The surveyed seabed, drawn at true 1:1 scale where it actually lies, over the camera.
name SHARM OBHUR
sonar KONGSBERG EM712
epoch 2018-03-04
horizontal datum EPSG:32637 — WGS 84 / UTM zone 37N
CHART DATUM (LOW-WATER, CD/LAT - INFERRED) · TIDE APPLIED
LAT vs LOCAL CD UNRESOLVED · ABSOLUTE VERTICAL UNVERIFIED
Depths are metres below chart datum. The seabed is placed by adding the water level above that datum and your eye height above the water, both adjustable below. Because the datum itself is only inferred, the whole surface can sit up to about 0.4 m too high or too low. At 100 m range that is 0.2° of angle, which is nothing next to the compass error — but it is not zero, and it is not verified.
ground sample 1.5 m (decimated 8.4× from the 0.5 m surface)
points 315,535
triangles 621,228
depth range 5.34 – 51.65 m
selection block-shoalest: every vertex is the shallowest measured cell in its 3x3 block
Each point is the shallowest measured cell in its block — the hydrographic convention, generalised toward safety. Depths therefore read shallower than the survey surface by 0.12 m on average (0.28 m RMS): 0.05 m on flat ground, 0.49 m on slopes over 20°, up to 10.8 m at the worst single point on a reef wall.
Points sit on a regular 1.5 m lattice, so a value can be up to 0.75 m from the cell it was measured in — 20× smaller than the GPS error this is positioned by.
The survey's least depth is 4.805 m, and it is NOT in this model. Gridding lost it before anything else happened; the shallowest ground any surface product can show is 5.340 m.
Phone GPS gives the observer, projected into EPSG:32637. The compass gives heading, the accelerometer and gyro give tilt. There is no visual tracking of any kind, so accuracy is whatever the phone's GPS and magnetometer give — typically ±5–15 m and ±5–20°.
Grid convergence and the UTM point scale factor are both applied, so the render is true ground metres rather than grid metres.
Heading is the weak link. Use the arrows at the bottom to rotate the overlay until the drawn survey edge sits on the real channel edge, then report the number.
Field of view is assumed to be 67.5° across the long axis of the camera frame — the iPhone main rear lens. iOS exposes no way to read the real value. If the overlay is consistently too wide or too narrow, trim it here.
True 1:1. No exaggeration on any axis, horizontal or vertical. From the shore, the far end of the survey sits about half a degree below eye level — it is meant to look like a thin band under the horizon.
NOT FOR NAVIGATION.
Stand at the water's edge and hold the phone upright, pointed across the water. The surveyed seabed is drawn where it actually lies.