Probably realizing this, the Zodiac Killer issued his Little List Letter to the San Francisco Chronicle on July 26th 1970, just one month later. Within this lengthy correspondence he added two large crosshairs, the first contained the number 13 on the east axis and a bold circle placed in the southwest quadrant, along with the acronym SFPD.
The second crosshair had ' P.S The Mt Diablo code concerns radians + inches along the radians '.
Obviously this was attempting to elaborate on the insufficient information he provided on the Phillips Road Map.
The Zodiac Killer referred to radians in the Little List Letter. An angle's measurement in radians is numerically equal to the length of a corresponding arc of a unit circle and one radian is approximately 57.3 degrees.
The Zodiac Killer mentioned radians (ie more than one), If we take a look at his placement of the bold zero, next to SFPD on the Little List Letter, it lies well past 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.
4 radians = 229 degrees, but is slightly shy of the positioning of the bold circle, that is until we add back in 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 right. It matches exactly.
Therefore this bold circle the Zodiac drew, already incorporated the 17 degrees of Magnetic North and this is in all likelihood the correct angle away from Mount Diablo. ie
4 radians + 17 degrees = 246 degrees.
Do we have the possible location of the bomb set clockwise of True North. 246 degrees points towards Alameda and the Bayview area of San Francisco, but the bold circle could fall anywhere along this line.
The only problem is scale, is the bold circle at true scale from the center of the crosshair, and because the bold circle is anything but pinpoint, 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'.