Alum Rock Airport operated in the era of barnstorming and early commercial aviation, when airfields were little more than cleared pastures with a windsock. Johnston's operation offered flight instruction, aerial rides, and charter services to a community that was still adjusting to the idea that human beings could fly. The airport served a dual purpose as both a commercial facility and a base for the aerial photography that documented the Santa Clara Valley during its agricultural peak. The photographs taken from flights out of Alum Rock show a valley covered in orchards of apricot, cherry, and prune, the landscape that earned it the name Valley of Heart's Delight.

As San Jose expanded eastward in the postwar decades, the former town of Alum Rock was absorbed into the city, and the airport's open land became too valuable to remain as an airfield. The airport closed, and the land was developed for residential and commercial use. Today the intersection of Alum Rock Avenue and Capitol Avenue is a busy urban crossroads with no trace of the airfield. The aerial photographs that Johnston and other pilots captured from their biplanes survive as the primary visual record of a valley that has been completely transformed, each image a document of a landscape that existed for only a brief moment between the Spanish ranchos and the semiconductor plants.