A traditional land-survey technique appears to be of high importance in digital marine mapping. The basic idea is that the 3D world can be modeled via a proper algorithm using 2D images and by defining the camera parameters during capture. Additionally ground control points are required for image referencing along with Digital Elevation Model for image rectification and production of final measurable data.
Main components of digital photogrammetry include:
1) Interior camera orientation, which includes the sensor attributes such as dimensions, pixel size, principal point, focal length and radial lens distortion.
2) Exterior camera orientation, which expresses position of camera during acquisition and tilting around X and Y axis.
Taking into advantage the water clarity, it has been applied since the 70s as bathymetric and habitat mapping technique yielding valuable high resolution datasets. Close range photogrammetry or in our case, low altitude aerial photogrammetry reveals seabed structures in centimeter resolution and assists discrimination even of most restricted habitat units.
The basic field requirements for seabed photogrammetry are:
- Acceptable water clarity
- Sun’s elevation and wind velocity
Consequently enclosed – semi enclosed oligotrophic waters comprise the ideal locations for photogrammetric survey once they have good transparency and they are less affected by turbidity. Another useful parameter that needs to be taken into consideration is light conditions during acquisition of imagery. The light conditions depend on sun’s elevation and sea state. Generally calm sea is ideal for aerial photogrammetry once sea surface roughness may cause noise due to the sun glitter effect. During sun positions in zenith (midday ± 2hrs) light beams penetrate seawater deeper and cause better underwater illumination. The seabed reflection is brightest with some colors saturated. When sun is at lower angles relatively to the horizon most of the incoming light is being scattered and absorbed. However the seabed appears more detailed and shaded due to oblique incident light. Low altitude aerial photogrammetry of the coastal zone allows for mapping of the sublittoral zone geomorphological units up to 15 meters depth depending on water conditions. For that purpose a digital calibrated camera is being required along with ground control points. The control of external camera geometry is relied on the aerial platform’s positioning and inertia sensors or alternatively it may be presumed and fully depended on the amount of ground control points.
Another parameter that applies in through water photogrammetry is the correction of refraction effect. In this occasion we are talking about two-media (air/water) photogrammetry. For extraction of 3D information regarding shallow seabed morphology it is required that the images will be corrected for the refraction effect. The refraction effect needs to be corrected when we require high accuracy extraction of seabed Digital Elevation Model. This will allow monitoring of volumetric changes in short term scales thus assisting erosional or accretion processes to be identified and precisely measured for shallow seabed.