The Agreed Boundary Doctrine...

The Agreed Boundary Doctrine explained in Ernie v. Trinity Lutheran Church, 51 Cal. 2d 702:
"The requirements of proof necessary to establish a title by agreed boundary are well settled by the decisions in this state. ( Mello v. Weaver, 36 Cal.2d 456, 459 [224 P.2d 691]; Hannah v. Pogue, 23 Cal.2d 849, 856-857 [147 P.2d 572]; Martin v. Lopes, 28 Cal.2d 618, 622-627 [170 P.2d 881]; Young v. Blakeman, 153 Cal. 477, 481-483 [95 P. 888]; see also 4 Cal.L.Rev. 179; 14 Cal.L.Rev. 138; 56 Mich.L.Rev. 487 et seq.) The doctrine requires that there be an uncertainty as to the true boundary line, an agreement between the coterminous owners fixing the line, and acceptance and acquiescence in the line so fixed for a period equal to the statute of limitations or under such circumstances that substantial loss would be caused by a change of its position. It is not required that the true location be absolutely unascertainable ( Price v. De Reyes, 161 Cal. 484, 489 [119 P. 893]); that an accurate survey from the calls in the deed is possible ( Silva v. Azevedo, 178 Cal. 495, 498 [173 P. 929]), or that the uncertainty should appear from the deeds ( Mello v. Weaver, supra, 36 Cal.2d 456, 460). The line may be founded on a mistake. ( Nusbickel v. Stevens Ranch Co., 187 Cal. 15, 19 [200 P. 651].)"

This is the part Bryant vs. Blevins overturned:
"The court may infer that there was an agreement between the coterminous owners ensuing from uncertainty or a dispute, from the long-standing acceptance of a fence as a boundary between their lands."

This explains that the boundary so determined is the boundary described in the Deeds (which is what Land Surveyors survey):
"Under the foregoing circumstances the line so agreed upon became in legal effect the true line. In Young v. Blakeman, supra, 153 Cal. 477, 482, it was said: "It is stated by the authorities that the line so agreed on becomes in legal effect the true line, that the agreement as to the line may be in parol and that it does not operate to convey title to the land which may lie between the agreed line and the true line, but that it fixes the line itself and the description carries title up to the agreed line, regardless of its accuracy . . . that 'the division line when thus established, attaches itself to the deeds of the respective parties, and simply defines, not adds to, the lands described in each deed,' and that if more is thus given to one than the calls of his deed actually requires, he 'holds the excess by the same tenure that he holds the main body of his lands.'" Each coterminous owner is deemed to have paid the taxes according to his deed. ( Price v. De Reyes, supra, 161 Cal. 484, 489-490; Caballero v. Balamotis, 144 Cal.App.2d 58, 61-62 [300 P.2d 363]; Carr v. Schomberg, 104 Cal.App.2d 850, 860 [232 P.2d 597].) It is therefore not material to the defendant's claim of title by agreed boundary that the plaintiff paid the taxes assessed upon the land according to her deed."

Adverse Possession is a completely different doctrine. It requires hostility while the Agreed Boundary Doctrine requires agreement. Adverse Possession involves a transfer of title while the Agreed Boundary Doctrine does not.

from Bryant v. Blevins, 9 Cal. 4th 47:
"Notwithstanding the conclusion we reach in this case, we decline to limit application of the agreed-boundary doctrine to instances in which existing legal records are inadequate to settle a boundary dispute. As previous cases have explained, such an inflexible rule would risk destabilizing long-standing agreements--made in good faith by coterminous property owners in order to resolve uncertainty as to the location of their common boundaries--that might, for any one of several reasons, be at variance with legal property descriptions or survey results. Instead, we reaffirm the vitality of the requirements necessary to establish the applicability of the agreed-boundary doctrine, set forth in Ernie v. Trinity Lutheran Church (1959) 51 Cal.2d 702 [336 P.2d 525]. (See also Martin v. Lopes (1946) 28 Cal.2d 618, 624 [170 P.2d 881] [discussing the policy in favor of according stability to boundary agreements adopted in good faith by coterminous landowners and acquiesced in for a period longer than the statutory period of limitations for adverse possession].)"

and quoting Young v. Blakeman (1908) 153 Cal. 477, 481-482 [95 P. 888]:
"The object of the rule is to secure repose, to prevent strife and disputes concerning boundaries, and make titles permanent and stable…. If a measurement is made and the line agreed on and acquiesced in as required by this rule, it is binding on and applicable to all parties to the agreement and their successors by subsequent deeds."

The doctrine is intended to reduce litigation. In other words, the property owners have a valid agreement and it doesn't need to be confirmed by a Judge just as we don't litigate every Deed and Contract.

I think this is the key part of Bryant:
"Thus, a fence--which, in Ernie v. Trinity Lutheran Church, supra, 51 Cal.2d at page 708, we observed "might in and of [itself] be of an uncertain, temporary or equivocal nature"--is not the type of "substantial structure[]" from which an agreement to accept an agreed boundary reasonably may be inferred in the absence of evidence that uncertainty on the part of the property owners led to their agreement to rely upon the fence as evidence of their common boundary. (1c) On the record before us, we conclude that defendants, as the parties invoking the agreed-boundary doctrine as the basis for their claim of title to the disputed strip of land, have not met their burden of proof under the test we set forth in Ernie."

In the following the Bryant Court more or less says it would decide Ernie the same today, that is, if the established boundary was based upon a Survey long enough ago then the Doctrine would be activated. I take from this that we should be retracing old Surveys, not staking new lines:
"A comparison of the facts of the case before us with those underlying Ernie v. Trinity Lutheran Church, supra, 51 Cal.2d 702, is instructive. Ernie involved a property dispute over the ownership of a strip of land, slightly less than one foot wide and one hundred forty feet in length. In 1925, the defendant's predecessor in interest (like the defendant, a church), purchased a parcel of land adjoining the property owned by plaintiff's predecessor in interest. Shortly after having a survey made of the property, the defendant's predecessor constructed a rectory and a cement walkway (with a fence embedded in it) upon the strip of land in question. These improvements remained in place for more than 26 years without objection from the adjoining owner, the plaintiff's predecessor. At some point after purchasing the lot adjacent to the defendant's property, the plaintiff commissioned a new survey of the land, based upon the descriptions set forth in the recorded deeds, and, when that survey indicated the disputed land fell within the description contained in the plaintiff's deed, the plaintiff sued to establish her ownership of that strip. On these facts, the court in Ernie held: "It may be inferred that there was an uncertainty as to the true [boundary] at the time the structures were erected [presumably because the defendant's predecessor had a survey conducted immediately prior to construction of the rectory, walkway and fence], which uncertainty was settled by practical location on the ground at that time and was agreed to by the then coterminous owners." (51 Cal.2d at p. 708.) Accordingly, the court in Ernie held that the defendant properly held title to the disputed strip of land under the agreed-boundary doctrine."

A fence by itself with no evidence that it was built to resolve uncertainty in the boundary would probably fail the boundary test in California.