Peterson devised his system "so that live birds could be identified readily at a distance by their 'field marks' without resorting to the bird-in-hand characters that the early collectors relied on.
Another application of this system was made when Roger Tory Peterson was enlisted in the US Army Corps of Engineers from 1943 to 1945.
“...plane identification—the aircraft spotting technique—was based on Roger’s bird identification method-the Peterson system.”.
[3] Created for use by amateur naturalists and laymen, rather than specialists, the "Peterson System" is essentially a pictorial key based upon readily noticed visual impressions rather than on the technical features of interest to scientists.
The technique involves patternistic drawings with arrows that pinpoint the key field comparisons between similar species.