How To Find Hidden Reefs
Kalukalukuang Group - Landsat Image (S-50-05_2000)
I am currently mapping the above group of islands. They looked interesting. Also, their images were very sharp making the map making process relatively easy. The challenge when mapping an atoll is always the reef system. My early effort at reef mapping was to create a polygon that encompassed all visible reefs and call it EVS-Reef. Now I use a complex series of layers, a minimum of 10 different reef layers, that are often readily discernable.
Kalukalukuang Group - Map
I have created two of the island maps for this group. The other islands in the group are shown on the above map as WVS red outlines. The two islands have detailed renderings of their respective reef systems that are discernable from the Landsat image. (See previous post for Palau Kalukalukuang ID).
Kalukalukuang Group - Map (AMS SB 50-7 Series T503)
However, I was curious after studying the AMS map of the Kalukalukuang Group. I saw a much larger reef system that was not too far beneath the surface of the water. Where was this reef system using Landsat imagery. Global Mapper gives you options to create effects on your images that can sometimes allow you to see what was previously not visible. The initial effect is always "No Blend". It is how I typically work when digitizing shorelines. I have used the other effects (16 different ones) and have varied the darker-to-lighter effect as well. I still prefer "No Blend" for almost all of my projects. But what about those reefs? Where were they?
Kalukalukuang Group - Map Reliability (AMS SB 50-7 Series T503)
The AMS map relied on aerial photos to construct their original map. The bulk of their work was based on imagery gathered between 1922 through the 1950s. Using that imagery they showed outlines of the reefs. But it appeared Landsat didn't find them.
Kalukalukuang Group - Image Effect Color Dodge
I tried an effect called "Color Dodge" and the following showed up. The clouds show up as bands flowing up or down the image. However, the curious yellow signatures were part of that hidden reef. In fact, it is the reef system and it shows up clearly using the "Color Dodge" effect.
Kalukalukuang Group - Comparison of Effects
The final image shows the same area using both effects (Top - No Blend; Bottom - Color Dodge). So the hidden reefs were unhidden. My future maps of atolls will require that I employ the "Color Dodge" or "Color Burn" effects to display and map any hidden reefs.
