Lake Depth at Your Actual Boat Position

A lake's maximum depth is not your depth.

If a lake reaches 24 metres in its main basin, that does not mean your boat is sitting over 24 metres. You might be on a shallow shelf, a channel edge, a soft slope, or outside the mapped coverage entirely.

That sounds obvious. A surprising number of fishing map experiences still blur the difference.

Gilli's lake-depth work is designed around a stricter idea: return the estimated depth at the actual point, not the lake's headline number.

Bathymetry Has Shape

A useful lake map is a surface, not a label.

Contours, shoreline constraints, source charts, georeferencing, and interpolation all build a depth surface. When you tap a position, the app should query that surface at the coordinate and return the sampled value.

That means two points inside the same lake should be able to return different depths. A point near shore should not borrow the basin maximum. A point outside water should not pretend coverage exists.

This is the practical difference between a lake profile and a working fishing map.

Why Anglers Care

Fish respond to structure.

Drop-offs, shelves, saddles, basins, weed edges, shallow bays, and thermocline-adjacent bands all matter. A species prediction that knows only the weather is incomplete. A map that knows only the lake maximum is incomplete too.

If Gilli can sample depth at a tap or live boat position, it can connect ecology and structure more directly.

The app can answer: what is the water doing here?

The Current Integration Direction

The architecture is intentionally simple for the app.

Gilli should ask one backend endpoint for point depth. The backend decides whether the coordinate falls inside a mapped lake. If it does, the backend queries the lake-specific bathymetry surface. If it does not, it can fall back to the existing marine bathymetry path or return no depth.

That keeps lake registry changes, map metadata, fallback behavior, and depth semantics out of the mobile UI.

The app gets one normalized answer.

Three Göteborg Lakes First

The first lake-depth scope focuses on the Göteborg-region lakes already moving through the mapping workflow: Rådasjön, Härlanda Tjärn, and Stora Delsjön.

The goal is not to fake coverage. A lake should only return point-depth when the published bathymetry package exists and passes the required checks. Until then, the registry can know about the lake without pretending sampled depth is available.

That distinction matters because trust is built on not making up depth.

Not Sonar

This is chart-derived bathymetry, not live sonar.

It reflects the source chart, georeferencing, interpolation surface, and publish quality. It can be extremely useful for planning, but it should not be presented as real-time depth measurement.

That honesty makes the feature stronger, not weaker.

The point is not to claim the lake bottom is known perfectly.

The point is to stop treating a complex lake as one number.