Over the years, mariners have reported finding shoals and reefs, islands, and many other features during the course of their voyages. Unless these have been verified as actually existing and their position fixed with the least water depth over the associated shoals, they are depicted on published navigation charts as "doubtful data". Existence Doubtful (ED), Sounding Doubtful (SD), and Position Doubtful (PD) are marked alongside such features on the charts.
The documentation supporting many of these historical data is very sparse. Their verification by searching is an expensive and time-consuming matter during which every effort must be made to prove (or disprove) the reported feature conclusively; otherwise a great deal of money and resources will have been expended to no purpose.
The international Hydrographic Organization has recently made a study of how best to ensure that positive charting action can be taken after searches for doubtful features. The result is the publication of Standard Criteria by which it may be judged whether or not a search can lead to the doubt being eliminated. Recent developments in instrumentation and methodology for positioning and remote sensing by sonar systems and satellite imagery will enable some of the thousand od such features to be removed or confirmed on the charts.
"There is little chance of finding a reported shoal in the open ocean by directing the course of the searching vessel to the geographical position assigned in the report and endeavouring to re-discover the shoal by passing in its vicinity with look-outs posted aloft or with a sounding line suspended of this kind have sometimes been cited in disproof, and it is, therefore, of interest to arrive at an estimate of the expectation that would thus be justified.... it appears that there would not be one chance in hundreds of finding the shoal by thus performing the search for it."
Thus wrote Littehales in 1933.
Thirteen years after LittleHales wrote above in the International Hydrographic Review, early in the New Year of 1946, His Britannic Majesty's Landing Ship (Tank) 324 passed, westbounf and with a strong tidal stream under her stern, through the Balabak Strait on passage from the Pacific towards Labaun off the West coast of Borneo. It was dusk; as the tropical night fell under an overcast sky, it was decided to stand-on toward the west into deeper water before heading southward to avoid the congested and shoal-littered inshore passage down the coast of what is now Sabah.
The navigator an uncomfortable feeling that his dead reckoning was being influenced, either ahead or astern, by the strong streams that flow through the Balanak Strait. If the westing was too great the ship might find herself in the vast "not surveyed" area in the middle of the South China Sea. There were no radar or modern navigation aids other than the magnetic and (unreliable) gyrocompasses, chronometer, sextant and an antique Kelvin hand sounding machine; to make matters worse, the only rotor for the patent for had parted company.