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The charting of the great river St. Lawrence was a task which meant long days out in a small boat, taking soundings of the depth of the water. At the same time, whenever a sounding was taken, Cook had to know where he was.

This was not easy. In many places the St. Lawrence is miles wide, and when Cook found a very shallow place or a sandbank it was important that he should mark it in exactly the right position on the chart. This was so that ships sailing up or down the river could avoid it.

There was only one way to do this. Cook took the direction of the hills or other high points on the banks with a compass, and when he had two or more of these, he was able to draw lines on the chart which gave him his position.

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Day after day and month after month Cook went on patiently working out the exact position of each one of many thousands of soundings. When he had finished, his chart was so accurate that he was given a special reward by the Admiralty.