Digital Photogrammetric Image, Digital Image, Characteristics of Digital Images (Especially for GATE-Geospatial 2022)

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Nowadays, nearly all photogrammetric images have a digital format, since digital photogrammetry has achieved great popularity these days. Digital images are such either naturally (if they have been captured by a digital sensor) or because they have been transformed into a digital format by using a photogrammetric scanner.

Photogrammetric Scanner

Photogrammetric scanner

Digital Image

  • A digital image is a bi-dimensional matrix where each minimum unit of information is a pixel with column and row coordinates (i, j) . Each pixel has a value known as Digital Number (DN) , which is represented by a grey level in a screen.
  • This digital image composition corresponds to an image with only one band; in other words, it is composed of just one matrix, and it would be displayed on greyscale. Nevertheless, colour images (RGB) , as shown in the figure, are composed of three matrixes: the first one for the colour red, the second one for green, and the third one for blue.
Digital Image

Digital Image

RGB Digital Image

RGB digital image

Characteristics of Digital Images

Resolution

  • The quality of a digital image depends on the pixel՚s size, which is the resolution. If that size is too large, the image loses information, but, on the contrary, if the size is too small, the image would have high quality, with the drawback of, in addition, needing a lot of storage space.
  • The resolution is expressed in pixels per inch (PPI) or dots per inch (dpi) . In the case of aerial images, the resolution is called spatial, and it has a direct correspondence with the size of the pixel in the ground (GSD = Ground Sample Distance) . Below figure shows graphically what the term GSD means.
The Resolution is Expressed in Pixels Per Inch

Dimension

The dimension of the image indicates the width and height of the image. It is usually expressed in centimeters, inches, or pixels.

Colour Depth

  • The colour depth corresponds to the number of bits used to describe the colour of each pixel. As shown in below figure, the greater the depth, the more colors that would appear in the image.
  • The relation between the number of colors and the depth is expressed in the following table.
Colour Depth
Table Supporting: Characteristics_of_Digital_Images
DepthColours (2nºb)
1 bit2
4 bit16
8 bit256
16 bit65,536
32 bit4, 294,967, 296
Depth of colour

File Size

  • The size of the file is the amount of memory needed to store the image՚s information. The number of bits needed to store it depends on the resolution of the length, width, and depth of colour.
  • Size of an image (bits) =
    • R = Resolution (PPI)
    • L and W = length and width (inch)
    • D = Depth of colour
  • In addition, the size of the image will be tripled if it is stored in colour. One of the drawbacks of digital images is related to the necessity of having a large storage volume, given the amount of available information. It should be noted that a photogrammetric project without high dimensions is composed of numerous images.

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