It is a commune and sub-prefecture of the Pas-de-Calais department 68 km west-northwest of Lille on the railway to Calais. The town is named after Saint Audomar, who brought Christianity to the area.
The canalised portion of the river Aa begins at Saint-Omer, reaching the North Sea at Gravelines in northern France. Below its walls, the Aa connects with the Neufossé Canal, which ends at the River Leie .
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