Table of Contents

Architecture: ENC Standards and API Layers

Electronic Navigational Charts (ENCs) are built on a stack of international standards. EncDotNet mirrors this layered architecture, with each library mapping to a specific standard layer.

Standards Stack

┌─────────────────────────────────────────────────────────────┐
│                    S-52 (Display)                            │
│         Chart symbology, colors, and display rules          │
│                                                             │
│  "How should a depth area be colored?"                      │
│  "What symbol represents a lateral buoy?"                   │
├─────────────────────────────────────────────────────────────┤
│                    S-57 (Data Model)                         │
│     Features, attributes, topology, and spatial records     │
│                                                             │
│  "This is a depth area (DEPARE) with min depth 5m"          │
│  "This buoy (BOYLAT) is at position 37.2°N, 76.1°W"        │
├─────────────────────────────────────────────────────────────┤
│                 ISO/IEC 8211 (Container)                     │
│       Binary encoding format for structured records         │
│                                                             │
│  "Record 42 has 6 fields; field 'VRID' is 5 bytes"          │
│  "The DDR defines field structures and subfield formats"    │
└─────────────────────────────────────────────────────────────┘

Each layer builds on the one below it, and the EncDotNet libraries follow the same pattern.

How the Libraries Map to Standards

Standard Library What It Does
ISO/IEC 8211 EncDotNet.Iso8211 Reads the binary container format — records, fields, and raw byte data
IHO S-57 EncDotNet.S57 Interprets the data model — features, vectors, attributes, and topology
IHO S-52 (ChartViewer app) Renders charts with symbology and display rules (not a library)
NOAA Products EncDotNet.Noaa Accesses the NOAA ENC product catalog for chart discovery and download

ISO/IEC 8211 — The Container Format

ISO 8211 is a general-purpose binary format for encoding structured data. It is not specific to nautical charts — it is used by several geospatial standards. An ISO 8211 file consists of:

  • A Data Descriptive Record (DDR) — a schema record that defines the structure of all fields in the file, including subfield names, types, and sizes
  • One or more Data Records — each containing a leader, a directory of field entries, and the field data itself

EncDotNet.Iso8211 provides two ways to read this format:

  • Iso8211Reader — a high-performance, forward-only token reader (ref struct) that emits tokens like StartRecord, DirectoryEntry, Field, and EndRecord with zero intermediate allocations. Supports streaming for large files.
  • Iso8211DocumentReader — builds a complete in-memory Iso8211Document object model containing all records and fields.

At this layer, you see raw tags like "DSID", "VRID", "SG2D" and byte arrays — the data has no nautical meaning yet.

IHO S-57 — The Nautical Data Model

S-57 defines how nautical chart data is encoded within ISO 8211 files. It specifies:

  • Features — real-world objects like depth areas (DEPARE), buoys (BOYLAT), lights (LIGHTS), and soundings (SOUNDG), each identified by an object code (OBJL)
  • Attributes — properties of features, such as minimum depth, light color, or buoy shape
  • Spatial records — geometry stored as isolated nodes (points), connected nodes, edges (polylines), and faces (polygons)
  • Topology — chain-node topology linking features to spatial records via pointers (FSPT), and edges to nodes (VRPT)
  • Coordinate encoding — positions stored as integers that must be divided by the Coordinate Multiplication Factor (COMF) to get decimal degrees

EncDotNet.S57 provides two levels of abstraction:

  • S57DocumentReader → S57Document — parses an ISO 8211 file into an S-57 domain model with typed feature records, vector records, dataset metadata, and coordinate conversion helpers
  • S57Chart — a higher-level model that categorizes features by geometry (point, line, area, meta) and indexes spatial records by type (isolated nodes, connected nodes, edges, faces), with lookup methods and reverse-pointer indexes

NOAA ENC Products

NOAA publishes an XML product catalog listing all available U.S. ENC cells with metadata like chart name, scale, coverage, and download URLs.

EncDotNet.Noaa provides EncProductCatalogClient to download and deserialize this catalog into typed objects (EncProductCatalog, Cell, Coverage, etc.).

Data Flow

A typical workflow follows the standards stack from bottom to top:

  .000 file on disk
        │
        ▼
  ┌─────────────────┐
  │  Iso8211Reader   │  Tokenize binary data
  │        or        │
  │ Iso8211Document- │  Build record/field
  │    Reader        │  object model
  └────────┬────────┘
           │ Iso8211Document
           ▼
  ┌─────────────────┐
  │ S57Document-     │  Parse S-57 semantics:
  │    Reader        │  features, vectors,
  │                  │  attributes, topology
  └────────┬────────┘
           │ S57Document
           ▼
  ┌─────────────────┐
  │   S57Chart       │  Strongly-typed chart:
  │                  │  point/line/area features,
  │                  │  spatial indexes
  └────────┬────────┘
           │
           ▼
      Application
   (display, analysis,
    export, etc.)

Each step adds meaning to the raw bytes:

  1. ISO 8211 turns a byte stream into records and fields
  2. S57DocumentReader turns fields into features, vectors, and metadata
  3. S57Chart organizes features and spatial records into a navigable, strongly-typed model

Choosing the Right Layer

Goal Use
Parse non-S-57 ISO 8211 files Iso8211Reader or Iso8211DocumentReader
High-performance scanning of chart files Iso8211Reader (token-level)
Read chart features and metadata S57DocumentReader → S57Document
Work with typed geometry and indexes S57Chart.FromFile()
Discover and download NOAA charts EncProductCatalogClient
Apply incremental chart updates (.001, .002, …) S57Document.ApplyChanges()
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