Catellix GPS positioning accuracy degrades with anchor geometry; the effect is quantified by the well-known Geometric Dilution of Precision (GDOP). Multi-vantage anomaly detection systems face the DUAL problem: how does anchor geometry affect DETECTION SENSITIVITY rather than localisation accuracy? This document formalises Geometric Dilution of Detection Precision (GDDP) for the Multi-Vantage Path Snapshot (MVPS) framework [I-D.melegassi-ippm-mvps-bundle]. The minimum displacement that a multi-vantage detector can reliably distinguish from measurement noise is NOT a single number: it is an anisotropic scalar field d*(v, theta) over the Earth's surface, governed by the geometry of the anchor set relative to each vantage. Three main results are proved: (1) GDDP Theorem (T-GDDP-1): d*(v, theta) admits a closed-form expression in terms of the Fisher Information of the anchor- to-vantage RTT-ratio vector. The directional detection threshold is d*(theta) = sqrt(chi2_crit / I(theta)), where I(theta) is the Fisher Information in direction theta. This is the Cramer-Rao bound applied to detection. (2) Anisotropy Lemma (L-GDDP-2): every vantage has a "blind cone" -- a set of directions in which displacement barely changes the RTT-ratio vector and detection sensitivity degrades. The blind cone is quantifiable and, for isolated vantages, can span over 70% of the compass. (3) Monotonicity Theorem (T-GDDP-3): adding an anchor NEVER reduces the Fisher Information of the system (Shannon chain rule applied to detection channels). There exists a principled anchor- placement optimisation: minimise max_theta d*(v, theta) over candidate sites. All three are validated to three layers of proof: canonical (math), empirical (deterministic scripts, seed=1337), and real data (RIPE Atlas measured RTTs, 92,067 D-squared values from 40 probes, 11/11 checks PASS). No simulation-only claim is made. The GDDP/GDOP duality has not, to the author's knowledge, been previously formalised. VerLoc (Kohls and Diaz, USENIX Security 2022) observes the directional effect empirically but does not derive d*(v, theta) or propose a geometric defence.