Probably realizing this, the Zodiac Killer issued his 'Little List' letter to the San Francisco Chronicle on July 26th 1970, just one month later. In this lengthy correspondence, the Zodiac Killer added two large crosshairs, likely to give us a hint regarding the design of the Button letter code. The first contained the number 13 on the east axis and a bold circle placed in the southwest quadrant accompanied by the acronym SFPD. This can be viewed on the left. The second crosshairs had "P.S. The Mt Diablo code concerns radians + inches along the radians" scrawled across its base. If we can decode their meaning, it may point us towards the area of interest "where the bomb is set". It is highly unlikely the Zodiac Killer ever buried a bomb, nevertheless, we can still unearth the intended threat.
The Zodiac Killer referred to radians in the 'Little List' letter. An angle's measurement in radians is numerically equivalent to the length of a corresponding arc of a unit circle, with one radian measured at approximately 57.3 degrees. If we take a look at his placement of the bold zero next to SFPD on the 'Little List' letter, it lies well beyond the 180 degrees line, but before the 270 degrees line. If we now assume this was the area of interest or "where the bomb is set", it has to be 4 radians.
5 radians = 286.5 degrees, so is too much.
3 radians = 172 degrees, so is too small.
Radians are usually measured counterclockwise, but if we look at the placement of the bold circle, it may indicate that Zodiac's intention was to to create an angle running clockwise, concurrent with magnetic north.
4 radians = 229 degrees, which is slightly shy of the positioning of the bold circle, that is, until we add back the magnetic north value from 1970, which was approximately 17 degrees off center to the right. Adding this to the 4 radians value of 229 degrees, we get 246 degrees. Now look at the image of the compass rose to the left. It matches exactly.
Therefore, the bold circle that Zodiac drew, already incorporated the 17 degrees of magnetic north. This, is in all likelihood, the correct angle away from Mount Diablo. In other words, 4 radians + 17 degrees = 246 degrees.
We now have the possible location of the bomb (or intended location) set clockwise of 'true north'. 246 degrees from Mount Diablo points towards the vicinity of Alameda and the Bayview area of San Francisco, but the bold circle could fall anywhere along this line. The only problem is scale. Due to the lack of pinpoint accuracy regarding the bold circle, only a rough location could ever be determined. So, until the 32 symbols on the Button letter are decoded, we are likely to wait yet another "fall to dig it up"..