Prospecting and fishing on mid-ocean rises and other unknown areas, is in a period of rapid change. There are a series of exciting new technologies available to reduce the risk of prospecting and to improve fishing yields while exploring new potential fishing grounds outside the EEZs. Charting is much cheaper than diesel fuel when it comes to getting an idea of the ocean bottom and deciding where to shoot.

Marimsys provides custom Gravity (mGals), Bathymetric and Temperature Charts from various topological, trackline, vessel and satellite orbital databases. We include some of these Charts with new BRIDGE systems to provide a plotting backdrop until the vessel can collect sufficient data to make into their own Marimsys Charts. Additionally, we create these satellite based Charts to order, to give clients a perspective on existing fishing areas, or when they are exploring new areas.

All plotting features of BRIDGE are available when using these Charts. When used aboard in the BRIDGE system while fishing, the vessel appears in its correct geographical position placed over the Chart. These Charts have become indispensable for trip planning because – all Waypoints, Catch Icons, Routes and Marks, can be plotted over all Charts in the system, of whatever type. This means that all information, including paper charts and multi-beam bathymetry, gathered in the course of planning a voyage can be brought together, calibrated, datum-adjusted, and analyzed in one system. Using these different types of Charts, and examining Waypoints and Marks made over one chart against the backdrop of another type of Chart is state of the art for exploring new areas, especially mid-ocean ridges and seamounts.

Gravity Charts

The image at the left is a Gravity chart, which is measured is mGal’s of gravity off the East Coast of New Zealand. Click on it to open it up because it is blurry at the reduced size.   While there is a strong correlation between mGal level and depth, the correlation is not perfect. These charts are made from the measurement of sea surface height from the Topex / Poseidon GEOSAT / ERM satellite missions.  They provide a very good estimation of mGals (milliGals) of gravity level, and generally provide the best representation of the seabottom for little explored areas. They have become indispensable for exploration trips to investigate new fishing grounds such as mid-ocean rises and seamounts.  Their weakness is that they will tend to represent similarly a sharp spire of great height coming close to the sea surface and a lower large rock mass, as each manifest similar gravity readings at the ocean surface.  Likewise, in shallow water the micro-effects of gravity are difficult to observe.  However, in our experience the contours represented by these charts are the closest you can get to the actual bottom contours without gathering the data to make Marimsys Charts.

When a client places an order for gravity or bathymetric charts of a particular area, the order itself and the area information for the charting is protected under the Marine Imaging systems security policy. 

Bathymetric and Track Line Data Charts

At the left is a bathymetric or topographic chart of Banco O'Higgins approximately 100 nm NW of Valparaiso, Chile.  The rise was almost unknown until recently when heightened interest in the Orange Roughy Fishery has made it a popular subject for investigation.  This image is not a particularly good copy of the original, which is in Marimsys file format - but it is probably the best cartography in existence of the Bank.  Going into this area to trawl or set a long line is far advanced with this bathymetry aboard.  You are far along in your analysis of where to place trial sets.

The accuracy of this dataset varies with the location of the data, however these images provide a good general background reference for the location of bottom features such as areas of suitable depth.  We recommend the use of both the mGal gravity and the  bathymetric charts aboard – there are differences.  Generally the Satellite mGal charts will have a better representation of bottom contours, while the Satellite Bathymetry provides an idea of the ranges of depths to be encountered.   We feel that it is better for the Captain on the scene to make the decisions as to which is more accurate.  Cartographers mathematically projecting bathymetry, with or without tracklines, tend to create as many errors as they smooth out. 

Marimsys Chart

Marimsys Charts are the charts we make from vessel data after the vessel has fished the area and collected data with the Marimsys BRIDGE system (or certain other types of plotter data). These charts can show contour lines down to 1-meter separation, and represent areas as small as 1 nautical mile square at a scale of approximately 1: 2500.  Security of both charting data and charts created from that data, is fundamental to private charting and are maintained securely by Marine Imaging Systems.

The satellite gravity surveys are a tremendous aid to fishermen. These are quite complex in actual calculation. What happens is that a satellite is put to circling the earth in an orbit that is mathematically precisely known. Plots are made of where the sea surface "should be" using mathematically flattened spheres - to imitate the Earth's bulge at the Equator. The satellite bounces a long radar wave off the ocean, which measures the distance from the satellite to the actual 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, which being denser than water, pull harder; and vice versa. These changes in sea surface height can range up to plus or minus 100 meters. Bumps like this are not noticeable at the surface of the ocean due to the long distances over which they occur - e.g. if you're going up a 15 meter hill that has a 150 kilometer hillside, you won't know it - especially if there are 5 meter waves washing through.

When the amplitude of these gravity forces are examined and huge amounts of interpolation and filtering is done, an image of the force of gravity at sea level can be created. 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 perhaps 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 and trackline may lose a bit of surface contour accuracy to excessive "number crunching".   The problem is very specifically with a good gravity plot over an area 500 miles by 500 miles, and 3 tracklines crossing this area - which data do you believe?  Where? Generally the bathymetric products which have been checked with trackline data, to verify cross-sections, but without the mathematical addition of the trackline data, 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, "micro gravity charting", bringing out the detail - perhaps areas of one half degree by one half degree. This zooming brings out the features of the seamount and does quite a good job of making its contours known e.g. it saves a lot of steaming around time looking at the echosounder contemplating the run of the first trial set.

The absolute best fishing charts are the Marimsys Charts made directly from data recorded by the vessel when it is fishing. With these charts it is possible to see down to the level of 1 or 2 meter obstacles. As such, it is possible to trawl the top of a seamount, the side of a ledge, or dodge around a "stubbie", directly from the chart - visually selecting the route. In the context of exploratory "first time" fishing, there is the opportunity to gather data, which will be extremely useful to make charts, should the vessel, or sister vessels return to the exploratory area. We sometimes place a charting technician aboard when the costs of the exploratory voyage, and the potential benefits justify it.