5.0 Point Cloud
5.1 File Formats
Point cloud data and their derived products submitted to the Office of Coast Survey must be able to be reviewed before they can be applied to Safety of Navigation products. The table below lists standard point cloud file formats that the Office of Coast Survey can review.
Table 5.1.0 OCS Reviewable Point Cloud Data Formats
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5.2 Required Data & Metadata
Point cloud data submitted to the Office of Coast Survey must contain, at a minimum, the 3D position of the points and horizontal uncertainties associated with the submitted data. Some data formats, such as ASPRS LAS, require additional data fields as part of the format specification. Please refer to individual format specification documents for further details.
In addition to the metadata requirements for the survey, additional metadata is required for point cloud data submitted to the Office of Coast Survey. Some metadata are required for all point cloud data types, while some are dependent on the technique of data collection.
Table 5.2.0 Point Cloud Data and Metadata Types
Field | Technique | Type | Mandatory/Optional/Conditional |
|
All | Data | M |
Y/ | All | Data | M |
Z | All | Data | M |
Vertical Uncertainty | Acoustic bathymetry | Data | M |
Vertical Uncertainty | Lidar | Data | O |
Horizontal Uncertainty | All | Data | M |
Classification | Lidar submitted in LAS format | Data | M |
RGB | Lidar | Data | O |
Intensity | Lidar | Data | O |
Geodetic Parameters (see CRS) | All | Metadata | M |
Interpolation | All | Metadata | C |
5.3 Coverage
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The surveyed 4.0 meter depth contour referenced to chart datum. Coverage must be sufficient to fully develop the 4.0 meter depth contour. Note that in cases when this contour surrounds a feature disconnected from the contiguous mainland coastline (e.g., offshore islet or rock), the feature must be investigated utilizing appropriate hydrographic techniques and included in the feature file. -
The line defined by the distance seaward from the observed SPOR line which is equivalent to 0.8 millimeters at the scale of the largest scale chart covering any portion of the survey area (e.g., for a 1:80,000 scale chart, this line would fall 64 meters seaward of the SPOR). This application is most common in regions of fjord-like or "steep and deep" bathymetry. For any other application, field units must consult with their OCS Project Manager and/or COR when applying this particular rule in the field. -
The inshore limit of safe navigation for the survey vessel, as determined by the Field Unit. If kelp, rocks, breakers, or other hazards make it unsafe to approach the coast to the limits specified above, the NALL must be defined as the shoreward boundary of the area in which it is safe to survey.
5.4 Interpolation
Point cloud data submitted to the Office of Coast Survey that includes interpolated data must include a flag for each point that indicates if the point has been interpolated or not. Note that the inclusion of interpolated data without identifying the specific data points that are interpolated will preclude the use of the data for charted sounding application but may be used for contour generation or other bathymetric modeling purposes as deemed appropriate by the Office of Coast Survey.
5.5 Resolution
Point cloud data submitted to the Office of Coast Survey must not be downsampled. The resolution of the point cloud data must be able to support creating a gridded product at ½ the size of the features claimed to be detected by the survey.
5.6 Corrections
Point cloud data submitted to the Office of Coast Survey must be corrected for accurate 3D positioning of the points. If the corrections below are not applied to the data, it will substantially degrade the accuracy. This degradation will be reflected in the quality assessment and subsequent application of the data to products generated by the Office of Coast Survey.
Corrections typically account for the following categories (listed in the sequence in which they must be applied):
- Offset corrections to account for measurement equipment installation offsets, including (if applicable) angular offsets to ensure sensor reference frame alignment.
- Instrument error corrections to account for sources of error related to the measurement equipment itself.
- If the measurement equipment is located below the water surface, draft corrections to account for the depth of the measurement equipment.
- If the measurement equipment is mounted to a vessel traveling through water, dynamic draft corrections to account for the vertical displacement of the measurement equipment, relative to its position at rest, when a vessel is underway.
- If the measurement equipment utilizes sound waves, speed of sound corrections to account for the two way travel time of the sound wave through the water.
- Attitude corrections to account for the effect of vessel motion caused by waves and swell (heave, roll, pitch) and error in the vessel’s heading.
- Datum corrections to account for the translation from a preliminary survey datum to the final, delivered datum.
When performing an assessment of the final reported uncertainty values for the survey, the hydrographer must account for the quality of the corrections applied to the data and ensure that the corrections applied to the data support the reported uncertainty values. For more information on corrections typically applied to hydrographic data, see the Field Procedures Manual.
5.7 Cleaning
Point cloud data submitted to the Office of Coast Survey must be clean of spurious data points that cause the gridded surface to be shoaler or deeper than the reliably measured seabed by greater than the maximum allowable Total Vertical Uncertainty (TVU) at that depth.
The seafloor under charted human features (e.g. piers, anchor chains, bridges) must be left in the data and included in charted products. However, the pilings (and seawalls) supporting these structures must be rejected. Submerged pilings unattached from visible structures are obstructions that must be retained in the data.
5.8 Uncertainty
In addition to the overall horizontal and vertical uncertainty reported as part of the survey metadata, all bathymetric point cloud data derived from acoustic sensors submitted to the Office of Coast Survey must contain the uncertainty associated with the data. Uncertainty estimates may be calculated individually point-by-point, or by an assessment of the uncertainty of the data as a whole. Submitting point cloud data with an assessment of the uncertainty of the data as a whole instead of a point-by-point basis may result in the Office of Coast Survey only applying a portion of the submitted data, assessing the data with a lower grade, or being unable to use the data.
Failure to provide uncertainty values will result in the Office of Coast Survey determining the appropriate uncertainty based on an evaluation of the data and metadata provided, as well as any other factors that are deemed relevant at the sole discretion of the Office of Coast Survey.
If using a topo-bathymetric airborne lidar system, vertical uncertainty values may be provided to the Office of Coast Survey based on the Eren et al. model (2019) if it has been computed.
Point cloud data uncertainty must be broken down into Total Horizontal Uncertainty (THU) and Total Vertical Uncertainty (TVU) estimates and reported at the 95% Confidence Level.
The uncertainty estimates for point cloud data must account for all component uncertainties in the submitted data due to residual systematic and system-specific instrument uncertainties such as the speed of sound in water; beam-forming precision; system latency; static vessel draft; dynamic vessel draft; heave, roll, and pitch; and any other sources of uncertainty in the actual measurement process, including those associated with vertical datum correction such as tidal measurement, zoning, or translation from one datum to another. For more information on uncertainty components as well as how to determine the uncertainty values for the components, see the Field Procedures Manual.
Total Horizontal Uncertainty
Table 5.8.0 Maximum Allowable THU Values
OCS Quality Metric | Fixed Value | Variable Value |
Exceptional | 1 meter | N/A |
Critical | 2 meters | N/A |
General 1 | 5 meters | 5% of depth (in meters) |
General 2 | 20 meters | 10% of depth (in meters) |
General 3 | 50 meters | N/A |
General 4 | 500 meters | N/A |
Total Vertical Uncertainty
Depth TVU
Where:
a represents the portion of the uncertainty that does not vary with depth
b is the coefficient which represents that portion of the uncertainty that varies with depth
d is the depth
For negative depths (i.e. heights), the maximum allowable TVU cannot be less than a
Table 5.8.1 Maximum Allowable Depth TVU Values
OCS Quality Metric | The portion of the uncertainty that does not vary with depth (a) | The coefficient which represents the portion that varies with depth (b) |
Exceptional | 0.15 meters | 0.0075 |
Critical | 0.25 meters | 0.0075 |
General 1 | 0.5 meters | 0.01 |
General 2/3 | 1.0 meter | 0.02 |
General 4 | 2.0 meters | 0.05 |
Height TVU
Table 5.8.2 Maximum Allowable Height TVU Values
OCS Quality Metric | Maximum Allowable Height TVU |
Exceptional | 0.15 meters |
Critical | 0.25 meters |
General 1 | 0.5 meters |
General 2/3 | 1.0 meter |
General 4 | 2.0 meters |
5.9 Data Quality Control
Crosslines
Crosslines should be geographically dispersed throughout the survey area.
For crosslines acquired and processed, the hydrographer must evaluate overlapping crossline and mainscheme coverage to ensure that the depth/height values from the two data sets do not differ more than the maximum allowable TVU for the depth/height. Any deviations must be investigated, the source of the error identified, and if possible, corrected. If unexplained or excessive discrepancies persist, additional crosslines must be reacquired to assist in the resolution of the issue. If deviations are not able to be corrected, they must be documented in the Report of Survey. This documentation must include a description of the deviation, any steps taken to attempt to resolve the deviation, as well as the hydrographer’s understanding of the cause of the deviation.
Data Overlap
Known Temporal Changes