What is measured by Horizontal Dilution of Position (HDOP)?

Horizontal Dilution of Position (HDOP) is specifically a measure of the accuracy of a positioning system, particularly one that relies on satellite navigation, such as GPS. HDOP quantifies how the geometric arrangement of satellites affects the precision of a determined position.

The value of HDOP gives insight into how the available satellite signals can contribute to a reliable position fix. A lower HDOP indicates a stronger geometry and thus a more accurate position, while a higher HDOP suggests potential inaccuracies in determining a position due to poor satellite arrangement.

In contrast, the other options involve different aspects of navigation and environmental conditions. The speed of water currents relates to hydrodynamics, the depth of water bodies deals with soundings and bathymetry, and the width of navigation channels pertains to navigational safety and infrastructure. None of these concepts directly relate to how a positioning system's accuracy is assessed, which is why they do not apply in this context.