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Salas y Gómez Expedition: Locating an Island Via Satellite

Marine ecologists use a new satellite image of Salas y Gómez and existing imagery of Easter Island to plan their dives and plot their data–and to correct long-standing cartographic errors in the placement of the former island.
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Alan Friedlander imports a high-resolution satellite image of Salas y Gómez into the GIS on his laptop and uses the coordinates of island landmarks from Google Earth to position the image.
By Alan Friedlander, U.S. Geological Survey and University of Hawaii
I’m glad we’re here now rather than in 2010, as originally planned, because if you looked on Google Earth last year, there was just a black square around the location of Salas y Gómez.
When we originally prepared to mount the expedition (before the devastating Chilean earthquake in March, 2010), I downloaded what imagery I could find–a really coarse Landsat image. Salas y Gómez is so small that by the time I got the resolution to where I could try to plot potential survey points, the image was so pixilated. It looked like a bunch of squares and you could hardly make the island out at all. So it’s been incredibly useful having a brand new, high-resolution satellite image courtesy of Google Earth and GeoEye.
We can actually make out lots of details in the image, both on land and in the shallow water surrounding the island. We can see some of the offshore, submarine features, such as the arch we dove on south of Salas y Gómez. It’s been helpful first in deciding where to sample, and then after the fact in re-projecting our sampling data back onto the image to give an idea of which places hold higher biodiversity, which places have higher fish biomass, which places are more productive than others. And it helps us do what scientists like to do with their data: Explore possible explanations for why things are the way they are.
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Breaking waves south of the island reveal the location of “The Arch,” one of the team’s favorite dive sites at Salas y Gómez.
Salas y Gómez is a little speck in a big ocean, but the environment around it has somewhat different geographies, different physiographies and different exposures. Seeing this and exploring the environments digitally on our laptops as we explore them physically in the water has been incredibly helpful.
Back here at Easter Island/Isla de Pascua/Rapa Nui as well, we’re using the high-resolution imagery we cached ahead of time in Google Earth both to choose our sampling locations around this island and then to project our data back in geographic space afterward. It lets us make geographic comparisons, not just between the two islands (which are 400 kilometers apart), but also within a ring of sites around this island.
We’re seeing different assemblages of fish and coral and algae on the north side of Easter Island versus the west side versus the south side. The south side is very exposed. The north side tends to have the only (what we’d call) “true” coral reefs.
The reefs around Easter Island and Salas y Gómez are reef communities but not really coral reefs. Even though the coral cover is often quite high–(40-percent-plus is my estimate) it’s really just corals growing on top of the volcanic rock substrate. It’s a veneer of coral on top of the underlying rock, not on the skeletons of prior corals as in a true coral reef.
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Tide pools visited by the team contrast with black basalts in a satellite image of Salas y Gómez.
Some geologists were here in 1997 and ’98. They looked all around the island and did a lot of coring, and they found that the only place that had true coral reefs was off the northwestern portion of the island, which we sampled on our first dive yesterday. That was pretty exciting. We were looking at a place where there’s actually been reef development forming on top of itself over many hundreds of years.
We use geographic information systems (GIS) and global positioning satellites (GPS) to explore marine community differences in geographic space. On Easter Island, reefs on the north side get more wind exposure, but much less wave exposure. The dominant wave energy at Easter Island comes from the south, from Antarctica. So the south side gets the full force with swells starting right about now. (The pounding will continue on throughout the austral winter.) The west side of the island is a mix of exposed and non-exposed sites, and the north side is relatively protected leading to a diverse set of coral and fish communities throughout.
The older maps we and many others had been using didn’t have the island in the right place! And this happens often. In Hawaii, where I live and do much of my work, many of the Northwestern Hawaiian Islands were off on the maps by several kilometers until quite recently.
New satellite imagery of Salas y Gómez allows us to be exact. When we took the very accurate GPS points we recorded at our Salas y Gómez sampling sites and projected them back on the existing maps we had been working with, there was no island. But when I took screen shots of the new satellite image in Google Earth, imported them, and used coordinates for specific features to anchor them the island lined right up with our sampling sites.
It should be helpful to the Chilean government as well. The island is a little further west than was depicted on some of the maps that were circulating when the new marine park’s boundaries were drawn. The island is even closer to the western fishing boundary–already very close to the island because of a carve-out made for fishing vessels from Easter Island. This has important implications that go beyond the science.
Photo by Ford Cochran
The science team will share frequent updates and media from the expedition, including photographs, videos and links to Google maps, here on the National Geographic News Watch blog. You can also follow the expedition on Google Earth by clicking on the blue ship icon located where the expedition begins near Easter Island, roughly 2,000 miles (3,300 km) northwest of Santiago, Chile. (Make sure the “Places” layer is turned on).
National Geographic and Oceana are members of Mission Blue
View all dispatches from the Salas y Gómez expedition here