You've been reading altimetry wrong this whole time
Altimetry isn't a heatmap. It shows ocean structure that most AU fishermen never actually use. Here's what the layer actually means — and how to read it for offshore game.
Open any AU fishing forum and watch how people talk about altimetry. Half the posts treat it like a slightly fancier SST layer — “the red bit is warmer, troll there.” The other half ignore it because they don't know what the contours mean. Both groups are missing the entire point of the layer.
Altimetry doesn't show temperature. It doesn't even directly show where bait is. What it shows is the shape of the ocean — the bumps and dips on the sea surface, measured to centimetre precision by satellite. And that shape, more than anything else on the BiteCast map, tells you where the fish are stacking up.
This post is about reading altimetry the way oceanographers read it — and why doing so will outperform “chase the warm patch” on every single offshore trip you take.
What altimetry actually measures
Satellites equipped with radar altimeters bounce a pulse off the sea surface and time the return. Combined with the satellite's own precisely-known orbit, that gives them sea surface height to centimetre precision. They subtract the long-term mean (a 20-year average of sea surface height at every point in the global ocean) and what's left is sea surface height anomaly — SSHA — the deviation from normal at this specific point, today.
The unit on the BiteCast Altimetry layer is centimetres. A reading of +15 cm means the sea surface at that point sits 15 cm higher than the long-term average for that location. −10 cm means 10 cm lower.
A 15 cm sea-surface bulge over 100 km of ocean isn't something you'd ever notice in person — the water still looks flat. But that gentle bulge is enough to drive massive geostrophic currents around its edges, which is what makes altimetry so powerful for finding eddies.
Why height matters: the warm-water bulge
Warm water expands. A column of 25 °C tropical water sits physically higher than the same column of 18 °C cold water next to it — the molecules are spread further apart. So whenever the EAC pinches off a piece of warm water and sends it drifting south as an eddy, that eddy literally sits taller than the surrounding ocean.
That's why warm-core eddies show up as positive SSHA — the +5, +10, +20 cm contours on the Altimetry layer. And why cold-core eddies (typically formed by upwelling, where cold dense water has lifted from depth) show up as negative SSHA — the −10, −20 cm contours.
The map isn't showing temperature. It's showing the gravitational fingerprint of warm-water structures. The temperature is correlated, but height is what the satellite can actually measure precisely from orbit.
The thing nobody reads: contour spacing
Here's where most anglers go wrong. They look at altimetry, see a red blob (positive SSHA), and head for the centre of the blob. The centre of the blob is the worst possible place to fish.
Pelagic species don't hold in the warm-water core of an eddy. They hold on the edges — specifically, on the side of the eddy where the rotating water meets the surrounding cooler ocean. That's the convergence front: a temperature wall where bait gets compressed and pelagics ambush from below.
Altimetry shows you exactly where those edges are, but you have to read the contour spacing, not the colour. Tightly-packed contours = sharp height gradient = strong geostrophic flow = sharp temperature wall. Widely-spaced contours = gentle slope = no real front.
On the BiteCast Altimetry layer, every contour line marks a 5 cm change in SSHA. So:
- Five contours within 5 km — that's a 25 cm height change over 5 km, which means a powerful, fishable convergence front. Drop a pin on it.
- Five contours over 50 km — same range, but the gradient is 10× weaker. Probably not worth a deliberate troll across.
- Two parallel contour lines following the same path — that's the front itself. Run your spread along it.
Tight contour packing matters more than absolute SSHA value. A +15 cm eddy with sharp edges fishes better than a +25 cm eddy with diffuse edges. Always.
The hill analogy
Picture the eddy as a hill on an otherwise flat plain. The SSHA contours are the topographic lines on that hill — they trace levels of equal height. A +20 cm eddy is a hill that rises 20 cm above the surrounding ocean over a region 100 km across.
Now think about where the fish hold on that hill. Not at the foot — the foot of the hill is the surrounding flat plain (SSHA ≈ 0), where there's no compression, no gradient, no front. Not at the top either — the peak of the hill is a plateau of warm, slow-rotating water with bait dispersed across the broad rotating interior. Both the foot and the top are flat zones.
The fish hold on the steepest part of the ascent — the slope where the contours pack closest together, where the hill rises fastest. That's where the geostrophic flow is strongest, where the rotating water collides hardest with the surrounding ocean, and where bait gets compressed against a temperature wall.
And that's exactly what the separate Eddies layer measures — the rate of change in horizontal temperature per kilometre. The Eddies layer doesn't show the hill itself (that's Altimetry's job); it shows the steepness of the hill. Bright halos in the Eddies layer = steepest part of the ascent = pinch zone where bait stacks.
Altimetry shows you the hill. The Eddies layer shows you the steepest slope on that hill. Fish the slope — not the foot, not the summit. The pinch zone is on the side of the hill where the contours are tightest.
The convergence front (the zero line)
Sometimes a warm-core (positive SSHA) and a cold-core (negative SSHA) eddy sit right next to each other. The boundary between them — where SSHA crosses through zero — is the zero line, and it's the most reliable convergence front on the map.
In the BiteCast layer, the bold teal contour at SSHA = 0 marks this exact boundary. When two adjacent eddies are pushing water at each other, that zero line is where the two flows collide. Bait stacks on it. Predators patrol it. If you see a tight zero line cutting between a +20 contour cluster on one side and a −20 cluster on the other, that's a top-five offshore lane.
Reading rotation direction
In the Southern Hemisphere, geostrophic flow rotates around SSHA highs anticlockwise (opposite to what NH-trained anglers expect). That means:
- A warm-core eddy spins anticlockwise. Bait gets pushed onto its western edge as the rotating water collides with cooler surroundings flowing north on that side.
- A cold-core eddy spins clockwise. Less reliable for pelagic stack but the upwelled nutrient water can fire chlorophyll on the edges (worth a chlorophyll layer cross-check).
This is why “target the western edge of the warm-core eddy” is the AU offshore truism. It's not arbitrary — it's the geostrophic flow direction in our hemisphere.
If you ever read an old American sport-fishing book that says “target the eastern edge of the warm-core eddy,” ignore it. Their warm-core eddies rotate clockwise (Coriolis flips with hemisphere). For AU waters, it's always the western/southwestern edge of warm-core eddies that fishes best.
The three-layer confidence stack
Altimetry alone tells you where the structure is. To know whether the structure has actually produced a fishable front at the surface today, stack it with two other layers:
1. Altimetry → finds the structure
Use altimetry to identify the eddy, the western edge, the convergence front. SSHA changes slowly (eddies drift 8–12 km/day), so the layer is reliable for trip planning 1–3 days out.
2. SST → confirms the front exists at the surface
Switch to SST and look at the same area. Is there a sharp temperature gradient (1–2 °C over 5–10 km) along the western edge you identified? If yes, the eddy edge is producing a surface front today. If the SST is uniform across the eddy boundary, the front has temporarily relaxed and the bite will be off. See how to read SST for the edge logic.
3. Subsurface temperature → confirms the front extends below
Pelagic species are usually 30–80 m down, not at the surface. Use the BiteCast Subsurface temperature layer at 50 m and 80 m to confirm the front extends below the surface. If the surface front exists but the subsurface is uniform, the fish are spread out. If both surface + subsurface fronts align, you've got a vertical compression zone — top-confidence offshore lane.
When all three layers agree, you can plan the trip. When they disagree, look elsewhere.
The Eddies layer (a different thing)
BiteCast also has a separate “Eddies” layer that's commonly confused with Altimetry. They're related but show different things:
- Altimetry shows the structure — where the eddy bulges + dips are.
- Eddies shows the edge — the rate at which subsurface temperature changes per kilometre. Bright halos = sharp temperature gradient = literally the edge of the eddy at depth.
Workflow: open Altimetry to find which eddy you're targeting, then switch to the Eddies layer (set depth slider to 50 m) to see exactly where its temperature edge sits. Drop a pin on the bright halo. That's your trolling lane.
AU-specific patterns to recognise
The Sydney summer eddy field
In a typical NSW summer, the EAC has separated from the coast around Coffs and broken into 3–6 warm-core eddies drifting south + east through the Tasman. Some park 25 km off Sydney; others sit 80+ km out. Open Altimetry from October onwards and you can see the field setting up — useful for planning weekends and trip targets.
Annie Pippard (Sydney offshore charter operator) once described it as “Pac-Man chasing dots,” which is roughly right — you're tracking which eddy has the sharpest western edge and lining up a run to it before it drifts past.
The Bermagui canyon delivery
Far South Coast NSW gets EAC water as eddy fragments rather than a continuous current. When a warm-core eddy parks within 15 km of the Bermagui canyons (visible as a +15 to +25 cm SSHA bulge on Altimetry), the fishery fires — striped marlin, yellowfin, even occasional southern bluefin all stack on the eddy edge meeting the canyon shelf-break.
The Gold Coast EAC mainstream
Further north, the EAC is still a continuous current rather than fragmented eddies. Altimetry still shows the structure — the EAC mainstream itself sits 10–25 cm higher than surrounding water. Look for the western boundary of that elevated zone (where it transitions to neutral SSHA over a tight gradient). That's the EAC convergence front, and it's where mahi + marlin stack year-round.
Common mistakes
- Reading the eddy interior instead of the edge. The +25 cm centre is where the warm water is — but it's also where the bait isn't, because there's no boundary to compress it against. Edges, always edges.
- Ignoring contour spacing. A diffuse eddy with widely-spaced contours has weak edges and won't hold fish well, even if it's big and warm. Tight packing = sharp front = bait + fish.
- Treating altimetry as a heatmap. It's not. Two adjacent +15 cm eddies could be 18 °C and 22 °C respectively — you can't tell from altimetry alone. Always cross-check with SST.
- Planning on yesterday's eddy position. Eddies drift 8–12 km/day. The eddy you found on Wednesday is somewhere else by Sunday. Re-check before you launch.
- Forgetting hemisphere rotation. If you read US offshore content, the rotation directions are flipped. AU warm-core eddies rotate anticlockwise; bait stacks on the western edge.
What to ask the AI companion
Some prompts that work well for altimetry-driven planning:
- “Where's the closest warm-core eddy with a sharp western edge?”
- “Show me the steepest SSHA gradient within 80 km of Browns Mountain.”
- “Is the convergence front off Sydney still tight, or has it relaxed?”
- “Which eddy will be in range on Saturday given current drift?”
The companion reads the live altimetry data + the day's SST + subsurface stack and gives you a one-paragraph answer with coordinates. See how it works.
Three things to do tomorrow
- Open the Altimetry layer in BiteCast and find the warm-core eddy with the tightest contour packing on its western edge — not the one with the highest peak SSHA.
- Switch to SST. Confirm the front exists at the surface (sharp temperature break following the same line).
- Switch to Subsurface temperature, set the depth slider to 50 m. Confirm the front extends below. If yes, drop a pin on the western edge and run it.
That's the whole workflow. It takes 90 seconds in the app, and it will outperform “troll the warm patch” on every offshore session you fish.