DRONEWORK
UAV Photogrammetric survey
Photogrammetric approaches are is in use for the
last 150 years. During that time, the photogrammetry experienced substantial growths
from analogue to today's digital techniques (Uysal et al., 2015). Photogrammetry helps in determining the
geometric properties of objects from images. The output of photogrammetry is
typically a map, a drawing or a 3D model of some real-world object or land
mass. It is the art of making maps and precise measurements from photographs,
especially from aerial surveying. Digital images captured from UAV are
frequently processed using conventional methods or using the new and efficient
techniques with the specific software. The advantages of using UAVs for
photogrammetric surveying are that their quickness, position, and stabilisation
can be controlled correctly, so successive, distortion-free above ground images
of a site can be acquired, which can then be processed to create 3D point
clouds, digital terrain models, contour maps, digital surface model or can also
be merged into a 2D or 3D orthomosaic image.
The fundamental principle used
by photogrammetry is aerial Triangulation. In this technique, we acquire
data from at least two dissimilar points. The persistence of taking images from more
than 2 points is to generate lines of sight. Once these lines of sight are arranged,
we linked them to trace a point where they encounter and thus analyse the
coordinates of the desired point (Castillo et al., 2017). The
ability to speedily take aerial image from vertical, horizontal, and oblique
angles allows a high degree of accuracy and flexibility which basically cannot be
achieved by any traditional means, or by terrestrial surveying (http://libguides.wustl.edu/drones4data).
Agisoft
PhotoScan, Pix 4d mapper and other existing software supports measuring distances
between control points, as well as of surface area and volume of the
reconstructed 3D model. All these measurement are generally performed on the
generated 3D mesh. For distance measurement the existing
softwares enables measurements of direct distances between the points of the
reconstructed 3D scene. The points used for distance measurement must be
defined by placing markers in the corresponding locations. Model coordinate
system must be also initialized before the distance measurements is performed.
Alternatively, the model can be scaled based on known distance (scale bar)
information. For measuring distance in the study site, we placed multiple
markers above the buildings and in the front lawn, created scale bar from 3D
view context menu and finally viewed the distance in estimated value mode in
the Agisoft PhotoScan. Distance between cameras are also measured in the
workspace in the similar manner. Surface area and volume means, or by terrestrial surveying (http://libguides.wustl.edu/drones4data).
Figure 1 : snapshot showing markers taken above building and in lawn
Table 1: Details
of the marker
Labels
|
X/East
|
Y/North
|
Z/Altitude(m)
|
Point 1
|
85.441448
|
23.412947
|
588.20
|
Point 2
|
85.441446
|
23.412949
|
574.12
|
Point 3
|
85.441471
|
23.412950
|
587.40
|
Point 4
|
85.441529
|
23.412773
|
587.14
|
Point 5
|
85.441621
|
23.412796
|
587.02
|
Point 6
|
85.441555
|
23.412972
|
587.95
|
Point 7
|
85.441508
|
23.413190
|
573.79
|
Point 8
|
85.441702
|
23.413247
|
573.65
|
Point 9
|
85.441476
|
23.413224
|
574.16
|
Point 11
|
85.441329
|
23.413388
|
575.14
|
Point 12
|
85.441412
|
23.413404
|
575.64
|
Table 2: Calculated distance
Labels | Distance(m) |
Point 1_Point 2
|
14.07
|
Point 7_Point 8
|
20.75
|
Point 4_Point 5
|
9.72
|
Agisoft
PhotoScan, Pix 4d mapper and other existing software supports measuring distances
between control points, as well as of surface area and volume of the
reconstructed 3D model. All these measurement are generally performed on the
generated 3D mesh. For distance measurement the existing
softwares enables measurements of direct distances between the points of the
reconstructed 3D scene. The points used for distance measurement must be
defined by placing markers in the corresponding locations. Model coordinate
system must be also initialized before the distance measurements is performed.
Alternatively, the model can be scaled based on known distance (scale bar)
information. For measuring distance in the study site, we placed multiple
markers above the buildings and in the front lawn, created scale bar from 3D
view context menu and finally viewed the distance in estimated value mode in
the Agisoft PhotoScan. Distance between cameras are also measured in the
workspace in the similar manner. Surface area and volume measurements of the
reconstructed 3D model of the study area also performed after defining the
scale and coordinate system of the scene. To measure surface area and volume,
one has to use Measure Area and Volume command from the Tools menu. Surface
area is measured in square meters, while mesh volume is measured in cubic meters.
Volume measurement can be performed only for the models with closed geometry.
Watch next part for more information .
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