flattening devices to achieve this. The camera body also carries reference marks known as fiducial marks that define a coordinate mesurement axis and allow film stretch to be determined. The camera is also very large, the size of the negatives is 23cm by 23cm, which is somewhat bigger than the average snapshot. The cameras are constructed in this manner so that measurements of a high accuracy and precision can be obtained.

3. The Geometry of the Aerial Photograph

The average scale of the aerial photograph can be computed either by taking the ratio of the flying height above the ground and the focal length (f/H), or by taking the ratio of a known distance in the photograph and the distance on the ground.

As the flying height above the ground is not usually known accurately, the second method is employed where a more reliable scale is needed. Neither of the two scale calculation methods give an accurate scale as there are two distortions that affect measurements made on a single aerial photograph.

a. Height distortion:

Because a lens and a photograph give a perspective or central projection, objects that are above or below the plane will be shifted by an amount approximately

Dr =

b. Tilt distortion

Although a lot of care is taken in the flying of aerial photography the photographs are rarely taken exactly horizontal. The tilts that occur in the aircraft, although kept to a minimum by the levelling of the camera system, do affect the position of objects on the photograph. This is most apparent with panoramic photographs that show parallel roads converging to a point on the horizon, this affect still appears in photographs that have small tilts but occur to a much smaller amount. The correction is approximately

Dr =

However as the tilts are generally unknown this correction is difficult to apply.

These distortions have been mentioned to indicate that an aerial photograph is rarely true to scale so cannot often be used as one would use a map. There are many situations however, especially in forestry or geography, where these distortions are minimal when compared to the accuracy of the data required, and can sometimes be ignored.

There are other distortions such as film shrinkage, earth curvature, refraction effects and so on but these are only significant when dealing with precise photogrammetry.

4. Geometry of a Stereo Pair of Photographs

Aerial photographs are normally flown in runs with a pre-determined amount of forward and side overlap.

This forward overlap, usually 60%, enables the aerial photographs to be viewed in 'stereo', or three dimensionally. The accurate geometry of the photography also enables three dimensional coordinate data to be extracted; the same height distortions that affect the single photograph can now be used to determine the height of the object in a stereo pair.

The detailed geometry of a pair of photographs will not be described here, however there is an approximatel method of extracting heights off a stereo pair which is of use.

5. Parallax Bar Heighting

The difference in position of an object with height displacement on two photographs is known as parallax. If this parallax can be measured the height of the object can be determined if the scale of the photograph is accurately known, the air base is horizontal and the tilts are minimal. The derivation of this formula is shown in most photogrammetric textbooks.

z = H where b' is the photo base, p' is the difference in parallax

An instrument known as the parallax bar is used to measure the parallax - p'.

6. Map Compilation

Topographic mapping of scales of 1:5000 and smaller is these days performed almost entirely using aerial photogrammetry. Overlapping photographs are acquired of the area of interest (this is done on a routine statewide basis by the Department of Property and Services), and certain points visible in the pair of photographs are coordinated to act ac scale and tilt c These points are used in the plotting instrument to scale and orient the photographs, to enable the operator to trace the necessary detail from the 'stereo-model'. A good machine operator is capable of providing line work of cultural features like roads, buildings and so on, as well as contours showing height. The production of contours relys on the operator's ability to perceive depth in the pair of photographs (it all happens in the operator's head). Each major theme, or printing ink colour, is usually plotted either on separate sheets or in differing colours to enable similar features to be distinguished. Standard series maps used certain ink colours to depict certain types of features, like blue for water and green for vegetation. Maps also show the standard plan features like north orientation and a scale, but also show coordinate reference systems like AMG or latitude and longitude.

7. Rectification

It is possible to optically remove tilt distortion from photographs of flat terrain by reprojecting the negative onto a tilted base board. This process is known as rectification (making right). A rectified photograph is true to scale only where the terrain is flat and even. These photographs can be used for planning works or projects, and can be annotated similarly to maps with borders and legends to become Photomaps.

8. Orthophotographs

Recent developments in photogrammetry have seen the development of instruments that can eliminate height displacement as well as tilt distortion from a photograph. The process is involved and consists of rectifying small strips of the photograph at a time and reconstituting this into a new photograph. The resultant image is true to scale, and is often annotated with contours, coordinate grids an a legend, and published as a standard series map.

9. Current Developments in Photogrammetry

The last decade or so has seen many advances made in the technologies associated with photogrammetry, and the microchip has had a major impact. It is now possible to achieve similar precision and accuracy of measurement using conventiomnal cameras and computer compensation for lens and film effects as was previously possible with special survey cameras and restitution equipment. This has enabled the application of photogrammetry in biometrics, architecture, geology, archaeology, mechanical and civil engineering, nuclear physics, zoology and other scientific disciplines where remote measurement is an advantage. Photogrammetry is now seen as another measurement tool available to the measurement scientist not just a way of making maps.

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