Baselines of the territorial sea
Learn how the Canadian Hydrographic Services (CHS) defines the seaward limits of a coast (baseline).
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Types of baselines
CHS determines normal baselines using points along the low-water line of the coast.
We draw straight baselines by joining points along a:
- deeply indented coastline
- border (fringe) of islands along the coast
We can use a combination of both methods to draw lines if needed.
Normal baseline points are called 'normal' because they follow the natural coastal landscape.
They're most often located on low-tide areas, which:
- drying sandbanks
- the limit of the low-water line
- are above water at low tide
- lie within 12 nautical miles of the coast or major islands
Normal baselines can also use lines from elevations that aren't low tide, such as:
CHS charts show where these low-water line points are located. The charts' vertical datum is the lower of either the low-water line from large tides or the lowest normal tide.
This 0 level of the charts applies to water depths and is referred to as chart datum.
All water depth measurements collected during hydrographic surveys are reduced to the chart datum before being included in the chart.
Canada's coastline is irregular, so CHS uses 'straight' baselines to:
- close legal bays and mouths of rivers
- preserve the general trend of the coastline
- define a fixed limit in a very dynamic environment (like a delta)
Straight baselines can only be drawn from areas that support a permanent construction above water, like a lighthouse. This includes:
- low-water elevations
Lines joining the turning points of straight baselines are geodesics (the shortest distance between 2 points on the surface of the Earth).
Charts and maritime enforcement
CHS takes care of the baseline data in our surveys and charts. However, changes to the baseline of the territorial sea can only be made on the recommendation of the minister of Global Affairs Canada. The territorial sea baseline is published in regulations made by the governor in council pursuant to the Oceans Act.
Our nautical charts are often used in court cases involving violations of Canadian laws in sovereign waters. Because CHS charts establish the territorial sea baseline and show Canada's maritime limits:
- courts of law accept them as evidence
Canadian law enforcement agencies rely on them to apprehend ships in violation of Canadian maritime law
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