PS Exam Preparation

Comprehensive preparation for the NCEES Principles and Practice of Surveying (PS) exam. 5 modules covering all 5 exam domains with 50 in-depth topics.

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Lesson 8

Hydrographic & Remote Sensing Surveys

Learning Objectives

After completing this topic, you should be able to:

Describe the purpose and methods of hydrographic surveys
Explain bathymetric measurement techniques and tidal datum references
Identify common hydrographic survey equipment and accuracy standards
Describe LiDAR technology, applications, and accuracy characteristics
Explain aerial photogrammetry principles, control requirements, and products
Understand remote sensing data types and their surveying applications
Compare the advantages and limitations of different data collection platforms

Overview

Hydrographic surveys and remote sensing technologies extend the surveyor's reach beyond traditional ground-based measurement. Hydrographic surveys map the underwater terrain and features of water bodies, while remote sensing technologies (LiDAR, photogrammetry, satellite imagery) capture terrain and feature data from airborne or spaceborne platforms. These methods are essential for projects involving water boundaries, coastal engineering, floodplain mapping, large-area topographic mapping, and environmental assessment.

The PS exam tests your understanding of the principles, equipment, accuracy standards, and appropriate applications of these technologies.

Key Concepts

Hydrographic Surveys

Hydrographic surveys measure and map the physical features of water bodies:

Primary purposes:

Navigation charting and harbor management
Dredging design and monitoring
Coastal and shoreline mapping
Water boundary determination
Flood studies and dam safety
Bridge and pier foundation design
Environmental and habitat mapping

Key elements measured:

Bathymetry (water depths)
Shoreline position
Bottom composition and features
Water levels and tidal characteristics
Currents and flow velocities
Underwater obstructions and structures

Bathymetric Measurement Methods

Single-beam echo sounder:

Emits a single acoustic pulse downward
Measures the two-way travel time of the reflected signal
Depth = (Speed of Sound x Travel Time) / 2
Speed of sound in water varies with temperature, salinity, and pressure (approximately 1,500 m/s in salt water)
Produces depth profiles along survey lines
Simple, reliable, and widely used

Multi-beam echo sounder:

Emits a fan of acoustic beams across the track
Measures depths at many points simultaneously across a swath
Provides complete bottom coverage between survey lines
More efficient for full-coverage bathymetric surveys
Requires careful calibration (roll, pitch, heading, latency)

Side-scan sonar:

Emits acoustic energy to both sides of the survey vessel
Produces an acoustic image of the bottom surface
Excellent for detecting objects, structures, and bottom texture
Does not provide accurate depth measurements (complementary to echo sounders)

Acoustic Doppler Current Profiler (ADCP):

Measures water current velocity and direction at multiple depths
Can also estimate bottom depth
Used for flow studies, discharge measurements, and navigation

Tidal Datums and Water Level Reference

Hydrographic surveys must reference water levels to a known datum:

Datum	Definition	Common Use
Mean Higher High Water (MHHW)	Average of the higher high water over a 19-year epoch	Shoreline mapping, boundary determination
Mean High Water (MHW)	Average of all high water over a 19-year epoch	Tidal boundary line in many states
Mean Tide Level (MTL)	Average of MHW and MLW	General reference
Mean Sea Level (MSL)	Average of hourly water levels over a 19-year epoch	Vertical datum approximation
Mean Low Water (MLW)	Average of all low water over a 19-year epoch	Navigation reference (some charts)
Mean Lower Low Water (MLLW)	Average of the lower low water over a 19-year epoch	Chart datum (NOAA charts)

The 19-year tidal epoch (National Tidal Datum Epoch) accounts for the 18.6-year lunar nodal cycle. The current epoch is 1983-2001.

Relationship to NAVD 88:

Tidal datums are referenced to local tidal observations
NAVD 88 is a fixed geodetic datum
The relationship between tidal datums and NAVD 88 varies by location
NOAA publishes tidal datum-to-NAVD 88 relationships at tide gauge stations

Common wrong path — using the wrong tidal datum for a boundary. Tidal waters have multiple plausible datums (MHW, MHHW, MLW, MLLW, etc.), and using the wrong one can move a legal water boundary by tens of feet. The general rule: boundary determination in tidal waters uses MHW or MHHW (depending on state law); chart datum for navigation is MLLW (different purpose). Students sometimes apply MLLW or MSL for a boundary question because those datums are more familiar from nautical charts — producing an answer that is hydrographically defensible but legally wrong. Check the state's specific statute for the tidal boundary definition before computing. Similarly, do not mix a tidal datum (MHW, MLLW) with a fixed geodetic datum (NAVD 88) without applying the local offset; the two differ by amounts that vary spatially, typically by tens of centimeters to a meter.

Quick retrieval check — try before reading on.

▶A nautical chart shows a shoreline at −3.2 ft (relative to MLLW). You need to determine the tidal boundary of a property fronting this shoreline. The state defines the tidal boundary as MHW. At this location, MHW is +4.8 ft above MLLW. What is the position of the boundary relative to the nautical-chart shoreline?

The nautical-chart shoreline at MLLW = 0 ft elevation in the MLLW frame. The MHW boundary is at +4.8 ft above MLLW. Under typical sloping coastal topography, the MHW line lies inland of the MLLW shoreline (higher elevation = farther from the water). The horizontal distance depends on the beach slope. If the beach has a 2% slope (1 ft rise per 50 ft horizontal), the MHW line would be approximately 4.8 / 0.02 = 240 ft inland of the MLLW shoreline. If the slope is 10% (steeper), it would be 4.8 / 0.10 = 48 ft inland. For a steeper slope still (1:1, 45°), approximately 5 ft inland.

The key practical point: the MLLW-based chart shoreline is not the legal tidal boundary — it is a navigation reference. The legal boundary (MHW per state statute) is measurably different in position, and that difference can be tens to hundreds of feet horizontally depending on slope. A surveyor who uses the chart shoreline as the boundary places the boundary in the wrong position.

Hydrographic Survey Equipment

Positioning systems:

GNSS (RTK or DGPS) for vessel position
Heading reference systems (gyrocompass, GNSS heading)
Motion reference units (heave, roll, pitch compensation)
Vessel-mounted inertial navigation systems

Depth measurement systems:

Single-beam and multi-beam echo sounders
Sound velocity profilers (SVP) for measuring speed of sound in the water column
Bar check equipment for echo sounder calibration

Data acquisition:

Hydrographic survey software integrating position and depth data
Automatic tide gauge connections for real-time water level correction
Data logging at high rates (multiple soundings per second)

Figure PS.5.8 — Remote Sensing Methods for Surveying

LiDAR (Light Detection and Ranging)

LiDAR is an active remote sensing technology that measures distances using laser pulses.

Airborne LiDAR operation:

An aircraft-mounted laser emits rapid pulses toward the ground
A scanning mechanism directs the laser across the flight path
A receiver measures the time for each pulse to return
GNSS and IMU (Inertial Measurement Unit) determine the sensor position and orientation
Point coordinates are computed from the range, scan angle, and sensor position

LiDAR returns:

First return -- Reflects from the top of the canopy or first surface encountered
Last return -- Reflects from the ground surface (penetrates through vegetation)
Intermediate returns -- Reflect from intermediate surfaces (understory, branches)
Full waveform -- Records the entire return signal, enabling detailed analysis

Point classification:

Raw LiDAR data contains millions of points that must be classified
Common classes: ground, low vegetation, medium vegetation, high vegetation, buildings, water, noise
Ground classification is essential for producing bare-earth terrain models
Automated classification algorithms require manual quality checks

LiDAR accuracy:

Component	Typical Accuracy
Horizontal	10-30 cm (depending on altitude and system)
Vertical (open terrain)	5-15 cm RMSE
Vertical (vegetated terrain)	15-30 cm RMSE
Point density	1-100+ points per square meter

Bathymetric LiDAR:

Uses green wavelength laser (532 nm) that penetrates water
Simultaneously collects topographic (near-infrared) and bathymetric data
Effective in clear, shallow water (typically less than 40 meters depth)
Used for coastal mapping and nearshore surveys

Aerial Photogrammetry

Photogrammetry extracts three-dimensional measurements from overlapping photographs.

Principles:

Stereo pairs of photographs taken from different positions enable 3D measurement
The parallax difference between corresponding points in stereo pairs provides depth information
Ground control points (GCPs) tie the photo model to the ground coordinate system
Interior orientation (camera calibration), relative orientation (photo-to-photo), and absolute orientation (model-to-ground) establish the geometric relationship

Key parameters:

Parameter	Definition	Typical Value
Forward overlap	Overlap between consecutive photos along the flight line	60-80%
Side overlap	Overlap between adjacent flight lines	25-40%
Flying height	Altitude above mean ground level	Varies by scale
Photo scale	Ratio of photo distance to ground distance	1:3,000 to 1:40,000
Ground Sample Distance (GSD)	Ground dimension of one pixel	3-30 cm

Ground control requirements:

Minimum of 3-4 GCPs per photo model (more for larger blocks)
GCPs must be visible in the photographs and precisely surveyed on the ground
Aerial triangulation (AT) extends control across large photo blocks using tie points
GNSS-aided aerial triangulation reduces (but does not eliminate) the need for ground control

Photogrammetric products:

Orthophotography (rectified, georeferenced aerial imagery)
Topographic contour maps
Digital terrain models
Planimetric feature mapping
3D building models

Accuracy depends on:

Flying height and camera focal length
Quality of ground control
Terrain relief relative to flying height
Operator skill (for manual compilation)
Software algorithms (for automated extraction)

UAS (Drone) Photogrammetry and LiDAR

Unmanned Aircraft Systems (UAS) have expanded access to photogrammetry and LiDAR:

Lower cost and faster deployment than manned aircraft
Higher resolution data collection (lower flying heights)
Suitable for small to medium-sized project sites
Require ground control or PPK/RTK GNSS for accuracy
FAA Part 107 regulations govern commercial UAS operations
Rapidly evolving technology with increasing capabilities

Satellite Remote Sensing

Satellite platforms provide data for large-area mapping:

Optical imagery -- Multi-spectral data for land use, vegetation, and planimetric mapping
Radar (SAR) -- Synthetic Aperture Radar for terrain and change detection, works through clouds
Satellite-derived elevation data -- SRTM, ASTER GDEM, and commercial stereo satellite imagery
Resolution and accuracy are generally lower than airborne methods but cover larger areas
Useful for reconnaissance, planning, and monitoring at regional to global scales

Exam Tips

Speed of sound in water is approximately 1,500 m/s -- echo sounder depth = (speed x time) / 2
The 19-year tidal epoch accounts for the 18.6-year lunar nodal cycle
MLLW is the chart datum used on NOAA nautical charts; MHW or MHHW is typically used for tidal boundary determination
LiDAR last returns penetrate vegetation to reach the ground; first returns map the canopy surface
Photogrammetric accuracy depends on flying height, camera focal length, and ground control quality
Ground control points are required for both photogrammetry and LiDAR (GNSS/IMU provide some, but GCPs are needed for verification)
Multi-beam sonar provides full bottom coverage; single-beam provides profiles along survey lines
Side-scan sonar produces images of the bottom but does not provide accurate depth measurements
Bathymetric LiDAR uses green wavelength (532 nm) to penetrate water; standard topographic LiDAR uses near-infrared which does not penetrate water
Forward overlap in aerial photography is typically 60-80%; side overlap is 25-40%

PS Exam Preparation

Module 1: Legal Principles

Module 2: Professional Survey Practices

Module 3: Standards & Specifications

Module 4: Business Practices

Module 5: Areas of Practice