Render a Sailing Voyage from GeoJSON with a Map Snapshot API

A sailing club wants a quick image of each member's voyage for its newsletter. You have:

• voyage.json: a GeoJSON FeatureCollection exported from a GPS plotter. It may contain several features (for example a Point marking the home harbor), but exactly one feature has a LineString geometry: the voyage, in travel order. Positions follow the GeoJSON convention [longitude, latitude] and may include a third elevation value.
• A snapshot renderer HTTP service at RENDER_URL. You POST a JSON body describing the map, and the response body is the PNG bytes of the image (Content-Type: image/png). On bad input it returns a 4xx status with a short text explanation.

{
  "width": 800, "height": 600,
  "markers": [{"lat": 36.14, "lng": -5.35, "label": "S", "color": "green"}],
  "lines":   [{"color": "#d62728", "width": 3,
               "points": [{"lat": 36.14, "lng": -5.35}, {"lat": 36.144, "lng": -5.34}]}]
}

Note that the renderer wants lat/lng objects, while GeoJSON stores longitude first. Use only the Python standard library.

Part 1: Extract the track

Write load_track(path) -> list[dict] returning the voyage as {"lat", "lng"} dicts. Find the LineString by geometry type instead of assuming its index, and fail clearly if there isn't exactly one. Print the first point and the count.

Part 2: Render the route

Send the whole voyage as a single red line and save the response to voyage.png. Handle non-2xx responses with a useful error, check that you actually got a PNG, and write the file in binary mode.

Part 3: Markers and banded colors

Add an S marker (green) at the first point and an F marker (red) at the last. Draw the route as consecutive bands of 8 segments each, cycling through the colors #1f77b4, #ff7f0e, #2ca02c. A segment joins two consecutive points. Adjacent bands must touch, with no visible gaps.

Example

For a 20-point voyage starting at [-5.35, 36.14] and ending at [-5.16, 36.216]:

first: {'lat': 36.14, 'lng': -5.35} count: 20
segments per band: [8, 8, 3]
colors: ['#1f77b4', '#ff7f0e', '#2ca02c']
markers: S at (36.14, -5.35), F at (36.216, -5.16)

Constraints

• Tracks have up to 5,000 points. Assume the renderer accepts payloads up to 2 MB.
• Test your client without the real service by running a small mock HTTP server on 127.0.0.1.