Leaving the Basin: Terrain, Timing, and the Logic of the Exit

The first two articles establish where the spatial measurements in the “Bus Bomb” letter converge and how a pedestrian could reach and remain concealed in that location. The present article examines the next practical question: once inside the El Polin basin, what options existed for leaving it, and which of those options offered the greatest advantage.

Once inside the basin, the safest immediate action was to remain still. The surrounding ridges formed an acoustic bowl. Fire trucks at the southern street-end junctions, dog activity on the higher ground, and motorcycle or troop movement on the upper dirt and paved loops produced distinguishable sounds. As long as those elements remained active on the perimeter, movement carried higher risk than silence. Immobility allowed the Zodiac to continue mapping the search while expending no energy and creating no new noise or visible motion.

A practical window for that stillness can be estimated from the known timeline. The crime occurred at approximately 9:55 - 9:58 p.m. The Fouke sighting on Jackson Street followed at about 10:00 p.m., and the crossing of the Maple Street wall would have occurred within the next one to two minutes. Transit into the basin required roughly seven to twelve minutes depending upon experience. Intensive search activity was fully underway by approximately 10:15–10:30 p.m. Remaining in the basin for a further 30 to 75 minutes after arrival would have allowed the initial surge to organize into patterned sweeps while still providing continuous acoustic information. A shorter hold increases risk; a substantially longer hold is possible but not required by the evidence. A working central estimate of roughly 45 to 60 minutes of concealment before movement is consistent with both the letter’s knowledge of the search and the decision to move while activity was still audible.

The basin is a low point. Only a limited number of exit points exist, and each carry different costs.

West leads toward the Presidio Golf Course and higher interior ground. That direction moves deeper into the park, away from city streets and transit, and into areas more likely to see military foot or vehicle traffic. It offers no clear advantage.

South rises toward the Main Post. That point was more brightly lit and more actively used by military personnel and vehicles. The risk of encounter would have been higher.

East returns toward the Julius Kahn Playground, the original focus of the dog search. Re-entering an area already swept and still close to the southern boundary wall would be the least attractive option.

Northeast follows the natural drainage as it rises gently and then flattens. The corridor leads toward open, lower-friction ground and ultimately toward the city rather than deeper into the reservation. Of the available choices, it combines the least resistance with the greatest practical benefit: progressive distance from the densest search activity and eventual access to civilian streets.

As the ground rises even modestly along the northeastern corridor, a distant illuminated landmark becomes useful. In 1969 the Palace of Fine Arts was lit at night. Its glow, visible as a halo above the trees, would serve as a directional confirmation once the terrain lifted the Zodiac high enough to clear intervening canopy. The landmark did not need to be seen in detail; the presence of the glow itself would orient a person moving northeast and confirm that the path was opening toward the Marina side of the park rather than back into the interior ridges.

The distance from the El Polin basin to the vicinity of the Palace of Fine Arts is approximately 0.8 to 1.2 miles by realistic walking routes. At a night-hiking pace of 2.5 to 3.5 mph on mixed terrain, the transit requires roughly 15 to 30 minutes. Combined with a 45-to-60-minute hold inside the basin, the Zodiac could have been in the open ground near the Palace approximately one hour to one hour and forty-five minutes after first entering the tree line at Maple Street.

The sequence - entry at Maple, descent into the basin, prolonged stillness while the search is mapped by sound, then movement along the single most advantageous corridor - does not read as pure improvisation. It reads as the execution of a route that had been considered in advance.

Topographic maps of San Francisco were publicly available in the 1960s through USGS outlets, outdoor retailers, university bookstores, military surplus shops, and engineering suppliers. Those maps show contour lines, drainages, elevation change, and the standard symbol set used by the U.S. Geological Survey. A person comfortable reading them could identify the basin as a low point, recognize the northeastern rise and subsequent flattening, and note the relationship between rising ground and the visibility of a prominent illuminated structure.

The same familiarity with USGS conventions appears elsewhere in the Zodiac case. All of the non-alphabetic symbols in the ciphers correspond to forms found on period topographic and planimetric maps. That correspondence suggests the Zodiac drew from a visual vocabulary he already knew rather than inventing shapes at random. Comfort with topographic maps and their symbol systems does not require military training. It appears in a range of civilian roles and recreations of the period: Boy Scout leaders teaching map-and-compass work, amateur radio operators, forestry and park workers, utility planners and survey crews, civil-engineering draftsmen, and serious amateur outdoorsmen. These roles train the same habits of spatial abstraction, contour interpretation, and symbol recognition that surface both in the Presidio movements and in the cipher material.

The overall sequence is therefore consistent with prior study of the ground and with a broader pattern of map-based thinking, rather than with last-moment invention.

Summary

The Zodiac’s first decision inside the basin was to stay still and listen. A working estimate of roughly 45 to 60 minutes of concealment is consistent with the timeline and the acoustic detail recorded in the letter. When movement became the better option, the northeastern corridor offered the clearest combination of topographic ease and practical advantage. Elevation along that path would progressively reveal the glow of the Palace of Fine Arts as a directional check, and the walking time to that vicinity is approximately 15 to 30 minutes. The choices and the information used to make them are consistent with advance familiarity with the terrain and with the kind of map literacy that also appears in other parts of the case.