Cal. Government Code § 23137

The boundaries of San Diego County are as follows: Beginning at the southwest corner of the State of California as described in Article XXI of the Constitution of the State of California; thence easterly along the international boundary line between the United States and Mexico to its intersection with the range line between R. 8 E. and 9 E. of San Bernardino meridian; thence northerly along the range lines between said R. 8 E. and 9 E., which is also the westerly boundary of Imperial, to the northeast corner of T. 9 S., R. 8 E., which point is also on the southerly boundary line of Riverside; thence west along the second standard parallel south, San Bernardino base, which is also the south boundary line of Riverside, to the range line between T. 8 S., R. 3 W. and T. 8 S., R. 4 W.; thence north along said range line to the southerly boundary of the Rancho Santa Rosa; thence northwesterly and northerly along the boundary line of said Rancho Santa Rosa to the township line between T. 7 S., and T. 8 S., S. B. B. & M.; thence west along said township line to the easterly line of the Rancho Mission Viejo or La Paz, which is also the southeasterly boundary line of Orange; thence following said southeasterly boundary of Orange southerly and westerly along the easterly and southerly line of said Rancho Mission Viejo or La Paz to the most westerly line of the Rancho Santa Margarita y Las Flores; thence southerly along said westerly line of said Rancho Santa Margarita y Las Flores to the shore line of the Pacific Ocean, and continuing in the same direction to a point three English miles in said Pacific Ocean, which point is on the westerly boundary line of the said State of California; thence southerly along said westerly boundary line of the State of California to the place of beginning.