Hydrographic Survey Standards – Still up to date?

Hydrographic surveys are categorized in quality standards defined by the IHO in the Special Publication 44. These standards have been adopted by various national hydrographic offices, including the Dutch Hydrographic Service who even added a stricter standard.

Even though, the standards are widely used as a reference to check hydrographic surveys, it is hard to find out how to use the standards’ definition as a benchmark.

The main reason for this is that the standards main reference is standard deviation (SD), but it forgets to mention for which area this value is applicable. In general it is assumed that the standards are defined for a 1x1 meter bin size, but reducing the bin size will improve the SD. The same data, better and more detailed results? No!

Another influence to the value of the SD which is left out of the picture in the survey standards is the shape of the earth, the morphology.

In no case the defined bins will be perfectly flat and aligned perfectly parallel to the earth’s gravitational plane. This will already cause a SD on a slope, even when the measured points are perfectly aligned.

What we need is a slope corrected SD by deducting the SD introduced by the slope from the SD calculated from the measured points, as shown in the right in Figure 1. This slope corrected SD is a far better quality indication than described in the official documents.