Vision-based relative positioning in targetless environments via datumconstrained forward intersection

Hazard-adjacent sites such as seawalls, bridge edges, quaysides, and fenced industrial perimeters routinely require metric positioning without in-zone fiducials, survey control, or repeated traversal, yet still demand decision-ready uncertainty on a site-meaningful datum. We present a datum-constrained forward intersection (DFI) pipeline that completes metric anchoring in a reachable safe subregion and transfers the uncertainty of scale, pose, and datum plane across the boundary to a targetless zone. In the safe-zone calibration stage, standard ChArUco imagery drives a prior-constrained bundle adjustment that jointly estimates multi-camera extrinsics and the site datum plane under weak but auditable tape-measure priors (inter-camera baseline and optical-center heights). In the target-zone measurement stage, undistorted pixel rays intersect the datum in closed form and are fused by incidence weighting; first-order propagation yields a 95% confidence region on the datum plane for each reported coordinate and distance. To tolerate consumer-grade instabilities (modest refocus/zoom and mixed resolutions), the pipeline supports BA-anchored refinement and a minimalist one-parameter in-situ metric correction (DFI–MScale) based on a few taped distances. In the test pair, DFI–MScale attains a median/p90 distance error of 62/106 mm, reducing p90 by 88.5% versus ORB, 87.6% versus LoFTR, and 78.4% versus RoMa under identical intrinsics and target points.