🔍 Key Concepts

• Chart datums used on National Ocean Service (NOS) charts • Differences between mean low water, mean lower low water, and mean low water springs • Regional practices for vertical datums on U.S. Pacific Coast vs. Atlantic/Gulf coasts

💭 Think About

• Think about which tidal level is commonly used so that charted depths are safely usable most of the time on the U.S. Pacific Coast. • Ask yourself: in a mixed semidiurnal tide regime (common on the Pacific Coast), which tidal datum best represents the average of the lower of the daily low waters? • Consider how NOAA/NOS tends to label the sounding datum in the chart’s title block for Pacific Coast charts.

✅ Before You Answer

• Look up a typical NOAA Pacific Coast chart and read the note about "Soundings in fathoms/feet – Datum is…" • Verify the definition of mean lower low water (MLLW) and mean low water springs (MLWS) to see which is standard in U.S. practice. • Confirm which datum is specified in the NOS Chart No. 1 explanation of vertical datums for U.S. coastal charts.