Lets Understand the overall working of photogrammetry. Get ideas about photogrammetry branches. Also, check the difference between photogrammetry and remote sensing.

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Working of Photogrammetry
and Remote Sensing

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Introduction
• Photogrammetry, as the name suggests, is a 3-dimensional coordinate
calculating technique. It uses images as the primary medium for
metrology. The unrevealed principle used in photogrammetry is
triangulation or more accurately called Aerial Triangulation. By taking at
least two pictures from the so-called “lines of sight” get developed from
the particular camera. They later become points on the object. To produce
3-dimensional co-ordinate plan of the points of interest, they
mathematically intersect these lines of sight (which are of optical nature).

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• The Prussian architect Albrecht Meydenbauer in 1867 coined the term
photogrammetry.
• He fashioned some of the earliest topographic maps and elevation
drawings. Photogrammetry services in topographic mapping are famous,
but in recent years the technique has widely grown in the several fields.
Various industries like engineering, architecture, underwater, forensic,
geology, medicine, movies, games, and many other areas require precise
3D data.

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Branches of photogrammetry
There are two broad-based categories in photogrammetry:
1. Metric Photogrammetry: It comprises of the specific measurements and
calculations on photographs about the size, shape, and position of
graphics features. For obtaining other data such as relevant locations (co-
ordinate) of areas, features, and volumes, these calculations are
mandatory.
The particular technical camera takes the photographs, and that camera
gets used in engineering fields like surveying.

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2. Interpretive Photogrammetry: This type deals with perception and
identification of the particular features on a photograph such as its size,
shape, shadow, texture, pattern, etc. This type interprets the image to add
importance and intelligence.

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Remote Sensing
• Remote Sensing technology is an essential part of photogrammetry. It also
collects data from images. The term comes from the fact that you don’t need
to physically go to the place from where you want the data.
• So, then what’s the difference between remote sensing and
photogrammetry?
• The difference lies in the final information which we get. The colors specify the
differences. Therefore, land use/land cover is one of the primary outputs of
remote sensing process. The origin of remote sensing was to exploit a vast
number of color zones in satellite images and to create 2D data primarily
for GIS.

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• Nowadays, remote sensing devices are advanced which assist in 2D data
gathering and formulation. Software tools today have a vaster and
comprehensive range of technologies such as image mosaicing, 3D
visualization, RADAR, GIS, as well as the softcopy photogrammetry.
• Concepts to focus:
 Radiometric resolution.
 Spatial resolution.
 Temporal resolution
 Spectral resolution.

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• The radiometric resolution represents the capacity of the sensor to
estimate the brightness of objects or the signal strength (acoustic
reflectance). The sensor which is more sensitive to the reflectance of an
object than its surroundings will be beneficial to sense the smallest object.
• The spatial resolution represents the capacity of a sensor to identify the
smallest dimension of a design on an image. In a sense, the separable
distance between different patterns or things in a photograph which we
can see in meters.

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• The temporal resolution relies on many factors– what time a satellite will
take to return to the approximately same location in space. Or, the area of
the sensor (associated to its ‘footprint’), and whether the sensor can set
can off-bottom. You can more formally call it as the ‘revisit period.’
• The spectral resolution uses the sensitivity of a sensor to give the
response to a particular frequency spectrum (commonly for airborne and
satellite sensors). The frequency scales include visible light, invisible light
and also electromagnetic waves. The different wavelengths reflected from
the objects (as different colors) make it possible to see the features. But,
they must be adequately reflected.

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Basis of Photogrammetry
• The basis of photogrammetry is perspective geometry. At the easiest level, you
can consider a camera lens as a center of the view and light rays piercing
straightly into the image through the center. If the calibration of the camera is
proper, i.e., if we know the focal length, then we can analyze the angles of the
rays from different points. Even if we have 3 points in the scenario and their
3D coordinates, then determining the position of the center gets easy. Further,
the orientation of the photo holds the importance. Alternatively, we can
accurately identify a location using GPS from initial data.
• Knowing the orientation and position of the image enables us to determine
the 3D vector from the image point. The center defines it per light ray.

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• When there are more than two overlapping images, there is an option to
intersect the 3D vectors from every image. Further, we can define the 3D
location of points in the scenario. Our brains perform this calculation of
intersection when watching a 3D movie or stereo pair. Even in computer’s
vision, more than two images are combined to produce better results.
• Hence, this article was all about the working of photogrammetry and
remote sensing.