Shortly before his death Joseph Smith began making plans to move the main body of the Saints to someplace in the American west. After his assination, Brigham Young and the Quorum of the Twelve continued to flesh out these plans, ultimately choosing to move the Saints to the Great Basin. In making their plans they depended on the reports of John C. Fremont and on the maps of the American west that his expeditions had created. Here is one of them.
Fremont was an officer in the United States Army and during the 1840s he was commissioned to explore the far west on behalf of the United States government, including territory that then belonged to the Republic of Mexico. In 1843 he published the results of his explorations in (among other places) a book entitled Profile of the Route from the Mouth of the Kansas to the Pacific. This map of the Great Basin is taken from this book. Either this map, or one very like it, would have provided Brigham and the other leaders of the Church their sense of Great Basin geograpy.
Notice that large portions of the map, particularlly those showing the interior topography of the Great Basin are blank. Fremont did not explore these areas. However, Fremont’s map does show a crecent of detail that extends from the Snake River valley in the north (what is today southern Idaho) down through the Great Salt Lake and thence in a long arc to the Pacific, ending in what is today in San Bernardino, California. In the end this arc of explored territory at the base of the mountiains (where importantly there was water) was almost entirely settled by Mormon pioneers.