Best practices and capabilities for PIX4Dcloud

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 as 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 appear very differently between overlapping images due to their complex geometry (thousands of branches and leaves). Therefore, extracting common characteristic points (keypoints) between the images is more difficult. To achieve good results, it is recommended to use a grid image acquisition plan like the one described in the General Case section by applying the following changes:

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

Flat terrain with agricultural fields

In cases where the terrain is flat with homogeneous visual content, such as agriculture fields, it isn't easy to extract common characteristic points (keypoints) between the images. To achieve good results, it is recommended to use a grid image acquisition plan like the one described in the General Case section by applying the following changes:

• Increase the overlap between images to at least 80% frontal and side overlap.
• Fly higher. In most cases, flying higher improves the results.

Building reconstruction

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

• Orient the pitch of your camera so that the majority of the image is filled with the object you want to reconstruct, and objects you do not want to reconstruct comprise the minority of the image. No single camera pitch angle can be applied to all missions.
• 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 5-10 degrees to ensure enough overlap, depending on the size of the object and its distance to it.

Figure 2. Ideal Image Acquisition Plan - Building.

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 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 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.
• Each image must include land or some other stationary object to reconstruct other water surfaces, such as rivers or lakes. Flying higher may help to capture more land features.

Corridor mapping

Mapping a corridor such as a railway, road, or river requires at least two flight lines (Figure 3). GCPs are not required but are recommended to improve the accuracy of the reconstruction. For more information about the number and distribution of GCPs in corridor mapping: Number and distribution of ground control points (GCPs) in corridor mapping.

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. 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 a zigzag pattern.

Figure 4. Single-track flight NOT RECOMMENDED.

Large Vertical Object reconstruction

The 3D reconstruction of objects that are tall and slender requires a specific image acquisition plan (Figure 8):

• Fly close to the structure.
• Fly several times around the structure at several heights.
• Images should be taken with high overlap: 90% overlap between images taken at the same height and 60% overlap between images taken at different heights.
• Everything in the image frame must be in focus, including objects in the background that are outside the project area.
• Having image geolocation is recommended.

 

 

Power Tower

Power Tower reconstructed in the rayCloud

Figure 8. Image Acquisition Plan - Power tower.

 

Consider the following aspects to capture images that are detailed, sharp, and contain enough contrast.

Blur due to slow shutter speed.

Noise due to high ISO sensitivity.

Overexposed or underexposed (wrong aperture and/or shutter speed).

Distortions due to electronic or mechanical image stabilization.

Distortions due to the rolling shutter.

PIX4Dcloud offers different processing options depending on the type of flight that the user would like to process. The recommended options are listed below:

(*) Only JPG grayscale images are supported but not recommended. The results may exhibit radiometric errors, scale problems, and calibration inaccuracies.

For more information, see New dataset > Processing options - PIX4Dcloud.

The expected absolute accuracy of the final results (orthomosaic, DSM, point cloud, and DSM) when using PIX4Dcloud depends on the following:

Accuracy of the geolocation of the images

*ppm: parts per million ( 1mm per km)

For more information, Flying camera to surveying tool: RTK/PPK drone upgrades.

Use of GCPs

If ground control points (GCPs) are used, the absolute accuracy of the results will be comparable to the absolute accuracy of the GCPs, even with a standard drone. For example, if the accuracy of the GCPs is about 2-3 cm, we can expect that same degree of absolute accuracy in our final results.

More information: RTK vs PPK drones vs GCPs: which provides better results?

PIX4Dcloud Advanced includes the AutoGCP feature, which allows the use of GCPs to improve accuracy. Please see the requirements for this feature.

PIX4Dcloud is a web-based software enabling the visualization of various outputs, including Orthophotos, Digital Surface Models, NDVI indexes, Point Clouds, and 3D meshes. It's important to note that these results cannot be edited within PIX4Dcloud.

We recommend using alternative products such as PIX4Dmapper, PIX4Dmatic, or PIX4Dsurvey for any necessary editing.

PIX4Dcloud can successfully process a certain number of images according to the characteristics of the project, the processing options, and the requested outputs.

PIX4Dcloud is a user-friendly web-based software designed for simplicity, eliminating the need for expertise in photogrammetry. It uses a set of default parameters tailored for various flight scenarios. These parameters are not adjustable.