[4] On 10 March 1744 Watson was placed on the establishment of the engineers as a sub-engineer, and that year he lay with the ordnance train for the most part inactive at Ostend.

He was actively employed in the Campaign of 1745, took part in the Battle of Fontenoy on 11 May, and was promoted on 21 May to be captain in the 21st Foot, the Earl of Panmure's regiment.

With the execution of this survey, or extended military reconnaissance, was combined an enlargement of Marshal Wade's plan of connecting the Highlands and Lowlands, and opening up the country by means of good roads.

[4]Watson was assisted, both in this work and the survey, by two very able young men, his nephew David Dundas and William Roy.

There also are preserved several mercator projections of North Britain, on which maps are indicated the posts in the Highlands which were occupied or proposed for occupation by the regular troops.

The survey was eventually reduced by Watson and Roy, engraved in a single sheet, and published as 'The King's Map'.

[5] An alarm of invasion caused the recall of Watson and his assistants to England to make military reconnaissances of those parts of the country most exposed to such attack.

[6] On 21 April 1758 Watson was given the colonelcy of the 63rd Foot, and was appointed quartermaster-general in the conjoint expedition, under the Duke of Marlborough, Lord Anson, and Admiral Howe, which sailed from Spithead for the French coasts on 1 June.

He landed with the troops in Cancale Bay, near St. Malo, assisted on the following day in the destruction of shipping and magazines of naval stores in the suburbs, embarked again on the 11th Foot, and, after ineffective visits to Havre and Cherbourg, returned to Portsmouth.

On 31 July 1759 he reconnoitred the country between the allied camp and Minden Heath, extending his reconnaissance beyond the village of Halen.

He died in London on 7 November 1761, while holding the appointment of quartermaster-general to the forces, after a long illness; he was tended on his deathbed by his housekeeper, Sophia Wilson, whose devotion he mentioned in his will.