"chart datum" meaning in English

{
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "chart datum",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Nautical",
          "orig": "en:Nautical",
          "parents": [
            "Transport",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "the level below which depths are indicated and above which heights of the tides are expressed; usually mean level of low water at a spring tide"
      ],
      "id": "en-chart_datum-en-noun-r5avMlEP",
      "links": [
        [
          "nautical",
          "nautical"
        ],
        [
          "level",
          "level"
        ],
        [
          "depth",
          "depth"
        ],
        [
          "height",
          "height"
        ],
        [
          "tide",
          "tide"
        ],
        [
          "spring tide",
          "spring tide"
        ]
      ],
      "raw_glosses": [
        "(nautical) the level below which depths are indicated and above which heights of the tides are expressed; usually mean level of low water at a spring tide"
      ],
      "topics": [
        "nautical",
        "transport"
      ]
    }
  ],
  "word": "chart datum"
}

{
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "chart datum",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English nouns with unknown or uncertain plurals",
        "en:Nautical"
      ],
      "glosses": [
        "the level below which depths are indicated and above which heights of the tides are expressed; usually mean level of low water at a spring tide"
      ],
      "links": [
        [
          "nautical",
          "nautical"
        ],
        [
          "level",
          "level"
        ],
        [
          "depth",
          "depth"
        ],
        [
          "height",
          "height"
        ],
        [
          "tide",
          "tide"
        ],
        [
          "spring tide",
          "spring tide"
        ]
      ],
      "raw_glosses": [
        "(nautical) the level below which depths are indicated and above which heights of the tides are expressed; usually mean level of low water at a spring tide"
      ],
      "topics": [
        "nautical",
        "transport"
      ]
    }
  ],
  "word": "chart datum"
}