Preuss and Frémont met in December 1841 when the unemployed Preuss approached Frémont to solicit work. The only work Fremont had at the time was to complete the interpretation of the astronomical observations taken during Fremont's last expedition with
Joseph Nicollet. According to Fremont, he had to do most of the work for Preuss, but he still retained Preuss as a topographer on the expedition he was to embark on in the spring of 1842. That trip, from June to October 1842, resulted in an accurate survey and map of the
Oregon Trail as far as South Pass. Preuss again served as topographer on Frémont's second expedition to the west, from May 1843 to August 1844. After completing the survey of the Oregon Trail from South Pass to the Dalles of the
Columbia River, they went south along the east side of the Cascade Mountains to Klamath Marsh, then on south to the
Carson River, over the Sierra Nevada to
Sutter's Fort (present
Sacramento, California), south to
Tehachapi Pass. They hit the Old Spanish Trail at present
Victorville, California, then went northeast to
Bent’s Fort on the
Arkansas River, and returned to Westport, Missouri. "Frémont's sensational report [of the 1943-44 expedition] included an excellent topographical map by Charles Preuss, The large sheet, which depicted the routes of both of Frémont's expeditions, was a cartographic milestone. By accurately representing the basic features of the new country, Preuss changed the course of western mapmaking. No longer would cartography be based on myth and speculation." After completing this map, Preuss was engaged by Congress to create another map. He declined to accompany Fremont on his third expedition and in 1846, Preuss completed the second map, more important for prospective emigrants than the first. On seven sheets he carefully traced the Oregon Trail, using Frémont's narrative to indicate campsites with essential grass, wood, and water and to show distances, climate, and Indian inhabitants. Widely popular among those who took the Platte River road to Oregon and California, this annotated atlas was one of the greatest contributions Frémont and Preuss made to the development of the West." Congress commissioned to Preuss create a third map in 1848, this time using information from records kept by Lt. Kern who was the topographer on Fremont's third expedition. ==Fourth Fremont Expedition and the Pacific Railroad Survey==