
In all three examples, the Zodiac Killer was using his crosshairs as a target on land mass. So, is it possible that the Zodiac Killer derived his symbol from a target on land mass, such as the mysterious crosshairs symbol found in the Nevada desert, measuring 1,000 feet by 1,000 feet? The crosshairs symbol just south of the Triangle Eye is believed to have been constructed from slabs of concrete and asphalt, and made as early as the 1950s by the National Aeronautics and Space Administration.

Triangle Eye is within the Nevada Test and Training Range. The area is closed off to the general public so probably only see it from the air. These are artillery firing targets meant for "Circular Error Probable" and Time on Target related calibrations and training. During WW1 and WW2, it was found that most artillery kills happen within seconds of the attack starting, because once the artillery has started to impact, the soldiers are quickly rushing for cover. Because of this, artillery soldiers found that the best way to get optimal impact from artillery was to ensure that all shells landed roughly in the same area and at the same time, like a high-powered shotgun blast. These circles are used to determine how closely grouped the shells are, and because the ground is completely flat, it also allows for easy monitoring of the impacts to verify how close they are in time. If you look over the hills to the west, there is a similar circle that has more obvious impact markings. The circles are roughly 100 meters apart so it's easy to gauge how many shells land within 100 meters of the target. Link.

On August 4th 1969 he wrote "What I did was tape a small pencel flash light to the barrel of my gun. If you notice, in the center of the beam of light if you aim it at a wall or ceiling you will see a black or darck spot in the center of the circle of light". When the phrase in red (corrected for spelling) was typed into the newspaper archive, this sequence of ten words was only found in one newspaper previous to August 4th 1969, spanning 279 years. This exact same phrase was used in the Cleveland Plain Dealer newspaper on June 17th 1942 about wartime precautions in the event of a bombing raid from enemy planes (see below).
On its own this means very little, but In three lengthy consecutive communications on November 29th 1966, July 31st 1969 and August 4th 1969, I found the quote "it was about time for her to die" from 1888, "man is the most dangerous animal of all" from 1932, and "darck spot in the center of the circle of light" from 1942. A newspaper article on November 24th 1966 mentioned Jack the Ripper and Cheri Jo Bates, and five days later in the Confession letter we had a Ripper style communication with "it was about time for her to die" from 1888 (found only once prior to 1966). The July 31st 1969 letters appeared to reference the movie "The Most Dangerous Game" from 1932, with the quote "man is the most dangerous animal of all" being uttered by Merian C. Cooper, the associate producer of the film, discovered in only three 1932 newspapers. Now we had the "darck spot in the center of the circle of light" from 1942. One could argue that the methodology of sourcing archival newspapers phrases on November 29th 1966, July 31st 1969 and August 4th 1969 for the purpose of fashioning a sinister letter (if this is what happened), was indicative of a single designer responsible for all three.