Swift describes the location of Brobdingnag, and its geography, in the text of Part II of Gullivers Travels, and provides a map showing where it is. However these accounts are somewhat contradictory.
The map printed at the beginning of Part II indicates that Brobdingnag is located on the northwest coast of North America, in probably what is now British Columbia. It shows (from south to north) Point Monterey, Port Sir Francis Drake, Cape Mendocino, Cape St Sebastian, Cape Blanco and the semi-mythical Strait of Anián, all locations on the Pacific coast of North America, and depicts Brobdingnag as a peninsula extending west into the Pacific to the north of the Straits.
In the book Gulliver describes his voyage from England. After wintering at the Cape of Good Hope, the ship reached a latitude of five degrees south, northward of Madagascar in March 1703, and the Moluccas, "about three degrees northwards of the line" in April. From there the ship is driven by a storm "about five hundred leagues to the east" (this would place the ship still in Micronesia), after which the crew determine to "hold on the same course rather than turn more northerly, which might have brought us to the north-west parts of Great Tartary". They sighted land, which Gulliver later discovers is Brobdingnag, on 16 June 1703.
Brobdingnag is claimed to be a continent-sized peninsula 6,000 miles (9,700 km) long and 3,000–5,000 miles (4,800–8,000 km) miles wide, which based on the location given by Gulliver would suggest that it covers most of the North Pacific. Contrariwise, his map shows Brobdingnag to be of a similar size and extent as the present-day Washington, and his description of the voyage puts it at a six-week voyage from the Moluccas.
Swift was highly sceptical about the reliability of travel writings and the unlikely geographic descriptions parody many unreliable travel books published at the time which Percy Adams describes as "travel lies".