Compute distance to a subtense bar from its length and the subtended angle.
输入横基尺长度 c 与所对夹角 θ（度），求距离。
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Subtense (parallax) distance | D = b / ( 2 · tan(phi/2) ), where b is the subtense bar length and phi the parallax angle | GB 50026-2020, Code for engineering surveying |
| Small-angle approximation | For small phi, D ≈ b/phi with phi in radians | GB 50026-2020, Code for engineering surveying |
| Accuracy | Relative error ΔD/D ≈ Δphi/phi; accuracy drops as the parallax angle grows | GB 50026-2020, Code for engineering surveying |
| Typical application | Suited to short distances across obstacles where direct taping is impractical | GB 50026-2020, Code for engineering surveying |