Step 1. Before Starting a Project > 1. Designing the Image Acquisition Plan > a. Selecting the Image Acquisition Plan Type

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Pix4Dmapper is an image processing software that is based on automatically finding thousands of common points between images. Each characteristic point found in an image is called a keypoint. When 2 keypoints on 2 different images are found to be the same, they are matched keypoints. Each group of correctly matched keypoints will generate one 3D point. When there is high overlap between 2 images, the common area captured is larger and more keypoints can be matched together. The more keypoints there are, the more accurately 3D points can be computed. Therefore, the main rule is to maintain high overlap between the images.

As the image acquisition plan has a high impact on the quality of the results, it is important to design it carefully.

 
Important: Pix4Dmapper allows the user to process using a Processing Template. The templates with the label Rapid/Low Res produce fast results at low resolution that can be used on the field as an indicator of how good the dataset is.

If the Rapid/Low Res processing does not yield to good results, very probably, the dataset is not adequate and it is necessary to acquire images again. In some cases, the Rapid/Low Res option fails while the Full processing successfully reconstructs the model.

For more information about the difference between Rapid/Low Res and Full processing: 202558949.

 

Ideal image acquisition plan

The ideal image acquisition plan depends on the type of terrain / object to be reconstructed.:

 
Important: Below it is described the recommended different image acquisition paths and overlap for different cases, despite it may work with lower overlap than the suggested ones, in order to obtain the best possible results, it is suggested to use the recommended overlaps.


General case 

The recommended overlap for most cases is at least 75% frontal overlap (with respect to the flight direction) and at least 60% side overlap (between flying tracks). It is recommended to take the images with a regular grid pattern (Figure 1). The camera should be maintained as much possible at a constant height over the terrain / object to ensure the desired GSD.

Figure 1. Ideal Image Acquisition Plan - General case.

 

Forest and dense vegetation

Trees and dense vegetation often have a very different appearance between overlapping images due to their complex geometry (thousands of branches and leaves). Therefore, it is difficult to extract common characteristic points (keypoints) between the images. In order to achieve good results, it is recommended to use a grid image acquisition plan as the one described in the General Case section by applying the following changes:

  • Increase the overlap between images to at least 85% frontal overlap and at least 70% side overlap.
  • Increase the flight height: At higher altitude, there is less perspective distortion (therefore causing less appearance problems) and the dense vegetation has better visual properties. In other words, it is easier to detect visual similarities between overlapping images in such areas. The flight height in combination with the image pixel resolution and the focal length determine the Ground Sampling Distance (spatial resolution) of the images. Best results are obtained with a GSD higher than 10cm/pixel.

For more information about how to improve the results of dense vegetation areas selecting the correct processing options: 202560159.

 

Flat terrain with agriculture fields

In cases where the terrain is flat with homogeneous visual content such as agriculture fields, it is difficult to extract common characteristic points (keypoints) between the images. In order to achieve good results, it is recommended to use a grid image acquisition plan as the one described in the General Case section by applying the following changes:

  • Increase the overlap between images to at least 85% frontal overlap and at least 70% side overlap.
  • Fly higher. In most cases, flying higher improves the results.
  • Have accurate image geolocation and use the Agriculture templateFor more information about the Agriculture (Ag) template: 205319155.

 

Building reconstruction

Reconstructing 3D buildings requires a specific image acquisition plan (Figure 2):

  • 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. 
 
Note: For more information about oblique imagery: 202559859.
  • It is recommended to take one image every 5-10 degrees to ensure enough overlap, depending on the size of the object and distance to it. Shorter distance and larger objects require images every less degrees.
 
Note: 
  • The flight height should not be increased more than twice  between the flights, as different heights lead to different spatial resolution. For more information: 202558979.
  • Pix4Dmapper generates a high quality point cloud for oblique images of buildings. However, no orthomosaic is generated, when the selected template is 3D Models205319155.
 
Important: By default, Pix4Dmapper generates orthomosaics that are parallel to the (X,Y) plane. Therefore, to generate mosaics of facades, the Orthoplane tool needs to be used. For more information: 202559889.
Figure 2. Ideal Image Acquisition Plan - Building.
 
Note: It is possible to combine aerial nadir and/or aerial oblique and/or terrestrial images.

The images should have enough overlap in each dataset and between datasets. For such cases it is strongly recommended to use GCPs or Manual Tie Points to properly adjust the different sets of images. For more information: 202561599

 

Special cases

This section presents some hints for terrain that is difficult to map such as terrains with snow, sand, lakes, etc.

Snow and sand

Snow and sand have little visual content due to large uniform areas. Therefore: 

  • Use a high overlap: At least 85% frontal overlap and at least 70% side overlap.
  • Set the exposure settings accordingly to get as much contrast as possible in each image.
Water

Water surfaces have almost no visual content due to large uniform areas. Sun reflection on the water and waves cannot be used for visual matching.

  • Oceans are impossible to reconstruct.
  • To reconstruct other water surfaces such as rivers or lakes, each image needs to have land features. Flying higher may help to include more land features.

 

Corridor mapping

Mapping corridors such as railways, roads or rivers requires at least 2 flight lines (Figure 3). GCPs are not required, but are recommended to improve the georeference and accuracy of the reconstruction. For more information about the number and distribution of GCPs in corridor mapping: 202559299.

For a dual track it is recommended to use at least 85% frontal overlap and at least 60% side overlap.

It is possible to use nadir images or oblique images (with an angle between 0º and 45º pointing in both tracks to the center of the corridor). For flat terrain it is recommended to use nadir images. 

Figure 3. Dual track image acquisition plan for corridor mapping.

 

If a dual track image acquisition plan is not possible, a single track image acquisition plan can be used if (Figure 4):

  • Overlap is high enough: At least 85% frontal overlap.
  • Ground control points (GCPs) are defined along the flight line in zig zag.
Figure 4. Single track flight NOT RECOMMENDED.


Multiple flights

Pix4Dmapper can process images taken from multiple flights. When designing the different image acquisition plans, make sure that:

  • Each plan captures the images with enough overlap.
  • There is enough overlap between 2 image acquisition plans (Figures 5 and 6).
  • The different plans are taken as much as possible under the same conditions (sun direction, weather conditions, no new buildings, etc.).
 
Important: The flight height should not be too different between the flights, as different height leads to different spatial resolution. For more information: 202558979.

There is a special way to process datasets taken from multiple flights, for step by step instructions: 202558579.

Enough overlap between 2 flights Not enough overlap between 2 flights
Figure 5. Overlap between 2 flights.

 

Figure 6. Recommended image acquisition plan for 2 flights.

 

City reconstruction (visible facades)

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

Figure 7. Double grid image acquisition plan.

For the facades to be visible, the images should be taken with an angle between 10º and 35º, (for more information about the definition of the angles: 202559859) and not pointing to the nadir. If much detail is needed, aerial and terrestrial images should be combined.

 
Note: It is possible to combine aerial nadir and/or aerial oblique and/or terrestrial images.

The images should have enough overlap in each dataset and between datasets. For such cases it is strongly recommended to use GCPs or Manual Tie Points to properly adjust the different sets of images. For more information: 202561599.

 

3D Interior reconstruction

For interior reconstruction, it is strongly recommended to use terrestrial images. High overlap is needed (90%). Therefore, it is recommended to use a fisheye lens camera.

Manual Tie Points improve the reconstruction and help to properly adjust the model. For more information: 202970309.

 

Mixed reconstruction

It is possible to combine interior/exterior and/or aerial/terrestrial and /or nadir/oblique. Any combination is possible.

The images should have enough overlap in each dataset and between datasets. For such cases it is strongly recommended to use GCPs or Manual Tie Points to properly adjust the different sets of images. For more information: 202561599.

 

Large Vertical Objects reconstruction

The 3D reconstruction of objects like power towers, wind turbines etc requires a specific image acquisition plan (figure 8):

  • Fly close to the structure.
  • Turn several times around the structure at several heights.
  • Images should be taken with high overlap: 90% of overlap between images taken at the same height and 60% of overlap between images taken at different heights. 
  • The optimal camera angle for the top circle is 45 degrees. By pointing to the ground, the content of the images is easier to be matched and the results are better.
  • The images should be as focused as possible (both the main object and the background should be focused).
  • Having image geolocation is recommended. For more information about the image geolocation: 202557499.

 

   

Power Tower

Power Tower reconstructed in the rayCloud
Figure 8. Image Acquisition Plan - Power tower.

 

 
Note: For more information on how to map and measure pole and tower structures: 202560479.
 

 

Tunnel reconstruction

Pix4Dmapper can reconstruct tunnels. The biggest challenge for tunnel reconstruction is the lighting conditions. If the lighting is good either with natural light (if the tunnel is not too long) or with artificial light, the reconstruction could be very good.

In case of very dark tunnels, a tripod is recommended.

 
Tip: It is recommended to:
  • Use fisheye lens camera.
  • Take images in more than one line (avoid single track shooting). If a multiple tracks image acquisition plan is not possible, a single track could work. GCPs are highly recommended in this case.

 

 

 

Figure 9. Automatic Tie Points of a tunnel.

 

 

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