Altimetry = the height of the sea surface compared with mean sea level.

• "Upwellings" (areas below mean sea level) bring nutrients from the deep to the surface

• "Neutral areas" (between high/low spots) is where the food chain will have time to develop — attracting targeted species

• Cloud cover isn't an issue
• Use Altimetry/SSH data in conjunction with other overlays & imagery to identify hotspots

Relatively small changes in sea level can ultimately influence the feeding patterns of big game fish. Modern satellites orbiting the Earth can accurately measure ocean surface height, to within a few centimeters.

The surface of the ocean isn't flat - there are high spots and low spots. By studying the latest altimetry imagery you can location areas of upwelling and downwelling and the location of ocean current features and eddies.

Once a day, we receive data from satellites providing contour lines we can overlay on a map.

Cloud-cover doesn't affect altimetry measurements in the same way it affects other types of satellite imagery. The altimeter works by sending out a microwave pulse, bouncing it off the ocean's surface and measuring the return signal.

An upwelling is a phenomena where water from the deep ocean rises to the surface. This cold, deep ocean water is densely packed with nutrients and when it rises to the surface and is hit by sunlight, microscopic plants (phytoplankton) and animals (zooplankton) are able to grow quickly. Squid and baitfish feed on these microscopic animals, and in-turn attract larger pelagic species.

Upwellings can be identified by areas on the SSH overlay that are lower than surrounding areas. It's a bit counter-intuitive, but basically water in these "low spots" is rising to the surface in an effort to achieve equilibrium.

Generally speaking the best areas to fish are in the intermediate zones between areas of upwelling and areas of convergence. Use altimetry data to find areas of upwelling, downwelling and the convergence zones in between that often hold fish. Look for a thicker "0" line! These neutral areas - between the highs and away from the center of the low - is where the food chain will have time to develop.

Because of the Coriolis effect, currents will flow clockwise around areas where the SSH is higher in the Northern Hemisphere, and anti-clockwise around a high in the Southern hemisphere.

The real secret - if there is one - is to understand how the altimetry image you see is created and how that will affect the data's accuracy. Once you understand the data sources' limitations you will be able to understand when and how to use it properly.