San Diego’s Presidio was built on the camp-site picked out by Captain Fernando Rivera y Moncada, leader of the advance party of the expedition which brought Don Gaspar de Portola and Fr. Junipero Serra here from the tip of Baja California in the summer of 1769.
So far as is known, no map or plan of the Presidio exists, and various historians of bygone years have produced varying sketches of what the plan might have been. There is, however, a tiny indication of the Presidio on the 1859 chart of San Diego Bay, prepared by the U. S. Topographical Engineers. Despite its minute scale-it shows on the opposite page in its full size, near the upper right corner and just under the word “Valley” — it is possible to measure it in a general way. It is also possible to determine, roughly, the direction followed by the walls of its longer axis.
Curious about this, the staff of the Serra Museum went down and, with a reconnaissance compass, took the bearing of the east-west axis of the row of low mounds which were the north part of the old Presidio; it came out to approximately 270o magnetic. Then they took off the bearing of corresponding walls on the chart — and got the same thing. The overall width of the Presidio, as shown on the chart, also agreed in general with the same measurements taken on the site. It hinted strongly that here was, albeit microscopically, a plan of the old Presidio.
The rough sketch below the reproduction of the chart indicates the location of the buried ruins with respect to prominent landmarks in the park.