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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:

We can use a combination of both methods to draw lines if needed.

Normal baselines

Normal baseline points are called 'normal' because they follow the natural coastal landscape.

They're most often located on low-tide areas, which:

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.

Straight baselines

Canada's coastline is irregular, so CHS uses 'straight' baselines to:

Straight baselines can only be drawn from areas that support a permanent construction above water, like a lighthouse. This includes:

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:

Canadian law enforcement agencies rely on them to apprehend ships in violation of Canadian maritime law

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