In order to obtain the appropriate imagery the Group utilizes remote controlled or unmanned aerial platforms deployed at low altitude (50-500 m). The most significant advantages of these platforms include:
- Negligible logistics
- No trained personnel required
- Simple/no legal issues regarding flight (depending on country)
- Electrical power source or wind momentum lifters
- Low altitude flights beyond cloud effect.
- High frequency acquisitions depending on project demands.
These unique features make the above platforms ideal tool for observation and monitoring of large scale, shallow, dynamic areas in very high temporal and spatial resolutions. Regarding our projects we apply several kinds of the above techniques customized for imaging of the shallow waters. These techniques involve:
a) KITE AERIAL PHOTOGRAPHY
This technique comprises a passive method of flight relied on the wind momentum. Kites have aerodynamic shape therefore are capable of takeoff and lift some weight (1-3 kgs) depending on the wind speed. The platform is consisting of the kite and the rig suspension with the camera. It is controlled via the kite line which is operated by a person on the ground. The main directions of camera movement are (Figure 1 also):
- Along Z axis, actively by the ground operator that releases or rewinds the line and passively by wind changes in momentum that looses or tightens a given length of line.
- Along X, Y axis, actively by horizontal movements of ground operator (e.g. walking backwards or sideways) holding the line and passively by small changes in wind’s direction.
Figure1: a) GPS, b) waterproof case, c) Delta kite, d) rig and camera, e) measure tape, f)snorkel surveyor, g)inflatable boat and h) artificial bottom control.
Let us now have a look at the main components and functionalities of the Kite aerial platform.
i. Kite, we utilize a Delta type kite with 2 m wingspan (Figure 1c). It operates in various winds from 4-7 Bf. It is capable of lifting up to 1-2 kg in case of deployment in strong winds. It does not require any complex assemblage.
ii.Rig, it is a metallic frame being held by suspension attached on the kite line. Usually it is attached a few meters below the kite. It holds the camera and with a servo triggers the button at an interval (Figure 1d). The rig performs 2 main rotations. It is rotatable in Z axis during acquisition and the camera can be mounted at angle 0-90 degrees relatively to the ground. The rig adjustments contribute in obtaining oblique or near vertical imagery and improve overlap between successes of images.
iii.Line, the kite line anchors the kite to the ground and provides control of movements in all directions by the ground operator. The material is Dacron with >90kgs capacity. Special gloves are required for the ground operator whereas the line must be secured in an appropriate winch with brake.
Having calibrated and properly adjusted the camera we can obtain appropriate imagery for good accuracy projects such as routine surveys, preliminary investigations or GIS environmental applications. The camera calibration information is on Table1. For shallow water operations ashore winds are most appropriate for the KAP survey because they minimize the noise of waves (fetch is negligible due to land obstacle) and ensures that the camera stands over the sub-littoral zone.
Table 1: Camera calibration parameters for the Olympus Fe-3010 compact digital camera, calculated with automated calibration using ad hoc calibration field.
|Final RMSE (pixels)||1.4|
|CCD sensor width (mm)||6.3|
|CCD sensor height (mm)||4.7|
|Focal length at infinity focus (mm)||6.7|
|Principal point Xo (mm)||3.1417|
|Principal point Yo (mm)||2.6166|
|Radial lens distortion K1 coefficient||1.293e-003|
|Radial lens distortion K2 coefficient||1.362e-004|
|Total photos used||11|
|Average point coverage||83%|
Each KAP survey is accompanied by GPS measurements onshore and offshore in order to produce control/check points and a simple DEM for image ortho-rectification. We deploy a portable GPS rover antenna in RTK mode with sub-meter accuracy (Figure 1a). For offshore control points measurements we secure the GPS device within a waterproof case (Figure 1b). In many cases we utilize white plastic plates as artificial bottom control points which are clearly identified in aerial imagery (Figure 1h). They are placed on seabed before KAP survey using a few pebbles and measured by the snorkel surveyor (Figure 1f). The bathymetric survey is conducted from a small inflatable boat using a weighted measure tape (Figure 1e, g).
b) UAV PLATFORM
Small Quad-copter (65cm axial length), equipped with GPS (meter accuracy), altimeter and gyro-accelerometer.
High legs for camera mount (either DSLR or compact)
Flight control and dynamic positioning (in-house software under developement)
"Black box" logging for reconstruction of external geometry