It therefore shouldn't be any surprise that his next letter on June 26th 1970 contained all three of these components in its design:  He claimed he had set a bomb by Ingleside Police Station,  claimed his "glory" by insinuating his involvement in the shooting death of Officer Richard Radetich and  his bus bomb diagram matched perfectly the bomb to be set by Ingleside Police Station. In addition, the history of Officer Richard Radetich is pertinent to the location of the bomb.
In the June 26th 1970 Button letter he stated "The Map coupled with the code will tell you where the bomb is set*. It is clear the bomb was to be set in San Francisco, the target for the majority of his communications and ire toward police. The code provided the inches (5 to be exact) and the map was to provide the radians, to be set at Mag. N (magnetic north) from his zero point clockwise. This was plainly evident when he provided the extra two clues in the following Little List letter on July 26th 1970.
The bus bomb diagram in the April 20th 1970 letter (shown above) depicted a school bus traveling from north to south in the morning, next to a steep embankment, with the sun rising to the east. This location is just 230 feet from Ingleside Police Station. School buses traveling from the northern and eastern edge of San Francisco to St.Thomas More Catholic School would travel the Southern Freeway, passing only 210 feet from Ingleside Police Station. From this location the bus would be traveling south in the morning to begin classes for the day, the sun would be rising from the east, and this location has a steep embankment mirroring exactly his bus bomb diagram of April 20th 1970. This portion of the Southern Freeway is flanked by the parking lot at the City College of San Francisco. This parking lot is the ideal location to access the embankment without parking on the freeway.
The Zodiac Killer was an avid reader of the newspapers and had previously mentioned the murder of Officer Brian McDonnell on February 16th 1970, killed by a bomb placed on the ledge of Park Police Station. In the June 26th 1970 Button letter, the Zodiac Killer claimed his involvement in the murder of Richard Radetich, while giving us a location for the bomb that bisected Ingleside Police Station and the City College of San Francisco - two of the places Officer Richard Radetich attended prior to his murder, while serving in the Accident Investigation Bureau. Had the Zodiac Killer read about the murder of the young officer, and cynically positioned his claimed bomb on the Mount Diablo map at a location pertinent to Officer Radetich's life. Exactly between Ingleside Police Station and the City College of San Francisco? And the design of the bus bomb diagram was just a fortunate coincidence to this location.
The April 20th 1970 bus bomb diagram was penned two months before the murder of Officer Richard Radetich, so if this location was deliberately earmarked well in advance of his murder, it is unlikely the Zodiac Killer chose it because of the history of the young officer in the newspapers subsequent to June 19th 1970. But it wouldn't rule out the deliberate targeting of an officer connected in some fashion to Ingleside Police Station or an advance knowledge of their history, however implausible this may seem. Whatever the case, the primary focus of Ingleside Police Station and an attack on an officer, because "there is more glory in killing a cop" is a viable one. The location of a proposed bomb on the Southern Freeway by Ingleside Police Station, matches both the Mount Diablo map and code of 4 radians set to magnetic north, and 5 inches along the 4th radian. The location also matches perfectly the bus bomb diagram within the April 20th 1970 letter. These still stand, irrespective of any connection to Officer Richard Radetich, which may have no bearing whatsoever.
The Ingleside Police District encompasses the area south of Cesar Chavez Street to the San Mateo County line and west from Highway 101 to Faxon Avenue. It is home to the San Francisco City College campus and many diverse residential neighborhoods including Sherwood Forest, Mt. Davidson, Glen Park-Sunnyside, Diamond Heights, Bernal Heights, Upper Noe, Mission Corridor, the Excelsior and St. Francis Wood, St. Mary's Park, Visitacion Valley, Precita Park, Cayuga, Outer Mission and Holly Park.
The Codes Of Zodiac Never Required An External Solve.