Principle of Aerial Photography

And Types of Photos

By T.SrinivasaRao

Contents

• Introduction -> How it works?(planning, common terms and

formation of photo)

• Types of aerial photographs

• Aerial photo vs Map - 1 point for now

• orthogonal vs perspective projection

• Principle of vertical photograph (dissection of taking aerial

photograph)

• Scale

• Stereoscopy

• Map vs Stereoscopy

• Map vs Mosaic

Introduction

• Aerial photography has been defined as the science of taking photograph from a point in the air for the purpose of making some type of study on earth surface.

• The net result of any photographic mission is photographic negative.

• photographic negatives are the result of favorable and unfavorable factors acting simultaneously.

• A good aerial photgraph must have a certain standard of geometrical accuracy.

• Aerial photography and its planning generally includes selection of types of aeroplane and camera, trend of run, film and filter combination, which is of great importance in aerial photo interpretation.

• Most of the conventional aerial photography is done at 1:30000 to 1:60000 scale on a conventional black and white panchromatic film.

• The scale of photography further depends on objectives of the work, which governs the speed of the aircraft and its flying height, also depends on the type of camera lens being used.

• optimum scale for geological photo interpretation are 1:25000 to 1:40000

• For regional geological surveys a small scale photograph of 1:60000 or even smaller is suitable.

• For extensive ground coverage satellite imagery are most useful.

• In aerial photography for more specific and detailed information such as geotechnical studies, mineral exploration, ground water surveys, land-use planning, and town planning, large scale photographs on photographs of scale 1:15000 to 1:10000 are most suitable.

• In such type aerial photographs, low speed aeroplanes (200 to 350kmph) are used and flying height is normally not more than 8000 to 10000m.

• low flying speed and heights are one of the most important aspects in obtaining sufficiently large scale photographs with sharp images of objects.

• There should be complete stereographic coverage of terrain, i.e., all features of the terrain should occur on atleast 2 photographs for stereoscopic viewing.

• Photographs are taken in runs in the direction of flight in such a way there will be minimum 60% overlap between adjacent photos and 30% sidelap between adjacent runs.

• the quality of photographs depends on– flight and weather conditions– camera lens– film and filters– developing and printing process

• as far as possible photograph should be vertical and should be free from elements of tilt and tip. Also free from defects like drift and crab.

• Drift -> 2 Successive flight lines should be parallel in

ideal case.

• At high altitudes because of strong wind currents called

sidewinds influence the aeroplane in maintaining

predetermined direction and straightness of run.

• This deviation from the original intended flight path is

called as drift.

• uncorrected drift will result in subsequent photographs

covering more area in the prevailing wind direction.

• During sidewinds if pilot tries to maintain original path, he has to turn the nose of the aeroplane slightly against the wind.

• this makes the aeroplane to rotate on its vertical axis.• in this case, original path is maintained but the area

covered by photograph is much different than that planned in the original.

• the aerial photograph is rotated in the direction opposite to wind direction here.

• This defect is called as crab.• The above 2 defects causes reduction in stereoscopic

coverage of the terrain.

Types of Aerial Photographs

• Extreme growth in the technology and tecniques used for acquiring aerial photographs.

• Aerial photographs are classified according to attitude of camera axis, lens system, types of cameras, types of films and filters or some special equipment employed in the camera or techniques to record some special effect on the film.

• Classification of techniques is done here according to 3 main criteria which are discussed in the next few slides.

Types of Aerial Photographs

• Based on orientation of camera axis– Vertical photographs– Low oblique photographs– High oblique photographs

• Based on lens system– Single lens photography– Three lens photography(trimetrogon photography)– Four lens photography– Nine lens photography– Continuos strip photography

Types of Aerial Photographs

• According to special properties of films, filters or photographic equipment– Black and white photography (Panachromatic

photography)– Infra red photography– Color photography– Color infra-red photography– Thermal infra-red photography– Radar imagery– Spectrazonal photography

Based on oientation of camera axis

• aerial photographs are classified according to the orientation of camera axis at the time of photography.

• Vertical photographs are the ones which are taken with the axis of camera being vertical at the time of exposure.

• It is virtually impossible to take true vertical photographs.

• Deviation of optical axis from the vertical, if it is in between 1 to 2 degree, the photographs are called tilted photographs.

Based on oientation of camera axis

• Oblique photograph is taken with camera intentionally

tilted from the vertical.

• If horizon is captured in the photograph it is called high

oblique photograph.

• If horizon is not captured in the photograph it is called

low oblique photograph.

According to Lens System

• Single lens system is most commonly used lens system in

most of the aerial photo interpretation work.

• 2, 3 and multi lens became almost obsolete.

• But some of the multi lens techniques are still being used in

war reconnaissance and in photography researches.

• Trimetrogen system of photography is developed to obtain as

much as coverage for the preperation of maps during the

worldwar - II.

– The system got the name as it uses 3 cameras were

assembled with trimetrogen lens

– In this system 2 of the 3 cameras are obliquely mounted at 60 degree to vertical.

– This will cover photograph upto horizon and some area in the middle also by the camera which is mounted vertically.

– all cameras are exposed simultaneously.– Resulting photographs will cover area from horizon to

horizon in the direction perpendicular to flight line.– covers more area but scale becomes smaller towards

horizon– the vertical photgraph has 60% while there is 10 to 30

percent sidelap between vertical and oblique photographs.

• multi lens system consists of 2 to 9 lens in the camera, and all aquire images simultaneously.

• some times different filters are placed in cameras depending on requirement.

• in the past continuos strip photographs are also attempted– negative is made to pass through a narrow slot in the focal

plane of camera.

Aerial photo vs Map

• Aerial Photo:– Central projection– Non-Uniform scale– actual features

• Maps– orthogonal projection– uniform scale– symbols– imaginary objects presence

orthogonal vs perspective projection

Vertical photograph

vertical photograph - characteristics

• Tilt < 3 degree

• Scale is approximately constant throughout the photo

• Most common format available is 9x9 square inch

• within limitations a vertical photograph can be used to

substitute a map.

• p = i = n for a vertical photograph

( p - principle point, i - isocenter, n - nadir point)

Vertical Aerial Photograph

Tilted photograph

• C - Perspective center

• c - focal length

• principal point - PP

• camera axis - c-PP

• N - Nadir point, is the intersection of the vertical and perspective

center in the photograph

• tilt angle, t - angle between camera and vertical axis

• swing angle, s - angle measured at the pricipal point from the +ve y-

axis counter-clockwise to N

• azimut α is the angle at the ground nadir N measured from the +Y-axis

in the ground system counterclockwise to the intersection O of the

camera axis with the ground surface. It is the azimut of the trace of the

principal plane in the XY -plane of the ground system.

• isocenter I is the intersection of the bisector of angle t with the

photograph. It is on the principal line.

Scale

More discussion on Scale

• Refer your class notes.