This week’s question has to do with the depth information found on all charts, be they paper or electronic.  It applies to NOAA charts, NOS charts (National Ocean Service, US charts outside of the coastal US and US territories) and BA charts (British Admiralty).

The question relates to the Soundings or Depth information provided on navigational charts.  We all use the soundings to decide whether or not there is enough water for our vessel to safely navigate and anchor.

This week’s question is actually several questions all having to do with exactly what do the soundings on our charts tell us about water depths.  We know that the depths displayed on our charts show us the depth at low water, but it is always the lowest depth we can expect?  That is where the Depth Datum used on any particular chart comes into play, and thus this weeks series of questions.

Question 1; Where on a chart do you find the information telling you what depth datum is used on that chart?

Question 2; Some charts use MLLW as the depth datum, what is MLLW and what does it mean?

Question 3; On which agencies charts will you find MLLW as the depth datum?

Question 4; Some charts use LAT as the depth datum, what is LAT?

Question 5; On which agencies charts will you find LAT as the depth datum?

Question 6; Of the two datums, MLLW and LAT, which one is the more conservative?

These questions are important because MLLW and LAT are just two of many Depth Datums that have been used over the years, these two are currently the most popular in use and tell us slightly different things about the charted depths we see on our charts.  This information can be critical, especially for those who sail in “skinny water” (shallow water, such as the Bahamas and the Florida Keys) and in areas with lots of rocks and large tidal ranges.