Here's how Airmail pilots were directed from Reno to San Francisco in 1921.
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0 Miles. Leaving the Reno field the pilot should head his ship southwest and gain altitude of at least 10,000 feet to pass safely over the Sierras. Practically all of this altitude should be obtained near the field before starting on the course.
20. Lake Tahoe—The northern edge of Lake Tahoe is 6 miles south of the course.
25. Truckee—On the Southern Pacific near the point where Lake Tahoe Railway joins the Southern Pacific from the south. Two and a half miles to the northwest of Truckee lies a very good summertime emergency landing field. All approaches are clear and a space available for a landing 600 by 2,000 feet. A big boulder painted white stands on the northwest side of the field and beside it is a white wind indicator. This field is to be avoided in winter, as snow gathers on it to a frequent depth of 4 feet. Soon after passing Truckee the Sierras are crossed. On the direct course 10,000 feet will clear the highest peak, but an altitude of 15,000 feet should be maintained. The Southern Pacific Railroad tracks veer to the west and north and from here on to Sacramento are at a varying distance of 5 to 20 miles north and west of the course.
65. Colfax—Seventeen miles northwest of the course on the Southern Pacific Railroad. Elevation here is 2,422 feet. A small level field lies one-half mile south of the city. The field should be used only in an emergency, as it is difficult to get into and during the rainy season is very soft. The field is 600 by 300 feet.
85. Shingle Springs—Seven miles south and east of the course, on the Placerville Branch of the Southern Pacific that runs from Placerville to Sacramento. There is a field here one-half mile west of Shingle Springs, bounded on the north by a highway running to Placerville and on the south by the Southern Pacific tracks. The field is 1,500 yards long north and south and 300 yards wide east and west. The ground is level, hard, and smooth. The elevation here is approximately 1,000 feet.
95. The Southern Pacific, running from Placerville to Sacramento, is crossed at right angles 1 mile southeast of where it makes a right-angular bend and approximately parallels the course for the next 15 miles. The course lies from 1 to 3 miles southeast of this track.
112. Mather Field—Is the Army Air Service station in the Sacramento Valley, equipped like all Air Service flying fields. It is located to the east of Sacramento and near the small siding called Mills, 2 miles north and east of the course. A huge white water tower serves as an excellent landmark as well as the three lines of buildings on the ground. Three railroads are crossed in a stretch of less than 10 miles soon after leaving Mather Field. The Southern Pacific Railroad is to the northeast of the course at a varying distance of 10 to 15 miles after leaving Mather Field. Southwest of the course the Sacramento River will be seen soon after crossing the three railroad tracks at a distance of 5 to 10 miles.
152. Suison [Suisun] Bay—Into which the Sacramento River empties, a large oblong body of water parallel to the course. The pilot will fly along the southwest side of this bay.
162. Martinez—On the southeast corner of Suison bay. One mile northwest of the course.