Marine survey accuracy

Data published by the International Hydrographic Organisation shows up a surprising fact: the UK and Ireland are below Turkey in the league table of survey quality by area of national waters. Spain, Portugal and France score much higher than the UK.

 There is one obvious reason why the UK’s position is relatively low; with a much larger area of shallow continental shelf, the British Isles has proportionately a far bigger survey to do than a country such as Turkey, where depths in many places reach 200 metres relatively close to shore.

Even so, it is widely known that there are cruising areas off the Turkish coast where chart accuracy is poor. Rod Heikell’s pilot warns that they can be out be by up to 2 minutes of longitude, considerably more than the warnings for anywhere on the British and Irish coasts.

 UKHO chart Q 6090 looks more deeply into the situation in the UK and Northern Ireland. It colour codes areas of the seabed by survey quality.  In the January 2013 edition, large parts of the coast of the Scottish mainland, Hebrides and northern islands, Wales, the Channel Islands, Lancashire and Northern Ireland are still marked as surveyed by leadline, or unsurveyed.

Follow this link to see UKHO chart Q6090

In an earlier 2006 version published in the AIB report on the Octopus (see Orkney Roulette page), the entire coast of  Ireland was marked as leadline surveyed. The republic’s coast has been left out of the latest Q6090, but the Irish government has now produced its own version of the chart, which shows a very large surveying effort since 2006.

Link to Ireland survey coverage chart

Interactive chart with local detail of Irish surveys

One result of the new surveys is that there have been new editions of 22 of the 60 or so admiralty charts for Irish waters since 2009, according to the Irish Cruising Club.

For practical decisions during a cruise, we have to rely on what electronic and paper chart publishers tell us about the underlying quality of their products, backed up, of course, by pilot book advice. This information can be pretty patchy. For example:

• UKHO charts   have source data chartlets, a benchmark for quality.
• Imray charts have no source data.  Some Imray charts do have general warnings on chart accuracy, eg for the Ionian and Turkey, where recent editions have quantified errors in the area as up to one minute and half a minute of longitude respectively.
• Pilot books vary on the issue: Heikell has general warnings in his introductions of much larger errors in Greece and Turkey, of 1.5 minutes and 2 minutes longitude maximum respectively, but that covers a vast area.    
• The Irish Cruising Club pilot’s latest edition for the West and South coasts of Ireland has gone much further, with localised chart accuracy information for different chapters, a good benchmark to aim for.  
• Some other pilot books I have checked recently  do not seem to mention the accuracy issue at all, even where they cover cruising grounds which the tables above show have known  chart quality issues (for example parts of the Adriatic).
• Many leisure electronic charts lack information on source data or other indications of underlying survey quality. They also lack last correction dates, and there is no detail of how comprehensive the updating is. Memory Map does reproduce source data diagrams on its raster charts but not edition dates, even though it is not hard to find UKHO charts for sale now whose most recent edition was three decades ago, for example on the West of Ireland.
  
• To find comprehensive source information on an electronic  chart, you have to move to those used in Electronic Chart Display and Information Systems on ships, a different world,  where vessels equipped to a high enough standard are now allowed to dispense with paper charts altogether.

In preparation: articles on GPS accuracy and on what makes leisure charts unsafe compared with those used in ECDIS systems.