STEPS OF DATA PROCESSING FOR ANY UAV BASED WORK
All acquired images from
digital camera were downloaded into the computer after flight mission. Each
image was saved in jpeg file. The quality of images was checked before they
were used in the processing stage. Some of the images might have some quality
problem such as blurring image and colour balancing error which was caused
during flight mission. These problems usually arise from the attitude of the
UAV during flight. If the quality of all images were very bad, another flight
mission might need to be done. However, in this study, all acquired images were
in good quality and they were being preceded for the photogrammetric
processing. UAV Processing software is able to process aerial images and to
produce digital ortho-photo and digital elevation model (DEM) for the study
area. Photogrammetric technique involves many processes such as interior
orientation, relative orientation, aerial triangulation and bundle adjustment.
Interior orientation requires the information of camera parameters including
pixel size, focal length and principal points coordinates. All of these
parameters were being defined before the processing stage. Relative orientation
involved image correlation algorithm in order to transfer the tie points
between images. Tie points were responsible to align all acquired images in the
same condition in which the images were taken during flight mission. Ground
control points were established during image processing in order to project the
result into local coordinate system. Ground control points were collected by
using Real Time Kinematic-GPS (RTK-GPS).
In
the present work, some task are performed with the purpose to analyse the
possibility of acquire and use nadir images for 2D mapping and 3D modelling and
to perform measurements and analysis using photogrammetry on the UAV
acquired images. Compared different software that follows
different workflows, in order to evaluate their effectiveness and weaknesses. The accuracy of aerial data is directly related to the spatial
resolution of the input imagery. The high resolution images from UAV can
compete with traditional aerial mapping solutions that set on highly accurate
alignment and positioning sensors on board.
The acquired data were processed using different software tools: Agisoft Photoscan
Professional Pix4D, arc map 10.3 and drone deploy. The entire process is
carried out almost automatically by these software tools, based on algorithms
of photogrammetry and computer vision (CV) that allow to process a large amount
of images in a fast and easy way, with a limited influence of the user on the
resulting dense point cloud. All lead to the images
alignment, generation of dense point clouds and, subsequently, to the
production of a triangulated mesh and to DEMs and ortho-photo extraction.
Generally, the input data required by these tools to perform the 3D model
reconstruction process are only the acquired images and some GCPs, since it is
not even necessary to know a-priori the exterior orientation parameters of the
cameras. In this case the alignment performed by Photoscan was used as input.
Pix4Dcapture
will automatically start downloading images to our phone or tablet after capturing
the mission's final photo or we can directly take the images from the micro SD card.
We can keep our drone and remote controller on and connected to our phone or
tablet to wirelessly download our images.
1.
Add Photos: Processing of images with PhotoScan or any other software includes the following main steps: Loading
photos into PhotoScan, inspecting loaded images, removing unnecessary images,
aligning photos, building dense point cloud, building mesh (3D polygonal
model),generating texture, building tiled model, building digital elevation
model, building orthomosaic, exporting results.
2.
Loading and align photos: After adding Photos by given command from the Workflow menu we have to
optimize the added photos and then align
them according to the camera position
,it can be automatically performed by the software by clicking on ALIGN photo
command in workspace pane
I. Open Reference pane using the
corresponding command from the View menu.
II. Click
Import button on the Reference pane toolbar
and select the file containing camera positions information in the Open dialog.
The easiest way is to load simple character-separated file (*.txt,
*.csv) that contains x- and y-coordinates and height for each camera position
(camera orientation data, i.e. pitch, roll and yaw values, could also be
imported, but the data is not obligatory to reference the model).
In the Import CSV dialog indicate
the delimiter according to the structure of the file and select the row to
start loading from. Note that # character indicates a commented line that is
not counted while numbering the rows. Indicate for the program what parameter
is specified in each column through setting correct column numbers in the
Columns section of the dialog. Also it is recommended to specify valid
coordinate system in the corresponding field for the values used for camera
centres data.
III .Also to check our settings in the
sample data field in Import CSV dialog.
Click OK
button. The data will be loaded into the Reference pane.
3. Check Camera Calibration: Open Tools Menu → Camera Calibration
window
By default PhotoScan estimates intrinsic camera
parameters during the camera alignment and optimization steps based on the
Initial values derived from EXIF. In case pixel size and focal length (both in
mm) are missing in the image EXIF and therefore in the camera calibration.
window, they can be input manually
prior to the processing according to the data derived from the camera and lens
specifications.
If pre-calibrated camera is used, it
is possible to load calibration data in one of the supported formats using Load
button in the window. To prevent the pre-calibrated values from being adjusted
by PhotoScan during processing, it is necessary to check on Fix Calibration
flag.
PhotoScan can process the images
taken by different cameras in the same project. In this case in the left frame
of the Camera Calibration window multiple camera groups will appear, split by
default according to the image resolution, focal length and pixel size.
Calibration groups may also be split manually if it is necessary. In case ultra-wide
or fisheye angle lens is used, it is recommended to switch camera type from
Frame (default) to fisheye value prior to processing.
4.
Point cloud generation and Building dense cloud-PhotoScan allows generating and visualizing a dense point cloud model.
Based on the estimated camera positions the program calculates depth
information for each camera to be combined into a single dense point cloud.
5. Building mesh-PhotoScan
supports several reconstruction methods and settings, which help to produce
optimal re-constructions for a given data set.
7. Building DEM-PhotoScan
allows generating and visualizing a digital elevation model (DEM). A DEM
represents a surface model as a regular grid of height values. DEM can be
rasterized from a dense point cloud, a sparse point cloud or a mesh. Most
accurate results are calculated based on dense point cloud data. PhotoScan
enables to perform DEM-based point, distance, area, volume measurements as well
as generate cross-sections fora part of the scene selected by the user.
8. Building orthomosaic-Orthomosaic
export is normally used for generation of high resolution imagery based on the
source photos and reconstructed model. The most common application is aerial
photographic survey data processing, but it may be also useful when a detailed
view of the object is required.
9. Export Orthomosaic
Select Export Orthomosaic →
Export JPEG/TIFF/PNG command from File menu.
Set
the following recommended values for the parameters in the Export Orthomosaic
dialog
Projection:
Desired coordinate system
Pixel
size: desired export resolution (please note that
for WGS84 coordinate system units should be specified in degrees. Use Meters
button to specify the resolution in meters).
Split
in blocks: 10000 x 10000 (if the exported area is
large it is recommended to enable Split in Blocks feature, since the memory
consumption is rather high at exporting stage)
Region:
set the boundaries of the model's part that should
be projected and presented as orthomosaic. Also polygonal shapes drawn in the
ortho view and marked as boundaries will be taken into account for the
orthomosaic export.
TIFF
compression and JPEG quality should be specified according to the job
requirements. Big TIFF format allows to overcome the TIFF file size limit for the
large ortho-mosaics, but itis not supported by some applications.
III.
Click Export... button and then specify target file name and select type of the
exported file (e.g. GeoTIFF).
IV.
Click Save button to start ortho-mosaic generation.
15. Export DEM: Select
Export DEM → Export GeoTIFF/BIL/XYZ command from File menu.
Projection: Desired coordinate system
No-data
value: value for not visible points; should be specified according to the
requirements of the post processing application.
Pixel size: desired export
resolution
Split
in blocks: 10000 x 10000 (if the exported area is large, it is recommended to
enable Split in blocks feature, since the memory consumption is rather high at
exporting stage)
Region: set the boundaries
of the model's part that should be projected and presented as DEM. Also
polygonal shapes drawn in the ortho view and marked as boundaries will be taken
into account for the DEM export.
Click
Export... button and then specify target file name and select type of the
exported file (e.g. GeoTIFF).
Click
Save button to start DEM generation.
T
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