John Melish drew his 1816 map in the way he did in order to show the United States had territorial claims all the way across the continent to the Pacific Ocean.
John Melish, from Scotland, had emigrated to America and settled in Philadelphia in 1809, where he set up his map making and publishing business. In 1816, when his famous and influential continental map of the United States was published, much of the western portions of the map were still under Spanish control. But Melish included the western regions because, in his words, "part of this territory unquestionably belongs to the United States.” The Louisiana Purchase in 1803 had given the United States much western territory, and the USA also had already staked a claim to the Oregon Territory, thus stretching its holdings to the Pacific Ocean. Melish's map also showed portions of the Great Plains which were then held as British or Spanish possessions shaded in the way he indicated US territories. The map became a visual representation of what many Americans would come to believe as "Manifest Destiny" - that the United States was destined to control lands from the Atlantic to the Pacific across the continent.
