Image acquisition

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The image acquisition plan depends on the:

  • Type of terrain / object to be reconstructed.
  • Ground Sampling Distance (GSD): The GSD required by the project specifications will define the distance (flight height) at which the images have to be taken. For example a GSD of 5 cm means that one pixel in the image represents linearly 5 cm on the ground (5*5 = 25 square centimeters).
  • Overlap: The overlap depends on the type of terrain that is mapped and will determine the rate at which the images have to be taken.

A bad image acquisition plan will lead to inaccurate results or processing failure and will require to acquire images again.

All flight plans described below can be flown automatically with the flight planning app Pix4Dcapture available on Android and iOS.

Building reconstruction

For the reconstruction of a building a circular flight plan is recommended.

  • Fly around the building a first time with a 45° camera angle.
  • Fly a second and third time around the building increasing the flight height and decreasing the camera angle with each round.
  • It is recommended to take one image every 5 to 10 degrees to ensure enough overlap, depending on the size of the object and distance to it. More images should be taken for shorter distances and larger objects.

image4.png

Note: The flight height should not be increased more than twice between all flights, as different heights lead to different GSD.

City reconstruction - visible facades

The 3D reconstruction of urban areas requires a double grid image acquisition plan, so that all the facades of buildings (north, west, south, east) are visible on the images.

The recommended overlap is at least 75% frontal overlap (with respect to the flight direction) and at least 60% side overlap (between flying tracks). The camera should be maintained as much possible at a constant height over the terrain / object to ensure the desired GSD.

For the facades to be visible, the images should be taken with an angle between 10º and 35º (0° - the camera is looking down).
image5.png

 

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3 comments

  • David Bosko

    "Note: The flight height should not be increased more than twice between all flights, as different heights lead to different GSD."

    Does "should not be increased more than twice between all flights" mean that there should not be more than three altitudes at which the images are aquired from?

    Is there any maximum percentage change in GSD that is recommended?

  • Marco (Pix4D)

    Hi David,

    It means that the height of flight should not be too different.
    Usually we recommend that the highest GSD is smaller than twice the lowest GSD.
    However you should consider the the factor of 2 as guideline and not as a hard line.
    That factor 2 is based on intuition. It reflects the scale invariance of the feature detector we use in Pix4Dmapper: the same ATP can be matched even if the scale is different, but of course this scale invariance breaks if the scale is too different.

    Hope this will help.
    Best

  • David Bosko

    Thank you for the clarification.

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