basic concepts of photogrammetry

NAMITHA M. R.ID. No: 2015664502M. Tech, LWMETNAU

BASIC PRINCIPLES OF

PHOTOGRAMMETRY

Science and technology of obtaining spatial measurements and other

geometrically reliable derived products from photographs

Photogrammetry

Analysis procedures ranges form obtaining approximate distances, areas and elevations using hardcopy photographic products, unsophisticated equipment and simple geometric concepts to generating precise Digital Elevation Models (DEMs), orthophotos, thematic GIS data and other derived products through the use of digital raster images and relatively sophisticated analytical techniques

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Photogrammetric operation involving the use of digital raster photogrammetric image data – Digital / Softcopy photogrammetry

Most common use of photogrammetry- To produce hardcopy topographic maps

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Determining the scale of a vertical photograph and estimating the horizontal ground distances

Using area measurements made on a vertical photograph to determine the equivalent areas in a ground coordinate system

Quantifying the effect of relief displacement on vertical areal photographs

Photogrammetric activities

Determining object heights from relief displacement measurements

Determining object heights and terrain elevations by measuring image parallax

Using ground control points Producing maps and orthophotos Preparing a flight plan to acquire aerial

photography

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Geometric types of aerial photographs :-

i. Vertical photographs:

• Made with the camera axis directed as vertically as possible

• Unavoidable tilts cause slight (1 to 3°) unintentional inclination of the camera optical axis- Tilted photographs

Basic Geometric Characteristics Of Aerial Photographs

ii. Oblique photographs:

• Aerial photographs taken with intentional inclination of the camera axis

• High oblique photographs include an image of the horizon and low oblique photographs do not.

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Taking vertical aerial photographs :-

• Mostly taken with flight cameras along the flight lines or flight strips

• Nadir line- Line traced on the ground directly beneath the aircraft during acquisition of photography

• Successive photographs are generally taken with some degrees of end lap

• An end lap of 50% is essential for total stereoscopic coverage of a project area

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• Stereoscopic coverage consists of adjacent pairs of overlapping vertical photographs called stereo pairs

• Stereo pairs provides 2 different perspectives of the ground area in their region of end lap

• When stereo pairs are viewed through a stereoscope, the result is the perception of a 3D stereo model

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• Intervalometer: Device that automatically trips the camera shutter at desired time intervals.

• Stereoscopic overlap area: Area included in the overlap of successive photographs

• Air base: Ground distance between the photo centers at the times of exposure

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• The ratio between the airbase and the flying height above ground determines the vertical exaggeration perceived by photo interpreters

• Larger the base-height ratio, greater the vertical exaggeration

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• Adjacent strips are provided with a side lap of 30%

• Multiple strips comprise what is called a block of photographs

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Geometric elements of a vertical photograph:-

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• Light rays from terrain object Camera lens exposure station(L) Plane of the film negative

• The negative is located behind the lens at a distance equal to the lens focal length, f

• Paper print positive image positions can be depicted diagrammatically in front of the lens in a plane located at a distance, f

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• The x axis coincides with the line of flight and the y axis is located 90° counter clockwise from the positive x axis

• Photo coordinate point o is assumed to coincide with the principal point

• Principal point - Intersection of lens optical axis and the film plane

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• Ground principal point, (O)– Point where the prolongation of the optical axis of the camera intersects the terrain

• Images for terrain points A, B, C, D and E appear geometrically reversed on the negative at a’, b’, c’, d’ and e’ and in proper geometric relationship on the positive at a, b, c, d and e

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Photo coordinate measurement:-

• Engineer’s scale or metric scale

• Coordinate digitizer

• Comparator – Mono comparator or Stereo comparator

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The relationship between the photograph’s row and column coordinate system and the camera’s fiducial axis coordinate system is determined by a mathematical coordinate transformation between the 2 systems

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An expression that states that one unit of distance on the photograph represents a specific number of units of actual ground distance

Expressed as unit equivalents, representative fractions or ratios

Photographic Scale

Larger scale shows ground features at a larger, more detailed size (eg: 1:10000)

Smaller scale shows ground features at a smaller, less detailed size (eg: 1:50,000)

Same objects are smaller on a smaller scale photographs than on a larger scale photographs

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Scale S is computed as the ratio of photo distance d to the ground distance D,

Photo Scale, S = = For a vertical photograph taken over a flat terrain,

scale is a function of focal length f of the camera and the flying height H’ above the ground,

Scale = =

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Average scale computed for the entire photograph,

Savg =

where, havg Average elevation of the terrain

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All points on a map are depicted in their true relative horizontal positions, but points on photo taken over varying terrain are displaced from their true map positions

Result of photo scale variation- Geometric distortion Map is a scaled Orographic projection of the ground,

whereas a vertical photograph yields a Perspective projection

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On a map Top view of the object are seen in their true relative horizontal positions

On a photo Areas of the terrain at the higher elevations appear larger than corresponding areas lying at lower elevations. top of the objects are always displaced from their bases

This distortion is called Relief displacement

Relief Displacement

Causes any object standing above the terrain to lean away from the principal point of a photograph radially

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Relief displacement,

d =

where, r radial distance on the photograph from the principal point to the displaced image point

h height above datum of the object point

H flying height above the same datum chosen to reference h

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Parallax – Apparent change in relative positions of stationary objects caused by a change in viewing positions

This phenomenon is observable when one looks at objects through the side window of a moving vehicle

Image Parallax

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