Overview of Pix4D
Pix4D is the current premier software for processing
UAS data, mainly for constructing point clouds. The software can also perform
volume analysis and video fly-through. Before starting a project, the dataset
including images, images geolocation, and Ground Control Points (GCPs) have to
be obtained on the field. Pix4Dmapper requires a good
dataset to automatically produce results with high quality and accuracy. An
extension of this requirement is a certain level of overlap which depends on
the type of terrain that is mapped. The recommended overlap for most cases is
at least 75% frontal overlap with respect to flight direction and at least 60%
side overlap between flying tracks. Images should be taken in a regular grid
pattern. If the user is flying over sand, snow, or uniform fields, use a high
overlap of at least 85% frontal overlap and at least 70% side overlap.
Another useful feature of Pix4D is rapid check. Rapid Check
runs inside of Pix4D and is an alternative initial processing system where
accuracy is traded for speed. It processes faster in an effort to quickly
determine whether sufficient coverage was obtained and sacrifices accuracy to
do so.
Pix4D can also process images taken from multiple flights.
To do this, the pilot must maintain as similar of a flight height as possible
with the two flights. In addition, there must be enough overlap between the two
flights and both flights should occur under as similar conditions as possible. Pix4D
can process oblique images as well. The user will need images with enough
overlap in each dataset and between datasets.
GCPs are not necessary for using Pix4D. However GCPs are
recommended when overlapping images to properly adjust the different sets of
images. There are other instances where GCPs are recommended such as corridor
mapping to improve georeferencing and processing images without geolocation.
Finally, quality reports are created by Pix4D to inform the
user of the results of the processing completed. This includes any processing
steps that failed, a quality check, and a description of many different
parameters (computed GCPs, camera position, scale constraints, orientation
constraints, relative geolocation variance, etc.).
Using the Software
Volume Calculations
Volume calculations of several gravel/sand piles in an
Unmanned Aerial System (UAS) dataset within Pix4D were completed using the
volume tool. This tool is relatively simple to use. Once the project is loaded
go to the volumes tab and click on the volume tool. Next add control points
using mouse clicks to surround the object, in this case a sand/gravel pile.
Once the pile is surrounded right click to end the measurement. Click on
compute to get the final measurements. Figure 1 below shows the results page
and figure 2 shows three gravel/sand piles that were measured in the program.
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| Figure 1. Results of three different volume calculations on UAS data within Pix4D software. |
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| Figure 2. Three different volume calculations of sand or gravel piles in Pix4D software. |
Creating Animations
Within Pix4D one can also create a video that ‘flies’
through the project. To complete this click the raycloud tab and then the video
button. Using the mouse one can zoom and tilt the image and create waypoints along the way until the user deems the route
sufficient. Then the video must be rendered and saved in a format usable by
other platforms such as .mp4. The video below is an example of an animation. Take notice of the green line and dots. This is a different path but illustrates how a path is created for an animation using waypoints (green dots).
Map Creation Using Data Sets from Pix4D
Within ArcMap maps can be created using data sets generated
by Pix4D. The data sets used in this lab were already created by Dr. Hupy of
the University of Wisconsin-Eau Claire. Two maps were made; one of an orthomosaic image
and another of a digital surface model (DSM) file (figures 3 and 4). A DSM is a
file of all features on the surface and usually is derived from LiDAR data. By
showing all features on the surface, more accurate calculations can be done
based on the data.
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| Figure 3. Map of the orthomosaic image derived from UAS data |
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| Figure 4. Map of the DSM file derived from UAS data. |
To create the DSM map the appropriate .tif file from the
geodatabase was imported.
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| Figure 5. Geodatabase containing the two files required for the maps. |
Then the hillshade tool created a hillshade raster
of the same file. The transparency of the DSM file was set to 35% and placed on
the hillshade to add definition to the map. In ArcScene the same DSM file was
formatted to allow viewing from the side of the image. Side views allow a
better understanding of the topography of the image. Within ArcScene the image
was floated on a custom surface and set to render the layer at all times and
shade areal features relative to the scene’s light position. A red to green
color ramp was applied to both the ArcScene and ArcMap representation of the
DSM. The ArcScene image was imported into ArcMap as a jpeg.
Figure 3 and 4 both show the imagery obtained by a UAS. The
DSM however shows the topography more clearly. The red colors represent high
elevations and green represents low elevations. From the image one can discern
the gravel or sand piles and the trees in the surrounding area. The inclusion
of tree elevations indicates one shortcoming of UAS data, that thick canopy
cover obscures ground features. This should be taken into consideration when
planning a UAS flight.
Overview of Pix4D
This was a very short introduction to the Pix4D software.
According to Dr. Hupy this software began as a tool to interpret historical images
but saw uses in UAS data and moved its focus to fill the data processing gap.
The software works similar to ArcScene in its ability to move the image and
view it from multiple angles. There are many more nuances to this software such
as GCPs and oblique imagery analysis. Overall Pix4D appears to be a powerful
data processing tool for UAS data.
Conclusion
This lab was a great introduction to Pix4D. The lab walks
through the process of opening a project in Pix4D and doing basic manipulations
of the data within the software. It also includes how to utilize the data
created in Pix4D on another platform such as ArcMap. Data creation, processing,
and visualization are all important knowledge areas in geospatial fieldwork and
this lab introduces all three. The skills gained in this lab will provide the
basis for future labs involving UAS data processing.
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