EncDotNet.S100.Datasets.Cap

EncDotNet.S100.Datasets.Cap reads marine weather warnings in the OASIS Common Alerting Protocol (CAP 1.2), the format the US National Weather Service and many other alerting authorities publish them in, and turns each alert's areas into outlines a chart can draw. It reads:

one CAP 1.2 <alert> document;
an Atom feed of alerts, whether each entry holds a whole <alert> or, as api.weather.gov/alerts.atom writes them, the alert's fields as cap: elements of the entry;
the NWS GeoJSON form from api.weather.gov/alerts, one alert feature or a feature collection.

It depends on EncDotNet.S100.Core and EncDotNet.S100.Portrayals. The pipeline processor that draws it, CapDatasetProcessor, is in EncDotNet.S100.Datasets.Pipelines.

Not for navigation. CAP warnings aren't S-100 products or official navigational warnings. Hosts label them so wherever they appear.

The design is in Wind and wave data, §6.1.

Install

dotnet add package EncDotNet.S100.Datasets.Cap

Read a file and see what's in force

using EncDotNet.S100.Datasets.Cap;

var document = CapReader.Read("alerts.atom");    // CAP XML, Atom or NWS GeoJSON
var warnings = CapWarningSet.Build(document);     // zone codes resolved

foreach (var area in warnings.DrawnAt(DateTimeOffset.UtcNow))
{
    Console.WriteLine($"{area.Info.Event} ({area.Info.Severity}): {area.Area.Description}");
    Console.WriteLine($"  {area.ShapeSource}, {area.Surfaces.Count} surface(s)");
    if (area.UnmatchedZoneCodes.Count > 0)
        Console.WriteLine($"  zones not found: {string.Join(", ", area.UnmatchedZoneCodes)}");
}

Each CapWarningArea is one CAP area of one alert:

Outline. The area's <polygon>s; its <circle>s as 64-vertex polygons (CapGeometry.CircleToPolygon); or, for an area with only zone codes (the UGC geocodes, for example ANZ535), the NWS marine zones it names, from MarineZoneLayer.Default. A code the layer doesn't have is kept in UnmatchedZoneCodes, not dropped; an area with no outline at all isn't drawn but is still listed.
Time. In force from onset (else effective, else sent) to the later of expires and the hazard's end the NWS gives as ends (GeoJSON) or the eventEndingTime parameter (CAP XML). The NWS sets expires to when the message should be replaced, which can come before the hazard starts. The status uses WarningValidity, the rule S-124 navigational warnings use, so both start and end at the same instant. An Update or Cancel in the same file ends the alerts it references from when it was sent; a Cancel isn't drawn itself.

The marine zone layer

MarineZoneLayer.Default is the NWS coastal and offshore marine zone shapefiles (mz16ap26, oz16ap26; 699 zones), simplified once to about 500 m and bundled as GeoJSON (1.6 MB). It's a US Government work in the public domain. tools/cap rebuilds it. A host with a newer release can load its own with MarineZoneLayer.Load(stream) and pass it to CapWarningSet.Build.

CodesIn(south, west, north, east) lists the zones whose outline reaches into an area. The Library uses it to ask api.weather.gov for the alerts of the map view's zones.

Drawing

CapStyle builds the drawing instructions with the code-defined portrayal toolkit: each area hatched and outlined by severity, and labelled with its event:

Severity	Outline	Hatching
Extreme	violet, 0.9 mm	crossed, every 2 mm
Severe	red, 0.7 mm	every 2.5 mm
Moderate	orange, 0.5 mm	every 4 mm
Minor	ochre, 0.4 mm	every 6 mm
Unknown	grey, 0.4 mm dashed	every 6 mm

Labels are ordinary text instructions, so they follow the text scale and the renderer's label declutter, and aren't drawn at scales smaller than 1:3,000,000. Day, Dusk and Night palettes are in CapStyle.Palette.

Recognising CAP