Analysing an anchorage with Solano

1. What the anchorage analysis does

Finding a good anchorage means solving several questions at once: does the swell enter this cove? Does the wind come from a sheltered direction? Is there enough depth? And tonight, will it stay calm?

Solano's anchorage analysis answers these questions in one click. It combines:

2. Starting the analysis

3. The directional protection rose

The rose is made of two concentric rings, each evaluated for 36 directions (every 10°):

• Outer ring — wind: is the spot sheltered or exposed to wind coming from this direction? Colour according to the level of protection.
• Inner ring — swell: can swell coming from this direction enter the anchorage? Green = blocked, red = not blocked.

Colour palette

The direction-of-origin chevrons

On each ring, one or two chevrons indicate the direction of the main threat: the chevron's point faces the centre of the rose, meaning "the threat comes from there". It's not the direction the wind is blowing towards — it's where the wind or swell comes from.

The coverage sectors

Below the rose, the sheltered and exposed sectors are summarised as arcs. An arc can cross north: "280–60°" indicates coverage that passes through 360°, i.e. the shortest arc between 280° and 60° clockwise.

4. The 3-day comfort score

The comfort chart shows, hour by hour over 3 days, the predictable quality of the anchorage as a percentage between 0 % (very difficult conditions) and 100 % (dead calm).

How the score is computed

For each hour, Solano combines two factors:

Swell weighs 70 % in the score. At anchor, it's swell that determines pitch and roll — wind alone doesn't make a boat roll if the sea is flat. The 30 cm (Hs) threshold reflects that a short chop starts to be noticeable at anchor from 20–30 cm.

Reading the chart

On hover (or on tap on mobile), the bar shows:

• The direction and speed of the dominant wind + the gusts (e.g. 💨 NNW 334° · 12 kt · gust 18 kt)
• The significant swell height and its direction of arrival (e.g. 🌊 NW 308° · 0.8m Hs)

5. Depth and seabed type

Depth — EMODnet Bathymetry

Depth comes from EMODnet Bathymetry DTM 2022, the most complete European bathymetric database, with a native resolution of about 115 metres. The API returns three values for the nearest pixel:

• The smoothed value — the best point estimate (shown large)
• The min/max range — depth variability within the cell (useful for areas with marked relief)
• The sounding type — indicates the accuracy of the measurement: "precise sounding" (multi-beam), "estimated sounding" (single-beam), "LiDAR", "certified source"

Seabed type — EMODnet Seabed Habitats

The detected substrate is shown with an icon indicating the anchoring practicality:

• 🟢 Sand — excellent holding for most anchors
• 🟢 Mud — generally good holding, sometimes clinging on retrieval
• 🟡 Rock — uncertain hold depending on the profile, risk of fouling
• 🔴 Posidonia seagrass — anchoring prohibited (a protected species in the Mediterranean, fines apply)

6. The algorithm behind the scenes

The protection profile is computed by topographic ray casting — a technique borrowed from video games and robotics, applied here to coastal geography.

The ray-casting principle

For each direction (36 directions × 3 slightly spread rays in a ±5° fan), Solano casts rays from the anchorage point and measures the terrain height at 16 progressive distances: 25, 50, 75, 100, 150, 200, 300, 400, 500, 650, 800, 1000, 1200, 1500, 2000 and 3000 metres. That's 1,728 measurement points per anchorage.

These elevations come from the SRTM 30 m digital terrain model (global coverage), which returns 0 over the sea — meaning "no obstacle", exactly what the algorithm is looking for.

Breakwaters and harbour works

A 30 m terrain model has a blind spot: it is blind to narrow harbour works. A mole 10 to 20 m wide is too thin to be "seen" — it's read at water level, as if it didn't exist. Yet it's precisely the breakwater that protects a harbour anchorage.

Solano fills this gap with OpenStreetMap: around the point, it retrieves breakwaters and jetties (man_made=breakwater) as well as the coastline (natural=coastline) — because the solid moles of harbours are often mapped there as coast. Each ray then tests its geometric intersection with these structures (not a simple sampled point, which would miss a thin mole the way the terrain does). Any coast or breakwater encountered blocks the swell; a breakwater also adds wind protection. This input is purely additive: it reinforces protection where the terrain alone was blind, without ever removing any.

Wind protection

For each ray, wind protection is estimated by the H/D ratio: an obstacle of height H at distance D creates a shadowing angle of arctan(H/D). Above a ratio H/D > 0.20 (a 20 m wall at 100 m), protection is considered maximum.

Swell protection

Swell obeys different rules. A rock breaking at 50 m doesn't stop a 2 m swell — but a cliff at 200 m does. Solano applies an adaptive height threshold according to distance:

• Obstacle < 200 m → threshold 0.5 m (a nearby rock blocks even chop)
• Obstacle < 800 m → threshold 2 m (a concrete coastal obstacle)
• Obstacle < 2,000 m → threshold 5 m (headland or notable relief)
• Obstacle ≥ 2,000 m → threshold 15 m (major relief — hill, mountain)

This gradation lets it detect both small coves protected by nearby rocks and large bays protected by headlands 2 to 3 km away — like the bay of Portoferraio on Elba, whose flanking headlands are about 2 km from the anchorage.

7. What to keep in mind

• The protection profile is static. The land topography doesn't change, but the weather conditions and the swell height can make an anchorage uncomfortable even in a "sheltered" sector if the swell is very steep. Always cross the rose with the comfort score.
• Water is not modelled. The ray-casting rays pass through the terrain — they don't see the underwater reefs, shoals or sandbanks that can attenuate the swell. Real protection can be better than the algorithm predicts for anchorages with lots of shallows.
• Bathymetry at 115 m doesn't replace nautical charts. It gives an order of magnitude. In areas with complex underwater topography (passes, headlands, pocket anchorages), use the SHOM or Imray charts as a complement.
• Bay resonance is not computed. A bay can amplify certain swell periods by resonance even if it is geographically well closed. This phenomenon depends on the geometry of the water, not the terrain.
• The weather models have their own uncertainty. The comfort score is only as good as the swell forecast that feeds it — an underestimated Hs will give a score that's too optimistic. Check the stability score to assess the accuracy of the current swell forecast.