Module 7 - Charts & Publications
Choose a Suitable, Current Chart
What you will learn
Select chart coverage and scale for each passage stage, then establish the chart's edition, update status, units, datums and important cautions before using it.
Use the largest suitable scale for the immediate task, but only after checking that the chart is official, current, covers the whole decision area and contains the detail the vessel needs.
An official nautical chart is a special-purpose navigation product issued by, or on the authority of, a hydrographic office or other authorised government institution. It combines coastlines, depths, aids to navigation, hazards, routeing measures and source information, but it is not self-interpreting: the navigator must read its title, notes, symbols, datums, scale and status.
Large-scale charts cover a smaller area with more local detail; small-scale charts cover a wider area with less detail. A 1:20,000 harbour chart is therefore larger scale than a 1:200,000 coastal chart. Use the largest suitable scale for pilotage or a close hazard decision, while retaining the smaller-scale chart needed to understand the wider passage and the approach to the chart boundary.
Scale alone does not make a chart suitable. Check that the coverage includes the intended route and alternatives, the chart is the latest obtainable edition, subsequent applicable corrections are accounted for, the units and horizontal and vertical datums are understood, and cautions or source-quality information do not undermine the planned margin.
| Decision | Check | Why it matters |
|---|---|---|
| Coverage | Route, alternatives and margins lie inside the chart | Avoids navigating at an edge or beyond the product's intended area |
| Scale | Largest suitable scale for the immediate task | Provides the local detail needed without losing wider context |
| Status | Edition and applicable updates | A newly charted danger or changed aid can invalidate an old plan |
| Reference | Units, horizontal datum and Chart Datum | Prevents position, depth and clearance errors |
| Quality | Source or ZOC information and cautions | Sets a realistic margin around uncertain data |
Change scale as the navigation task changes
A passage begins with a 1:200,000 coastal chart and ends through a narrow harbour entrance covered by a current 1:20,000 chart.
1. Keep context
Use the coastal chart for the wider route, alternatives and approach to the harbour area.
2. Increase detail
Transfer the checked position and route to the 1:20,000 chart before the confined-water pilotage begins.
3. Read the margins
Confirm edition, update status, datums, units, projection and cautions on the chart actually being used.
4. Cross-check
Compare the charted plan with current notices, warnings, tides, weather, local information and what is observed.
Sense check: 1:20,000 has the smaller denominator and is the larger-scale chart; it should show more harbour detail, not more geographic area.
Common mistake or limitation
- Choosing the chart with the widest coverage for a close pilotage decision, even though it omits local detail.
- Treating 'largest scale available' as sufficient without checking currency, coverage, datums, cautions and source quality.
- Zooming a digital display beyond the data's compilation scale and assuming larger symbols mean more underlying detail.
Recap
- Large scale means smaller area and usually more detail.
- Use the largest suitable scale for the task, not one chart blindly for the whole passage.
- Coverage, status, datums, units, cautions and source quality are part of chart selection.
Optional quick check
Section 1 of 10
Which chart is normally the better primary chart for the final approach to a harbour: 1:20,000 or 1:200,000?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Charts & Publications
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