Dudley's chart of the southern Cornish coast. First map with latirudes and longitudes according to the Mercator projection:

first edition publication of the Act in 1714

Later edition of the Act of Longitude

Nova & accuratissima totius terrarum orbis tabula nautica variationum magneticarum index juxta observationes anno 1700 habitas constructa, byEdmund Halley, 1730
The compass variation was one of the possible ways to find longitude at sea!

The first nautical almanac to contain data dedicated to the convenient determination of longitude at sea.
It was the first to include tabulation of lunar distances as a tool to facilitate the determination of longitude at sea. By Nevil Maskelyne

Tobias Mayer's tables for the lunar distance edited by the Britisch admiralty and Nevil MaskelyneAct