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.

General case

For most cases it is recommended to acquire the images with a regular grid pattern. 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.
image7.png

The overlap and flight height have to be adapted depending on the terrain.

For forest, dense vegetation areas and flat terrains with agricultural fields it is recommended to increase the overlap to at least 85% frontal overlap and at least 70% side overlap and fly higher so that it is easier to detect similarities between overlapping images. Projects with thermal images require at least 90% frontal and side overlap.

For projects with multiple flights there should be overlap between the different flights and the conditions (sun direction, weather conditions, no new buildings, etc.) should be similar.

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.

image9.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 overlap should be the same as in section General Case.

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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15 comments

  • Mario brustia

    Hi to all,

    in advance,sorry for my poor english.

    My question : how can I modify camera angle (amanualy) in circular fly?

    I've tryed different altitude flight but I'm not able to find any reference to "change camera angle".

    Thank's to all for any suggested idea

    mario

  • Kapil (Pix4D)

    Hi Mario, 

    At the moment there is no option to change the camera angle and the camera angle is set automatically based on the flight height and Point of Interest. The drone translates to always face the Point of Interest (POI) which is the center of the flight. The camera angle is set to focus on the ground. In other words, the camera angle is closer to vertical when the flight altitude is higher. For more information, I would recommend you to go through our support article on  (Android) Circular Mission Settings.

    Regards,

  • Henrik Sivard

    Hi!

    Do you have any recommendation for City roof reconstruction? Not the facades, only the roof.
    Is a mix of two flights(double grid), one with the camera is looking down 0°, and one with the camera is 35º the best choice?
    The roof isn't flat.

    Best Regards
    Henrik

  • Kapil (Pix4D)

    Hi Henrik, 

    You don't need to have two flights with a different camera angle. You can have a double grid mission with the camera angle between 10º and 35º (0° - the camera is looking down).

    Regards,

  • Henrik Sivard

    Thanks for your answer! But in double grid you don't have the option to tilt the camera som "low", only between 45° to 80°?
    And in the Capture app 90° is he camera is looking down??

    Best Regards

  • Kapil (Pix4D)

    Hi Henrik, 

    There is the option to change the angle from 45° to 80° on the Pix4Dcapture app. The zero degree on the app means the camera is pointed in the horizontal direction. So, 80° would refer to be 10° from the vertical (the camera is looking down). So, if you wish to point the camera at 35°, you have to set the camera at 55°(90°-35°).

    Regards,

  • Henrik Sivard

    Thanks!

  • Henrik Sivard

    Hi again Kapil!
    What is the guidelines about height? The roof I'm going to measure isn't so big, so time is no factor. Is 10 meter above better then 20 meter above(higher quality)?

    Best Regards
    Henrik

  • Kapil (Pix4D)

    Hi Henrik, 

    I would still recommend you to fly 20 meters above the roof. If you fly closer to the roof, you might not have proper overlap and might have issue while processing the data. 

    Regards,

  • Henrik Sivard

    Thanks Kapil!

    Best Regards
    Henrik

  • Aaron Meyer

    Hi Pix4D,

    I am flying a large area of water recharge basins which will be at 300 ft (91 m).  The basins have levees in between them with culverts and gated structures.  I plan on flying the entire area with a regular grid pattern with the camera angle set to 0 deg. and then then getting more details of the structures by flying smaller double grids with the camera tilted at 30 deg. in order to get the vertical faces.  I plan on processing all flights together.  Do I need to adjust the flight height of the double grids to fly lower based on the 30 deg camera angle?

    Thanks!

    Aaron

  • Aaron (Pix4D)

    Hi Aaron, 

    Thanks for your question and interest in Pix4D.  Projects that include water add an extra challenge.  You'll want to maintain a minimum of 30% land in each photo so that the software has a better chance to find sufficient keypoints for a successful image calibration.  Flying higher will facilitate this as it will increase the field of view for each image. 

    The distance between the Camera/Drone and the object it is photographing will determine the project Ground Sampling Distance.  When capturing data we suggest that you avoid a doubling (or halving) of the image GSD.  This means avoid halving or doubling your distance from the Levees compared to your aerial grid images of the area. Some variance in distance is workable, but when the GSD between images doubles/halves the keypoint matching starts to reduce which leads to problems with calibration.

    When you capture images from an Oblique angle, the Drone altitude no longer dictates the image content GSD directly, but the linear distance from the camera/drone to the object being photographed. 

    I would also suggest capturing data the "links" any variable flight heights together (akin to Bread Crumbs)  Having "an informed transition" from a Nadir to lower level/altitude Oblique mission will help the software determine how the two datasets relate.  Avoid camera angle changes of 15 degrees or more, take some intermittent images as you adjust the drone altitude and oblique camera angle so as the software is matching up images, it will more easily find how the Nadir and Oblique data match. 

    Regards,

    Aaron.

  • Aaron Meyer

    Thank for the response Aaron (Great name, by the way :)

    Based upon your feedback, what I ended up doing was flying the whole site, which was only a couple acres, in the nadir, at 300 feet (91 meters).  I then calculated that tilting the camera to 30 deg and flying at 260 feet (79 m), should keep the GSD for the middle of the picture about the same and flew a smaller portion of the side, about 1/2 acre.  Unfortunately, I didn't have time to fly a 15 deg tilt (I will try that in a later flight).  I think it still generated pretty good results.  I'm going to try processing the nadir and oblique flight separately and compare the results to see if the difference was worth the additional flight.

    Your software is very intuitive for me and I enjoy using it.

    Cheers!

    Aaron

  • Aaron (Pix4D)

    Hi Aaron, 

    I wanted to try and clarify my comment about avoiding having a camera tilt change of more than 15 deg.  You needn't fly a separate mission at 15 vs 30-degree tilt, but rather capture a string of images that vary by just a few 5-10 degrees, so as to transition the drone from the 300' foot Nadir mission to the lower ~260' oblique mission.  Capture a picture at 285' with a 5-degree tilt, then another at 275' with a 10-degree tilt, then 270' with 15-degrees and 265 at 25-degree tilt and then when you get to 260', then the camera is at 30 degrees without having any large tilt changes between images greater than 15 degrees.

    Please report back how your project came out. 

    Cheers,

    Aaron W.

  • thomas koch

    Like the grid layout.

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