A chart is essentially a maritime map of the ocean and the coastline. Navigators use the term “chart,” but many 18th century writings, such as the notes of English hydrographic surveyor Murdock Mackenzie, whose work in the 1740s influenced explorers like Vancouver in their charting methods, called them “maps.” Although it shows enough land to give navigators their bearings and may sometimes even provide major landmarks and elevations for reference, a chart is concerned with information used at sea. Charts can show the depth of the water at various points, villages and outposts, and even the direction of prevailing winds.
Some of the earliest charts, called portolans, were hand-drawn by European navigators in the 13th, 14th, 15th and early 16th centuries. Captains could combine the portolans with the data in their personal notebooks to provide piloting information on harbours and trade routes. Portolans were first used in the Mediterranean and later adopted for larger stretches of ocean. Directional lines called Rhumb lines formed criss-crossing patterns that appeared as starbursts across the charts. Rhumb lines could be used to set a course by providing a record of compass bearings and wind direction.
A language of symbols and numbers that navigational pilots could read and interpret helped nautical charts advance as a way of communicating important information. Features included outlines of land, tide lines, sources of fresh water, and dangerous currents. During the 18th century, the increasing accuracy and sophistication of navigational equipment created better charts. Lines of latitude and longitude were placed like a grid over land and ocean, and charts were printed with presses for widespread access.
Before 18th century charting expeditions, many maps of the world used a blank area or an undulating line to represent the Pacific Northwest. The search for the Northwest Passage, the desire to claim resources such as sea otter pelts, and the eventual clash between the European powers that wanted to claim the region accelerated charting of the area. Accurate charts could be a way of expressing interest and even aspirations of ownership.
