We Digitised 70 Years of Lake Maps So You Don't Have to Guess Where the Deep Holes Are

Somewhere in a Belgian archive, there's a hand-drawn depth map of your favourite lake. We found it.

It was sitting in a cardboard folder, filed between a water chemistry report from 1971 and a typewritten note about pike stocking. The paper is thick and slightly yellowed. The contour lines are drawn in ink -- some freehand, some ruled with care. Depth soundings are written in pencil at odd angles, each one a number that someone earned by rowing to that spot, lowering a weighted rope, and writing down what they measured.

Nobody had looked at it in decades.

Who Made These Maps

Between the 1950s and the early 2000s, government agencies, fisheries boards, and university researchers across Europe surveyed thousands of inland lakes. The work was unglamorous. A team would go out in a small boat -- sometimes over several days -- and take depth soundings along transects. They'd plot the results by hand onto thick card stock, drawing contour lines the way a cartographer draws a mountain, except the mountain was upside down and underwater.

In Sweden, the limnological surveys were meticulous. Scandinavia has more lakes than almost anywhere on Earth, and the Swedes treated each one as a small scientific project. The resulting depth cards are beautiful objects -- precise, hand-lettered, sometimes annotated with notes about bottom substrate or vegetation. Each one is a portrait of a lake's hidden shape.

In Belgium and the Netherlands, the story is different but the outcome is the same. Waterway authorities, provincial fisheries services, and the occasional PhD student measured ponds, reservoirs, and canal-connected lakes that thousands of anglers fish every weekend. Those measurements ended up in filing cabinets, departmental archives, and occasionally someone's attic.

The data existed. It just wasn't anywhere useful.

What Was Lost

Here's the thing about a depth map drawn on paper: it helps exactly one person at a time, and only if they know it exists.

A depth card sitting in a provincial archive in Antwerp doesn't help the angler standing on the bank of that lake in Limburg. A Swedish survey filed in a university library in Uppsala doesn't help the weekend fisherman at the lake outside Gothenburg. The information was gathered, recorded, and then effectively lost -- not destroyed, just inaccessible.

And what information it is. These maps show you the shape of the world you can't see. Where the shallows end and the drop-offs begin. Where a 3-metre flat suddenly plunges to 15. Where the deep holes sit -- the cold-water refuges where fish stack up when the surface heats in July. Where the saddle between two basins creates a natural funnel, a migration corridor that pike and perch have been using for longer than anyone has been fishing there.

This is the kind of knowledge that used to take years of experience on a single lake to build. Or a fish finder and a lot of patience. Or knowing someone who knew someone who'd seen the old survey. Most anglers never had access to it at all.

What We Did About It

We went looking.

We contacted archives, libraries, government agencies, and fisheries departments across Scandinavia, Belgium, and the Netherlands. We asked the same question everywhere: do you have old depth surveys of inland lakes? The answer, almost always, was some version of "yes, but nobody's asked for them in years."

Then we digitised them.

Every contour line, every depth sounding, every pencil marking on every card -- we turned the paper into data, and the data into maps you can actually use. Live depth maps, on your phone, for lakes that haven't been properly mapped in the digital era.

Some of these lakes have never had a publicly available depth map before. The information existed, locked in a filing cabinet. Now it exists in your pocket.

Why This Matters More Than You Think

Depth data doesn't just tell you how deep a lake is. It tells you how a lake works.

A lake with a uniform 4-metre depth fishes completely differently from one with a 4-metre average that's actually 2 metres on the edges and 12 in the centre. The first is a bathtub. The second has structure -- thermal layers, oxygen gradients, feeding zones, and refuge areas. The fish in the second lake have a strategy. If you can see the depth map, you can read that strategy.

The old surveyors understood this. That's why they went out in their rowing boats with their weighted ropes. They knew that the shape of the bottom explains the behaviour of everything above it. They just didn't have a way to put that knowledge into the hands of the people who needed it most.

We do.

The Expedition Continues

We're not finished. Not even close.

The archive work is ongoing across three countries. Every week, more lakes go live -- depth maps that haven't seen daylight since someone filed them away in the last century. Some of these lakes are well-known. Some are the kind of place only locals fish. All of them have a shape underneath that most people have never seen.

We're starting with the lakes that anglers actually fish. The suburban lakes outside Gothenburg. The gravel pits in Flanders. The polders and reservoirs in the Netherlands. The forest lakes in Smaland. If people are casting lines there, we want them to see what's underneath.

See the Bottom

Open Gilli. Tap a lake. See the bottom.

No guessing. No asking around. No hoping that someone measured it once and that the information survived. It did survive. We found it. And now it's yours.

More lakes are going live every week. If there's a lake you want mapped, tell us.