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TEC-GIS45-07 · Lesson 10 · 12 clock hours · lab

Sounding Sheet and Depth Contour Deliverables

Objective. Students produce a final hydrographic sounding sheet and depth contour map from corrected data using GIS and hydrographic software, meeting basic NOAA chart symbology conventions.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Sounding Sheet and Depth Contour Deliverables

What Is a Sounding Sheet?

A sounding sheet is the formal deliverable of a hydrographic survey. It shows the seafloor, lake bed, or river bottom as a map of measured depths, presented with enough information that any qualified user can interpret and trust the data without returning to the field. A complete sounding sheet includes the measured depth values (soundings), depth contours, a chart border, all required marginalia, and clear statements of the vertical datum and horizontal projection used. NOAA's hydrographic survey standards define what a professional sounding sheet must contain, and those conventions have been widely adopted across the industry.

Vertical Datum: Why MLLW Matters

Every depth on a nautical sounding sheet must be referenced to a defined vertical datum. NOAA charts for US coastal waters use Mean Lower Low Water (MLLW) as the sounding datum. MLLW represents the average of the lower of the two daily low tides over a 19-year tidal epoch. Depths referenced to MLLW represent a conservative (near-minimum) water level, so mariners can trust that charted depths are close to the worst-case shallow condition. If your corrected XYZ file has already been reduced to MLLW in Lesson 9, your sounding values are ready to plot. Always state the datum explicitly in the marginalia — omitting it makes the sheet unusable and is a critical rubric failure.

Importing and Interpolating in ArcGIS Pro

Your corrected XYZ file from Lesson 9 stores each sounding as easting, northing, and depth. In ArcGIS Pro, import it using Add XY Data or convert it to a point feature class via the XY Table to Point geoprocessing tool. Confirm the coordinate system matches your project before running any interpolation.

To build a continuous depth surface you have two practical choices:

For most student projects with moderate point density, either method is acceptable. Choose one, document your choice in the map's data source block, and be consistent.

Once you have a raster or TIN surface, use the Contour tool (Spatial Analyst or 3D Analyst) to generate depth contour lines at a chosen interval.

Selecting a Contour Interval

The contour interval must be appropriate to both the depth range of the survey and the output map scale. A common approach: divide the total depth range by approximately 8–12 to get a starting interval, then round to a convenient number (0.5 m, 1 m, 2 m, 5 ft, etc.). A very shallow survey of 0–4 m depth might use a 0.5 m interval; a deeper harbor survey ranging 0–20 m might use 2 m. Intervals that are too small create an illegible tangle of lines; intervals that are too large hide navigation hazards. Label contours clearly with depth values and state the interval in the marginalia.

Sounding Density and Thinning

Plotting every measured sounding on the final sheet often produces an unreadable cluster of numbers. NOAA's Chart User's Manual provides guidance on sounding density — the principle that soundings should be thinned to show the mariner the critical depth information without overplotting. Retain shoal (shallow) soundings preferentially because they represent navigation hazards. Remove redundant soundings in deep, featureless areas where the depth contours already convey the shape of the bottom. In ArcGIS Pro, manually select and hide redundant features or use a display scale filter. Every sounding that remains on the sheet should be labeled with its depth value, formatted consistently (e.g., one decimal place in meters or feet).

Required Marginalia

A professional sounding sheet must include all of the following elements in the map layout:

| Element | Purpose |

|---|---|

| Chart border | Defines the map extent |

| Title block | Survey name, location, date |

| Scale bar | Graphic scale for the projection |

| North arrow | Orientation reference |

| Datum note | State MLLW (or other datum used) |

| Projection information | Coordinate system and units |

| Data source / date block | Instrument, operator, date collected |

| Contour interval note | e.g., "Contour interval: 1 m" |

Use the instructor-provided layout template to ensure all elements are in the correct positions. Missing marginalia elements each cost rubric points; datum and projection labels are weighted most heavily because errors there make the entire chart misleading.

NOAA Symbology Guidelines

NOAA nautical chart symbology is documented in the Chart User's Manual (free download at coast.noaa.gov). For a student sounding sheet, the key conventions are: depth contours shown as continuous lines, labeled with depth values; soundings labeled in the same depth units as the datum note; shoal areas or hazards may be tinted blue per NOAA practice. Your sheet does not need to match a printed NOAA chart exactly, but it should follow the spirit of the conventions closely enough that a mariner or client could read it without confusion.

Peer Review Process

Before final submission, exchange map layouts with a classmate. Use the provided rubric to check: Is the datum labeled? Is the projection stated? Is the contour interval appropriate and labeled? Are soundings thinned appropriately? Is every marginalia element present? Write specific, constructive comments. Revise your own map based on the feedback you receive. The peer-review step mirrors professional quality-control workflows used by hydrographic survey firms and government agencies.

Lab Safety Note

This lab is conducted entirely at a computer workstation. Follow standard ergonomic practices: adjust your chair and monitor height to reduce neck and eye strain during extended GIS work. Save your project file frequently to avoid data loss. Keep the corrected source data file in a read-only backup folder — never overwrite your only copy of the Lesson 9 dataset.

Key terms9

Sounding sheetA formal hydrographic deliverable showing measured water depths as labeled point values and contours, with full marginalia including datum, projection, and source information.
Mean Lower Low Water (MLLW)The average of the lower of the two daily low tides over a 19-year tidal epoch; the standard vertical datum for NOAA nautical sounding charts on US coastal waters.
Depth contourA line on a sounding sheet connecting points of equal depth below the datum surface, analogous to an elevation contour on a topographic map.
TIN (Triangulated Irregular Network)A vector surface model that connects measured data points with non-overlapping triangles; it honors every observed value exactly and is commonly used to interpolate bathymetric surfaces.
KrigingA geostatistical interpolation method that estimates unknown values at unmeasured locations by weighting neighboring observations according to their spatial correlation structure.
Sounding densityThe number and distribution of individual depth values shown on a chart; NOAA guidelines require thinning redundant soundings while retaining shoal (shallow) values that represent navigation hazards.
MarginaliaAll information placed outside the main map body — including title, scale bar, north arrow, datum note, projection information, and data source block — required to make a chart independently interpretable.
Contour intervalThe constant difference in depth between successive depth contour lines on a sounding sheet; must be chosen to suit the depth range and map scale.
ShoalA shallow area of the seafloor, lake bed, or river bottom that may be a navigation hazard; shoal soundings are given priority when thinning sounding density on a chart.

Worked examples4

1. A harbor survey covers depths from 0.4 m to 9.8 m below MLLW. Choose an appropriate depth contour interval for a chart at 1:5,000 scale.
  1. Step 1 — Find the total depth range: 9.8 m − 0.4 m = 9.4 m.
  2. Step 2 — Divide by a target of 8–12 contour lines: 9.4 ÷ 10 = 0.94 m.
  3. Step 3 — Round to the nearest convenient value: 0.94 m rounds to 1.0 m.
  4. Step 4 — Check: 9.4 m ÷ 1.0 m = approximately 9 contour lines — within the 8–12 target range and legible at 1:5,000 scale.
  5. Step 5 — Record the interval in the marginalia: 'Contour interval: 1 m (MLLW).'
Answer: A 1 m contour interval is appropriate, producing approximately 9 contour lines across the depth range.
2. You have 1,200 sounding points in a 400 m × 300 m survey area. Your final chart is printed at 1:2,000. Estimate the average printed spacing between soundings if all points are displayed, and determine whether thinning is needed.
  1. Step 1 — Calculate survey area: 400 m × 300 m = 120,000 m².
  2. Step 2 — Calculate average ground-area per sounding: 120,000 m² ÷ 1,200 points = 100 m² per point.
  3. Step 3 — Approximate average spacing on the ground: √100 = 10 m between soundings.
  4. Step 4 — Convert to printed distance at 1:2,000: 10 m ÷ 2,000 = 0.005 m = 5 mm on paper.
  5. Step 5 — Evaluate legibility: 5 mm spacing between labeled depth values is extremely tight; numeric labels (each ~3–4 mm wide) will overlap. Thinning is required.
  6. Step 6 — Apply thinning, retaining shoal soundings and removing redundant deep soundings until average printed spacing is at least 8–10 mm in congested areas.
Answer: At 1:2,000, undisplayed soundings would average only 5 mm apart on paper — labels would overlap. Thinning is required; retain shoal (shallow) values preferentially.
3. A sounding collected at NAVD 88 elevation −3.72 m must be converted to depth below MLLW. The local tidal offset from NAVD 88 to MLLW is −0.41 m (meaning MLLW is 0.41 m below the NAVD 88 zero). What is the charted depth?
  1. Step 1 — Understand the geometry: MLLW is 0.41 m below NAVD 88 zero, so MLLW elevation in NAVD 88 terms = −0.41 m.
  2. Step 2 — Depth below MLLW = (MLLW elevation) − (sounding elevation) expressed as a positive depth: Depth = −0.41 m − (−3.72 m) = −0.41 + 3.72 = 3.31 m.
  3. Step 3 — The charted depth is 3.31 m below MLLW (a positive depth value on the sounding sheet).
  4. Step 4 — Label this sounding as '3.3 m' (rounded to one decimal place) on the chart.
Answer: The charted depth is 3.31 m below MLLW, labeled '3.3 m' on the sounding sheet.
4. Your kriging raster has a cell size of 2 m. You want to generate depth contours at a 1 m interval. A classmate suggests the 2 m cell size is too coarse for 1 m contours. Evaluate this concern.
  1. Step 1 — Recall the Nyquist-style rule for contour generation from rasters: the contour interval should generally be no smaller than the raster cell size, and many practitioners recommend the interval be at least 2× the cell size for reliable results.
  2. Step 2 — At 2 m cell size, each cell represents a 2 m × 2 m ground area. Contours at 1 m interval ask the software to detect depth changes smaller than one cell width.
  3. Step 3 — The Contour tool will still produce lines, but they are interpolated within each 2 m cell and may not accurately represent real bottom features at sub-2 m scales.
  4. Step 4 — Options: (a) accept a 2 m contour interval to match the raster resolution, (b) reduce cell size by re-interpolating the kriging raster at 1 m if data density supports it, or (c) document the limitation in the data source block.
  5. Step 5 — For this student project, accepting a 2 m interval or documenting the resolution limitation are both defensible choices.
Answer: The classmate's concern is valid. A 2 m cell raster cannot reliably support 1 m contours; either increase raster resolution or increase the contour interval to 2 m, and document the decision.

The math in this lesson1

Learners divide a 9.4 m depth range by 10 and round to a 1.0 m contour interval.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Students import the corrected XYZ sounding file into ArcGIS Pro (from Course 2 skills); create a TIN or kriging interpolated raster from the depth values; generate depth contours at an appropriate interval for the project scale; symbolize contours following NOAA nautical chart style guidelines · Students place soundings as labeled point features on the map using NOAA sounding density guidelines (thinning to avoid overplotting); add chart border, scale bar, north arrow, datum note (MLLW), projection information, and data source/date block following a provided template · Peer review: students exchange map layouts, check each other's datum labels, projection metadata, and contour interval labels against a grading rubric, then revise before final submission
Materials: ArcGIS Pro (institutional license), corrected XYZ dataset from Lesson 9, NOAA Chart User's Manual (free, coast.noaa.gov) for symbology reference, map layout template (instructor-provided), color printer or PDF export
Check: Sounding sheet and contour map reviewed against rubric: datum and projection labeled correctly, contour interval appropriate to depth range, sounding density follows NOAA guidelines, all required marginalia present; minimum passing score 80% of rubric points

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Hydrography / NSPS CST knowledge
1. Which vertical datum is used as the sounding reference on NOAA nautical charts for US coastal waters?
Hydrography / NSPS CST knowledge
2. A student's final sounding sheet is missing the datum note. Which consequence is most significant?
Hydrography / NSPS CST knowledge
3. When thinning soundings on a final sounding sheet, which values should be retained preferentially?
Hydrography / GIS / NSPS CST
4. A survey covers depths from 1.0 m to 13.0 m. Using the guideline of 8–12 contour lines across the depth range, which contour interval is most appropriate?
GIS / Esri ArcGIS Pro
5. In ArcGIS Pro, which tool converts a table of XY coordinates and depth values into a mappable point feature class?
GIS / Esri ArcGIS Pro
6. Which interpolation method for creating a bathymetric surface honors every measured sounding value exactly, connecting points with non-overlapping triangles?
Hydrography / NSPS CST knowledge
7. Which of the following is NOT a required marginalia element on a professional sounding sheet?
Hydrography / NSPS CST knowledge
8. A sounding is recorded at NAVD 88 elevation −5.10 m. The local MLLW surface has a NAVD 88 elevation of −0.55 m. What is the charted depth below MLLW?
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