Gravity Charts
The satellite gravity surveys are a tremendous aid to fishermen. The preparation and actual calculation of these is quite complex, but the theory isn't too bad. A satellite is put to circling the earth in a precisely known orbit. Plots are made of where the sea surface "should be" which take into account the Earth's flattened Poles and bulged Equator. The satellite bounces a long radar wave off the ocean, which measures the distance from the satellite to the ocean surface. With a known satellite position, and a known distance to sea surface, the height of the sea surface above or below the "should be" level can be calculated. If the ocean is "too high" it means that there is reduced gravitational force pulling the water surface down, if the ocean is "too low" it means that there is a strong gravitational force pulling down the sea surface. If there's a strong gravitational force, there's rock down there not far from the ocean surface. Rock, being denser than water, pulls harder and reduces the level of the ocean surface. If the ocean surface bulges up, there is a lot of water beneath the surface. These changes in sea surface height can range up to plus or minus 100 meters, but are not noticeable at the surface of the ocean because of the long very small angle slope, and waves.
The data from many different satellite passes is put together to give a picture of the levels of the whole ocean. Gravity forces at sea level are extrapolated from the sea surface height. Comparisons are made from different satellite passes at particular points to calibrate the information. Then huge amounts of date filtering and finally interpolation is done. The result of this process is an image of the seabed. The first image created is the estimate of absolute gravity at the sea surface. This reflects quite well the actual contours of the seabottom. Subsequent image products, after more mathematical adjustment and sometimes the fitting of trackline data, are versions of the estimated bathymetry of the seabed. Generally, the raw gravity data is best for the contours you actually fish, as the bathymetric products may lose a bit of surface contour accuracy to excessive "number crunching". But the bathymetric products with the depths filled in are best for determining if the area in question has depths which are correct for your target species.
Gravity based charts are not perfect, they will represent a steep thin pinnacle that rises close to the surface similarly to showing a large mass of seamount which does not rise so close to the surface. Even with that reservation, they are by far the best images available for most of the deep ocean bottom. Most of these charts available on the market are in very large scales such as, "The North Atlantic Ocean". At Marine Imaging Systems we prefer to make these charts for quite small areas, bringing out the detail, perhaps one half degree x one half degree. This zooming brings out the details of features such as seamounts and stretches of the continental shelve, and does a quite good job of making the contours known.
Download a Marimsys Satellite mGal Gravity Chart, or look into the other satellite based gravity images a Marimsys Satellite Bathymetric Chart. Learn more about our approach to charting, and if you would like to purchase these charts, check out our chart pricing.