Geog336: Geospatial Field Methods: Introduction to Pix4D Software

Developed by Pix4D SA as a web-based standalone product for Microsoft Windows, Pix4D is one of the current premier software for photogrammetry. It’s many applications include creating professional orthomosaics, constructing point clouds, generating models and more. With it’s simple layout and automated tools, it is incredibly easy to use. The proper background knowledge is necessary to understand the information that is being generated and to ensure that the tools are used correctly and the results interpreted ethically.

Photogrammetry is the technique to extract geometric information from two-dimensional images or video. For a comparison of photogrammetry software, click here.

This post contains:

A basic introduction to the Pix4D software
A brief tutorial for performing basic functions in Pix4D Mapper
A final overview of the software with links on where to find more information

Introduction to Pix4D

Figure 1 The illustrated workflow of Pix4D software. Source: Pix4D User Manual

CAPTURE: The user can capture an image with any digital camera or use Pix4Dcapture flight planning app on your mobile or tablet for drone field operations, flight review, and optimal data capture.
PROCESS: Choose offline processing on Pix4D desktop for full control over data, without needing an internet connection. Choose online processing for fully automated, hardware free results on Pix4D cloud.
ANALYZE: On desktop, gain access to advanced editing features, quality control, and measurements. For the cloud, monitor projects over time, use the drawing overlay for construction, and automatic NDVI map for agriculture.
SHARE: Easily collaborate and annotate projects online, then share maps, models, and analytics with a simple URL.

Data Acquisition

The images that form the dataset must be obtained in the field before using Pix4Dmapper. It is recommended to use images that have geolocation and Ground Control Points (GCPs) for more accurate results.

In order to take a good dataset, there are four steps:

1. Design an Image Acquisition Plan: Considering key details ahead of time will mitigate problems and ensure that data collection is as thorough and efficient as possible. Is the project aerial, terrestrial, or mixed? What type of terrain or object will be photographed? What type of camera will be used? At what rate will the images be taken? What is the ideal flight height and angle? Draw out a path for ideal image collection. For aerial projects, this includes selecting the corridor path or regular and/or circular grid. Always keep the purpose of the project in mind.

2. Configure the camera settings: Wrong configuration can result in images with blur, noise, distortions, etc. There are specific requirements necessary to collect agricultural data, etc. so the specific project type should be researched ahead of time to avoid any problems.

3. Georeference the images: While this step is considered optional, it is highly recommended to avoid data mishaps later. The images can be georeferenced using a camera with built-in GPS or by external GPS devices.

4. Getting Ground Control Points (GCPs) on the field or through other sources: Again, though optional, this step is recommended to ensure data accuracy. Plan how many GCPs have to be acquired and include their measurement in the Data Acquisition Plan.

What are the recommended hardware and software requirements?

Windows 7, 8, 10 64 bits is recommended, as is a CPU quad-core or hexa-core Intel i7/Xeon. Your computer should be GeForce GPU compatible with OpenGL 3.2 and 2 GB RAM. Solid-state hard-drives are best.

Small projects (under 100 images at 14 MP): 8 GB RAM, 15 GB SSD Free Space.
Medium projects (between 100 and 500 images at 14 MP): 16GB RAM, 30 GB SSD Free Space.
Large projects (over 500 images at 14 MP): 32 GB RAM, 60 GB SSD Free Space.
Very Large projects (over 2000 images at 14 MP): 32 GB RAM, 120 GB SSD Free Space.

What is the overlap needed for Pix4D to process imagery?

At least 75% frontal overlap is required for most cases and at least 60% side overlap, though the desire percentage changes depending in the terrain type. This detail requires careful planning. The image acquisition plan depends on the required Ground Sampling Distance (GSD) in the project specifications. The GSD will define the flight height at which the images have to be taken. This is heavily dependent on the project specifications and the terrain type or object to be reconstructed.

Nowadays, advanced Unmanned Aerial Vehicles (UAVs) come with very good software that can design the image acquisition plan automatically when given some parameters. These smart little robots can automatically take the required images according to the selected acquisition plan with no need for the user to intervene.

What if the user is flying over sand, snow, or uniform fields?

The generally recommended flight path can be seen in Figure 2. Modifications on this path can be made for different types of terrain. For example, flat terrain with homogeneous visual content such as agriculture fields requires a frontal overlap of at least 85% between images and at least 70% side overlap. For this type of terrain, flying higher improves the results. Having accurate image geolocation is necessary and use of the Agriculture template is recommended (click here for more details).

For snow and sand, at least 85% frontal overlap and 70% side overlap is again recommended. To avoid washout, it is important to set the exposure settings to get as much contrast as possible.

For water surfaces (including oceans, lakes, rivers, and ponds), sun reflection and lack of visual content to aid in point matching are problematic. Pix4D cannot reconstruct water bodies as large as the oceans, but to reconstruct other water surfaces, each image needs to include land features. Flying higher may help.

Figure 2 This graphic demonstrates the flight path. over the area of interest. Each red circle represents an image that was taken during the flight. Overlap is necessary to piece the images together into a cohesive map of the area.

What is Rapid Check?

Rapid Check is a program that runs inside of Pix4D and is used as an alternative initial processing system where accuracy is traded for speed. It processes quickly in an effort to determine whether sufficient coverage was obtained, but the result has fairly low accuracy. The rapid check is for field use to verify whether the proper areas and coverage of data were attained.

Can Pix4D process multiple flights?

Yes, Pix4Dmapper can process images taken from multiple flights provided that four factors are accounted for:

The same flight height is maintained
Each plan captures the images with enough overlap
The image acquisition plans overlap one another adequately (Fig.3)(Fig.4)
The flights are taken under very similar conditions (sun direction, weather conditions, no new buildings, etc.)

Figure 3 In order for Pix4D to process data from multiple flights, the flight path must overlap at least as much as the diagram on the left. Source: Pix4D 3.0 user manual.

Figure 4 The recommended image acquisition plan overlap for multiple flights.

Can Pix4D process oblique images?

Yes. Oblique imagery (Fig.5) is recommended for use in building reconstruction and other 3D objects. This requires a specific image acquisition plan which includes flying around the object a first time at a 45° camera angle, followed by a second and third time around the building increasing the flight height and decreasing the camera angle with each round (Fig.6). Taking one image every 5-10 degrees is recommended 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.

Figure 5 Oblique imagery gives the viewer a reference to the height of an object. Source: Pix4D User manual.

Figure 6 An example of the ideal image acquisition plan for a 3D object, such as a barn. Source: Pix4D user manual.

Are GCPs necessary for Pix4D? When are they highly recommended?

While ground control points (points of known coordinates in the area of interest) are not necessary for processing in Pix4D, they are highly recommended in certain cases:

Corridor mapping with a single track (which is to be used ONLY when a dual track image acquisition plan is not possible). This requires at least 85% frontal overlap and GCPs that are defined along the flight line in a zig zag formation.
Images without geolocation. When images have no geolocation, Pix4Dmapper can use GCPs to locate, scale and orient the model.
When particularly high accuracy is needed.

What is the quality report?

The quality report is automatically generated for each project in Pix4D and can be opened by clicking Process>Quality Report on the main tab bar. This opens a report that can be downloaded and saved as a pdf (Fig.7).

Figure 7 The quality report pdf is generated automatically.

This report contains a summary of all aspects of the project, including: a preview (Fig.8), initial image positions, computed image/GCPs/Manual Tie Points Positions, GCPs/Check Points, absolute camera position and orientation uncertainties, overlap, internal camera parameters (including perspective lens and fisheye lens), 2D Keypoints Table and 2D Keypoint Matches graph, relative camera position and orientation uncertainties, 2D Keypoint Table for Camera, a table showing Median / 75% / Maximal Number of Matches Between Camera Models, 3D points from 2D Keypoint Matches, Manual Tie Points, Ground Control Points, scale constraints, orientation constraints, absolute geolocation variance, geolocation coordinate system transformation, relative geolocation variance, and rolling shutter statistics.

Figure 8 This image, taken from a quality report generated for the Litchfield Mine in Eau Claire, Wisconsin, shows the typical low resolution preview of the Orthomosaic and the DSM (digital surface model). They allow a visual inspection of the quality of the initial calibration.

To see an online example of a quality report, click here.

For more information on quality reports, click here to see the Pix4D support page.

A Tour of Basic Functions

Processing Data

Open Pix4Dmapper and click Project>New Project. Choose a folder to save to. Select Add images and open the data files. In this example, the data are aerial images taken by Dr. Joseph Hupy of UW Eau Claire using a DJI Phantom UAV. The study area is the Litchfield Mine, adjacent to the Chippewa River in Eau Claire, Eau Claire County, Wisconsin. The WGS 1984 UTM Zone 15N coordinates system is detected automatically by Pix4D. Upon opening a new project in Pix4D, the user is prompted with an option then to expand the project. For today’s purposes, a simple project (the default settings) will do (Fig.9). Click finish.

Figure 9 This menu displays options to expand the report. The basic, preselected options include a 3D model, an orthomosaic, a DSM, a 3D Mesh, and a Point Cloud. Using this information the user can measure volumes of objects, digitize objects, generate contour lines, and more.

The flight pattern is shown via points and lines over a basemap of the area (Fig.10).

Figure 10

Analysis of the quality report

After the initial processing is done, examine the quality report.

The following steps are recommended:

Quality Check. Verify that: all the checks are green, all or almost all the images are calibrated in one block, the relative difference between initial and optimized internal camera parameters is below 5%, and, if using GCPs, that the GCP error is below 3×GSD (Fig.11).

Figure 11 In the Litchfield Mine project report, the Quality Check revealed that all 68 images were calibrated (meaning that none were rejected for not overlapping enough) and Pix4D was able to automatically generate 8553.1 matches between images in order to overlay them accurately.

A red flag was raised by the Camera Optimization. This indicates that the focal length/affine transformation parameters, which are a property of the camera's sensor and optics which varies with temperature, shocks, altitude, and time, was not within 5% of the optimized value to ensure a fast and robust optimization.

The Georeferencing raised a yellow flag because no 3D ground control points were used, which means there may be a decrease in accuracy of the 3D model.

Examine the Preview. For projects with nadir images and for which the orthomosaic preview has been generated, verify that the orthomosaic does not contain holes or have distortions. If GCPs or image geolocation has been used, make sure that it has the correct orientation (Fig.12).

Figure 12 In the preview generated for the example project (right) compared to the aerial image of the Litchfield Mine (left), there are no holes in the data. Few distortions are apparent, though there appears to be an area of poor overlap in the center, indicated in this image by the orange rectangle. This may be because fewer photos were taken over the northern portion of the mine because there is less relief there.

Calculating surface area within the rayCloud editor

The use of the rayCloud is optional and it can be used to:

Visualize the different elements of the reconstruction and their properties.
Verify/ improve the accuracy of the reconstruction of the model.
Visualize point clouds / triangle meshes created in other projects or with other software.
Georeference a project using GCPs and /or Scale and Orientation constraints.
Create Orthoplanes to obtain mosaics of any selected plane.
Assign points of the point cloud to different point groups.
Improve the visual aspect.
Create objects and measure distances (polylines) and surfaces.
Create 3D fly-through animations (Video Animation Trajectories).
Export different elements, such as: GCPs, Manual / Automatic Tie Points, Objects, Video Animation Trajectories.
Export point cloud files using points belonging to one or several classes.

In order to calculate surface area of on object, such as a sand pile, click View>rayCloud on the Menu bar (Fig.13). Note: If the image does not resemble the figure below, then follow the steps above to process the data, then open the rayCloud.

Figure 13 The rayCloud view indicated the area of each image that was taken along the flight path and the angle each is associated relative to the ground.

In the left sidebar, click rayCloud>Surface (Fig.14). The measurements will be displayed in the right sidebar.

Figure 14 The surface box must be selected to view measurements such as projected 2D length and enclosed 3D area.

Calculate the volume of a 3D object

In order to create a new volume, click View>Volumes>New Volume on the Menu bar. On the 3D View, a green point appears beside the mouse. Left clicking the feature will mark the vertices of the base surface of the volume. With each click a vertex is created and the volume base surface is formed. Right clicking indicates that the feature is encircled and creates the volume base. On the sidebar, click Compute to compute the volume (Fig.15). For further instruction on this, click here.

Figure 15 This compilation of images shows the sequence of steps to compute the volume of an object in Pix4D. The calculations are displayed in image 4.

How to export a volume

On the Menu bar, click View > Volumes. On the sidebar, in the Objects section, click the symbol that resembles a square with an outward-facing arrow. Click All to select all Volumes to be exported, click None to unselect all Volumes or select the checkbox on the left of each Volume to select some of the Volumes to be exported (Fig16). Click export and save the image as a .shp file in order to open it in ArcMap (Fig.17). The metadata will need to be added manually.