Early hydrographic surveys consisted of depths calculated by sounding pole and hand direct line, with positions determined by 3-level sextant fixes to mapped reference details. Guide strains were being ropes, or traces, with depth markings and direct weights hooked up. These charts contain historic characteristics, like LORAN-C traces. Hydrographic survey https://hydrographic-survey-in-dh68912.mybuzzblog.com/12097096/new-step-by-step-map-for-hydrographic-survey