Early hydrographic surveys consisted of depths calculated by sounding pole and hand guide line, with positions determined by three-place sextant fixes to mapped reference points. Guide lines were ropes, or strains, with depth markings and direct weights attached. NOAA makes use of USCG-taken care of differential GPS beacons to Increase https://sergiodxmbb.blogsvirals.com/31720368/new-step-by-step-map-for-hydrographic-survey-in-dhaka