Case study: Monitoring reef restoration

with time-series ortho-mosaics

The Challenge

• The Nature Conservancy's Hawai'i Marine Program, Pono Pacific Land Management LLC, and Malama Maunalua undertook a large-scale reef restoration project. They sought a reliable method to monitor the progress of their efforts to remove leather mudweed (Avrainvillea amadelpha), a species of invasive alga that was smothering the reef in Maunalua Bay.

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  Aerial imaging of submerged features can be affected by both water column and sea surface conditions. The combination of light attentuation at depth plus waves and whitecaps hinders the ability to find stable matching features in overlapping images for photogrammetry.

   

RMH Solution

• Aerial surveys were conducted over six time periods, each carefully planned according to tides, swell, winds, turbidity, cloud cover and sun angle. At the lowest tide, water column effects were minimized. Sea surface roughness decreased during low swells and light winds. To avoid sun glint, we planned flights when sun angle was between 30-45 degrees or during overcast sky conditions.

• Due to the lack of high contrast features, the image processing and georeferencing required significant manual editing to identify ground control points.

 

Results

• We produced six sets of georeferenced imagery at 5cm GSD, providing TNC with a time-series of GIS data that they used for tracking and quantifying their reef restoration efforts. 

• We also produced high resolution oblique aerial photos that have been used a visual aids for the media and community outreach.