🌊 DAY 5: TIDES AND WATER LEVELS

From Theory to Practical Tide Reduction for Hydrographic Surveys
Instructor: Engr. Rokib Hossain | River Engineering Solutions
🏠 Course Homepage

📖 Today's Outline

🌊 What Causes Tides?

Tides are the periodic rise and fall of sea level caused by gravitational forces from the Moon and Sun, combined with Earth's rotation. In hydrography, understanding tides is essential because:

  • Depths measured from the vessel must be reduced to a common vertical reference (Chart Datum).
  • Tidal currents affect vessel positioning and sonar data quality.
  • Safe navigation depends on accurately predicted water levels.

The two main tidal constituents are:

  • M2 (Principal Lunar semidiurnal) – period 12.42 hours
  • S2 (Principal Solar semidiurnal) – period 12.00 hours

Other constituents (N2, K1, O1, etc.) refine the prediction. Most ports have a mixed, semidiurnal, or diurnal tidal regime.

📌 Key concept: Tide is NOT the same as water level variation due to meteorological effects (storm surge, atmospheric pressure). Hydrographic surveys require separation of astronomical tide and residual water level.
🌙 [Diagram: Gravitational forces of Moon and Sun creating tidal bulges]
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📐 Harmonic Constituents & Prediction

Tide predictions are based on harmonic analysis – decomposing observed water levels into a sum of sinusoidal components (constituents). Each constituent has an amplitude, phase, and speed. Common constituents:

ConstituentSymbolPeriod (hours)Description
Principal lunar semidiurnalM212.42Main lunar tide
Principal solar semidiurnalS212.00Main solar tide
Larger lunar ellipticN212.66Modulates M2
Lunisolar diurnalK123.93Diurnal component
Principal lunar diurnalO125.82Diurnal lunar

National oceanographic agencies provide harmonic constants for ports. With these, you can predict tides for any date using software like XTide, TPXO, or commercial packages.

🔧 Hydrographer's shortcut: For many surveys, you can use online tide predictions from NOAA Tides & Currents or the local hydrographic office. Always verify with a real‑time gauge for critical work.

📏 Tidal Datums – LAT, MSL, HAT

Tidal datums are reference levels derived from tide observations over a period of time (usually 19 years). The most important for hydrography:

  • Chart Datum (CD) – Usually Lowest Astronomical Tide (LAT). All charted depths are relative to CD, ensuring that the actual depth is never less than shown.
  • Mean Sea Level (MSL) – Average of all water levels over a long period. Used for land surveying and coastal mapping.
  • Highest Astronomical Tide (HAT) – Used for vertical clearance under bridges and overhead cables.
  • Mean Lower Low Water (MLLW) – Common in the United States as chart datum.

When performing a hydrographic survey, you will measure depths relative to the instantaneous water level. Then you apply tide correction to reduce depths to Chart Datum:

Depth(CD) = Depth(measured) - Tide_height(relative to CD)
📊 [Diagram: Relationship between measured depth, tide height, and Chart Datum]
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📉 Tide Reduction in Hydrography

The typical workflow for tide reduction during a survey:

  1. Deploy a tide gauge (or use a real‑time feed from a nearby port). The gauge records water level relative to a known vertical datum (e.g., MSL or local benchmark).
  2. During data acquisition, the echosounder records raw depths (relative to transducer). The GNSS provides heave and dynamic draft.
  3. Acquisition software (Hypack, QINSy, Teledyne PDS) applies:
    • Draft correction (transducer depth below water surface).
    • Tide correction – converts water level from gauge datum to Chart Datum.
    • Settlement and squat corrections (for moving vessels).
  4. Final corrected depth is stored as depth below Chart Datum.

If you are working in a remote area without a tide gauge, you can use global tidal models (e.g., FES2014, TPXO9) but with lower accuracy (typically 10–20 cm).

✅ Pro tip: Always run a tide staff check at the beginning and end of each day to verify that your tide gauge or online prediction matches the actual water level at your survey site.

📡 Real‑Time Tide Corrections

Modern hydrographic software can receive real‑time tide data via:

  • NTRIP – some NTRIP casters include tidal corrections as part of RTK network.
  • API feeds from national agencies (e.g., NOAA CO‑OPS, UK National Tidal and Sea Level Facility).
  • Local radio telemetry from a tide gauge deployed on site.

For post‑processing, you can download verified tide data after the survey and re‑reduce your depths – this is often required for final deliverables that demand the highest accuracy.

Example: Using Python to download real‑time tide data:

import requests import json response = requests.get('https://api.tidesandcurrents.noaa.gov/api/prod/datagetter?product=water_level&station=8574680&datum=MSL&units=metric&time_zone=gmt&format=json') data = response.json() print(data)

🔗 External Resources & Tools

Use these authoritative sources for tide predictions, harmonic constants, and real‑time data:

ResourceDescriptionLink
NOAA Tides & CurrentsReal‑time and predicted tides for US stations, plus APItidesandcurrents.noaa.gov
UK Admiralty EasyTideTide predictions for ports worldwideeasytide.ukho.gov.uk
XTide – Free tide prediction softwareOffline predictions using harmonic filesflaterco.com/xtide
TPXO Global Tide ModelGlobal ocean tide model (for remote areas)tpxo.net
🔗 Authority linking: NOAA, UKHO, and TPXO are trusted sources. Linking to them improves your SEO credibility.

📋 Homework & Fieldwork Checklists

📘 Day 5 – Homework Checklist

  • ✅ Read the entire Day 5 post.
  • ✅ Visit NOAA Tides & Currents and locate a tide station near your area. Download one day of predicted tides and plot them (using Excel or manually).
  • ✅ Identify the chart datum used by your local hydrographic office (e.g., LAT, MLLW).
  • ✅ Write a paragraph explaining why tide correction is essential for safe navigation.
  • ✅ Share this post with #Hydrography #Tides.

🛠️ Fieldwork Readiness Checklist – Tide Reduction

  • ✅ Before survey: request predicted tides for the project period (from official source).
  • ✅ Set up a tide gauge or verify real‑time feed (NTRIP/API) with a manual tide staff reading at known time.
  • ✅ Confirm the vertical datum of the tide gauge (MSL, local benchmark, etc.) and the required Chart Datum.
  • ✅ Configure acquisition software to apply tide correction in real time.
  • ✅ During survey: log raw tide heights every 5–10 minutes as backup.
  • ✅ After survey: if using predicted tides, compare with gauge and re‑process if necessary.
🖨️ Pro tip: Use the Print / Download PDF button to keep these checklists in your field binder.

❓ Frequently Asked Questions

🔹 What is the difference between a tide and a tidal current? ▼
Tide refers to vertical water level movement. Tidal current is the horizontal movement of water caused by tides. Both affect navigation and survey operations.
🔹 Can I use predicted tides from an online source without a gauge? ▼
For low‑accuracy work (reconnaissance), yes. For production surveys (dredging, navigation), you must use a real‑time gauge or verified post‑processed tide data because predicted tides can be off by 0.2–0.5 m due to meteorological effects.
🔹 How often should I record tide readings during a survey? ▼
If using a digital gauge that logs continuously, you can sample at 1‑minute intervals. For manual staff readings, every 10‑15 minutes is sufficient for most surveys.
🔹 What is a tide staff and how do I use it? ▼
A tide staff is a vertical ruler fixed to a structure (pier, bulkhead). You read the water level relative to a known benchmark. It serves as a backup and calibration for electronic gauges.
🔹 How do I convert tide heights from MSL to Chart Datum? ▼
You need the offset between MSL and Chart Datum for your location (obtain from local tide tables or hydrographic office). Then subtract that offset from MSL‑referenced tide height to get height above CD.

✅ Action Items & Internal Linking

🎯 Your Tasks for Today

  • 📌 Step 1: Bookmark the Course Homepage and NOAA Tides & Currents.
  • 📌 Step 2: Download the checklist PDF using the button at the top.
  • 📌 Step 3: Install XTide or explore a tide prediction app to understand harmonic constituents.
  • 📌 Step 4: Leave a comment on this post: "The chart datum for my local port is ____."
  • 📌 Step 5: Proceed to Day 6: Sound Velocity Profile Basics using the link below.
◀ Day 4: Horizontal & Vertical Datums Day 6: Sound Velocity Profile ▶
🔗 Internal linking note: Day 4 and Day 6 are correctly linked. This builds a strong internal network for SEO.

👨‍🔧 Engr. Rokib Hossain | River Engineering Solutions

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