In charting conventions for hydrographic surveys, which of the following statements correctly describe how soundings and spot heights are referenced and depicted on the map?

Introduction / Context:
Standardized cartographic conventions ensure that mariners and engineers interpret hydrographic charts correctly. Soundings, spot heights, and their reference datums must be consistently treated to avoid navigation hazards and design errors.

Given Data / Assumptions:

• Charts use a low-water reference (e.g., mean low water or lowest astronomical tide) as datum.
• Colour and symbol conventions distinguish depths from heights above datum.
• Plotting of the decimal point conveys both value and spatial location.

Concept / Approach:
Reducing soundings to a low-water datum provides a conservative depth reference for navigation. Conventions often use black for depths and red for elevations above datum to prevent confusion. Placing the decimal point at the sounding’s position is a traditional method to link the numeric value to its precise location on the chart.

Step-by-Step Solution:
Confirm reference: soundings are tied to a low-water datum.Depiction: soundings (depths) in black; spot heights (above datum) in red.Positioning: decimal point plotted exactly at the sounding location.Collectively, these statements make option “All the above” correct.

Verification / Alternative check:
Hydrographic chart legends and surveying manuals illustrate these conventions; while specific chart agencies may vary slightly, the principles remain consistent.

Why Other Options Are Wrong:

• Any single statement alone is incomplete and does not reflect the full set of conventions.

Common Pitfalls:
Confusing mean sea level with chart datum; misinterpreting colour conventions; ignoring the exact plotted position of the sounding.

Final Answer:
All the above.