Orthomosaics and drone footage of Pretty Beach reef (16-089) at low tide (2022-10-06)

This dataset consists of drone footage (DJI Mini 2) taken at the fringing reef off Pretty Beach, south of Port Douglas in Queensland at low tide (0.5 - 0.6 m LAT) on 2022-10-06. It contains 16 videos showing the reef from a broad range of angles including footage from 90 m and low footage from 6 m elevation. It also includes the raw photos to create two orthomosaics. The first corresponds to 310 photos that cover the whole reef from 90 m and the second consists of 176 photos covering a central strip of the of the reef taken from 6 m elevation. This dataset also includes an orthomosaic image covering the whole reef taken from 90 m height with approximately 5 cm resolution and a mosaic of a central portion of the reef taken at 6 m height with a resolution of 3 mm.

The video footage provides a snapshot of the state of this reef in 2022. It shows that there was live coral on the edges of the ribbed sections of the reef, and live coral in the channels between the ribs. It also shows that macro algae grows well in the channels between the ribs of the reef. The front of the reef was largely obscured by waves and turbid water. As a result, only the shallowest half of the reef is visible in the imagery.

All footage was collected under Australian Fly for Fun drone rules during a holiday using a DJI Mini 2 drone and was not conducted as part of any research project. It is provided by the author to be used freely under an open license by the research community.

Methods:

The imagery collected for the orthomosaics was flown manually without a planned survey. The high wind conditions, combined with the small drone, limited the flight paths, as the DJI Mini 2 drone cannot have its camera facing straight down in high wind conditions. The camera was set to take photos automatically every 2 seconds. Videos of the reef were collected between taking mosaic photos for the mosaics. 

Mosaic 1 - ~90 m height (2022-10-06-16-089-Pretty-beach-mosaic-1.tif):

This mosaic consists of an unplanned image collection process. As a consequence, the image mosaic created by Agisoft Metashape is somewhat geometrically distorted. In 3D, the 'flat' reef surface is bowed due to uncorrected errors in the drone lens and the biased flight path. This results in a small distortion in the scaling of the resulting image mosaic. Additionally, there is a run of oblique view images that are poorly handled by Metashape, resulting in a false view of the reef mapped into the offshore waters.

Comparing the orthomosaic to satellite imagery indicates that the reef appears smaller in the orthomosaic than in reality (possibly due to the bow effect of the 3D reconstruction). The width of the reef in the orthomosaic is 395 m, while the same width measured from Sentinel 2 imagery is 455 m. 

Even with all these faults the image mosaic is still a useful view of the whole reef. The calculated DEM was not saved due to the false curvature in the 3D reconstruction, making the DEM not accurate.

Mosaic 2 - ~6 m height (2022-10-06-16-089-Pretty-beach-mosaic-2.tif and 2022-10-06-16-089-Pretty-beach-mosaic-2-DEM.tif):

This is a small scale close up mosaic of a strip from the beach out through the middle section of the reef, taken from a height of approximately 6 m. This shows that the ribs of the reef are formed from the progressive growth of a massive coral species (possibly Coelastrea) up to a height where the coral is too exposed to air at low tide. This causes the top to die off, and the colony grows out laterally. The front edge of the colonies seems to grow more, possibly due to incoming waves washing sediment off the front surface of the colony. This might be the mechanism that causes the ribs to grow outward into the oncoming waves. The dead top surface of these ribs is covered in trapped sediment.

Note: Without Ground reference points, the elevation points in the DEM are just derived based on the GPS readings from the drone, which had an offset of approximately -14 m.

This mosaic is positioned approximately 23 m north north west of the location of Moasic-1.

Limitations of the data:

The positioning of the orthomosaics in this dataset was based solely on the GPS readings from the drone and so is only accurate to approximately 20 m. The large mosaic contains significant distortion (~15%) due to the bias flight path and oblique camera angle used. 

The tidal height was estimated from the tidal predictions for Port Douglas at the time of the observations. There was significant onshore wind that is likely to have slightly raised the actual water levels.

Format of the data:
- MP4 videos (4k 100 Mbps, 30 FPS)
- Photos (4000 x 2250 pixels)
- Orthomosaics Geotiff in WGS84

Data dictionary:

eAtlas Processing:
The orthomosaics were prepared by the eAtlas team from the images provided. No modifications to the source imagery were performed.

Location of the data:
This dataset is filed in the eAtlas enduring data repository at: data\custodian\2020-2029-other\GBR_EL_Pretty-beach-drone_2022

Tags: Coral reef
Organization: Australian Ocean Data Network