Altimetry charts explained by Thomas Hilton: how satellites measure sea surface height to about two inches, why fish turn up in the depressions and along a warm current's stacked edges, and why deepwater rigs break the rule. Captain Clayt James shows how small boats and weekend trips use it.
How Altimetry Charts Improve Fishing Forecast
(00:07:34)
Watch Full Video
View Short Trailer
Instructor:
Thomas Hilton
Description
/
Review
/
Instructor
How do you read the contour lines on an altimetry chart?
Altimetry charts map sea surface height. Satellites bounce radar off the ocean, and the heights become a daily contour chart around mean sea level. Higher water marks warm currents and eddies; lower water marks upwelling. Read the lows, and read where the contours bunch tightly, because that's where the current is strongest.
Thomas Hilton compares it to a state trooper's radar gun. On his chart each contour is 5 centimeters, about two inches, measured across 60 to 100 miles or more, and each line of latitude is 60 nautical miles. Somebody noticed more fish in the dark blue depressions than on the bulges, and the rule stuck. The Loop Current is the big bulge. The counterclockwise eddy beside it is the kind of feature he wants to see drift closer in, while the yellows and oranges are downwelling. He won't fish the middle of the Loop Current; he fishes its edges where the contours stack up, out to about +15 centimeters. "That's where the push is going to be."
Thomas Hilton compares it to a state trooper's radar gun. On his chart each contour is 5 centimeters, about two inches, measured across 60 to 100 miles or more, and each line of latitude is 60 nautical miles. Somebody noticed more fish in the dark blue depressions than on the bulges, and the rule stuck. The Loop Current is the big bulge. The counterclockwise eddy beside it is the kind of feature he wants to see drift closer in, while the yellows and oranges are downwelling. He won't fish the middle of the Loop Current; he fishes its edges where the contours stack up, out to about +15 centimeters. "That's where the push is going to be."
Why do deepwater rigs hold tuna and marlin even in bad altimetry?
Altimetry assumes free-flowing open ocean current and bait that drifts with it. Deepwater structure breaks that assumption. Bait with a rig to hide around stays put when the water changes, and the predators stay with it, so poor altimetry doesn't empty a rig the way it empties open water.
Hilton has heard it for years: a rig sitting in bad altimetry still producing good tuna and some marlin. The spars, semi-submersibles and drill ships in 3,000 to 10,000 feet of water attract bait and fish, and "the bait didn't leave, so the predators didn't leave." His rule is simple. Fishing an open-water rip, pay close attention to the altimetry. Fishing the rigs, don't worry much about it, and read the current, the water color and any rips instead.
How do you project a cloudy weekend from several days of charts?
Look back over the days before the trip to see which way the water is moving and how fast. If clouds hide the last pass before you leave, project the trend forward, start where the water should be, and work back toward where it was until you find it.
Captain Clayt James runs that play out of Orange Beach. If the water has been moving south, he starts about 10 miles south of the day before and works back north; if it's been moving east or west, he adjusts by 20 miles. He has friends with small boats that can't run 150 miles, let alone 240 one way, who now start where the charts put them instead of searching. Before satellites, he remembers a pilot flying a single-engine plane with a VHS recorder, filming weed lines and handing out GPS numbers at the dock. "We didn't even know what altimetry was back then."
Inside the video
- How satellites measure sea surface height, and the contour scale
- Depressions versus bulges, and a warm current's edges
- Counterclockwise eddies and downwelling zones on the same chart
- The deepwater rig exception
- Projecting the water's position through cloudy days
- Captain Clayt James on small boats and the spotter plane era
Altimetry Chart Questions
Do clouds affect altimetry?
No. Altimetry uses radar, which isn't blocked by clouds the way SST and chlorophyll images are, so it still works on overcast days. The tradeoff is that it's one chart a day and shows height, not temperature or color, so it's read alongside the other layers.
What does +15 mean on an altimetry chart?
It's 15 centimeters above mean sea level, about six inches. Thomas Hilton fishes the edges of a warm current where the contours stack tightly, and says that productive edge can reach into the positive readings up to about +15.
Does altimetry work close to shore?
It's built for open water. Most altimetry charts are more reliable farther offshore and less useful in the first 20 to 30 miles off the beach. Closer in, temperature, water color and salinity usually tell you more about where the fish are.
Reading Altimetry in the Gulf of Mexico
The Loop Current makes the northern Gulf of Mexico one of the best places anywhere to learn altimetry, and how it shapes the fishing is in altimetry charts and the Loop Current in the Gulf of Mexico.
Login
to leave a review.
User Reviews
There are no reviews yet.Thomas Hilton
Thomas Hilton is the founder of Hilton's Realtime Navigator and the pioneer of satellite fish mapping for offshore anglers. Nearly two decades ago he turned newly available ocean data into a service now considered standard by bluewater tournament teams and top charter captains. In his In The Spread videos, Hilton teaches how to read sea surface temperature, chlorophyll, altimetry, salinity, and currents, and how to combine those layers and plan fuel-smart trips that put you on tuna, marlin, and other pelagics across the Gulf, Atlantic, and Pacific.
Read moreVideos
We Recommend
0