Journey of a Sounding: Application of NOAA Soundings and Features to Navigation Products

Transcription

1 Journey of a Sounding: Application of NOAA Soundings and Features to Navigation Products Crescent Moegling, Peter Holmberg NOAA, Pacific Hydrographic Branch Seattle, WA Introduction: From the moment a sonar ping is emitted from a multi- beam transducer and reflects off the seafloor, to its final representation on a nautical chart product, it has undergone numerous transformations both vertically and horizontally. This paper will evaluate the progression of a sounding from ping to navigation product within the National Oceanic and Atmospheric Administration s (NOAA) Office of Coast Survey (OCS) workflow. Four examples were selected for this evaluation from hydrographic survey H11859 in Portland, Oregon. The survey was chosen based on the criteria that it contain a variety of features depicted by high resolution bathymetry, and that it has been fully applied to new editions of the raster and electronic navigation charts. This paper will describe the transformations of four discrete points: a sounding, an obstruction, a wreck and a rock. ENC RNC Scale Position (ENC) Depth Sounding US5OR15M (inset) 1:10, N, W 9.7m/32ft Obstruction US5OR16M :5, N, W 5.7m/19ft Wreck US5OR17M :20, N, W 6.7m/22ft Rock* US5OR15M :20, N, W 1.2m/4ft *Rock charted as a sounding Figure 1: Sounding, Obstruction, Wreck and Rock Examples H11859 was acquired in 2008 and 2009 by NOAA contractor David Evans and Associates. The project area was the subject of a paper presented at the 2009 US Hydrographic Conference titled GPS Derived Water Levels for Large Scale Hydrographic Surveys: Implementation of a Separation Model of the 1

2 Columbia River Datum, A Case Study that detailed the methodology and challenges to reduce the survey to Columbia River Datum using Ellipsoidally Referenced Survey (ERS) techniques. The Columbia and Willamette Rivers are busy, metropolitan waterways that support a vibrant Maritime Transportation System in the region. More than 35 various types of commodities travel up and down the Columbia River daily. This river system is the number one U.S. gateway for wheat and barley export, West Coast paper and forest products, West Coast mineral bulk exports and West Coast auto imports. This inland system supports 10 million tons of cargo annually and is connected to the deep draft channel and ocean shipping which supports over 40 million tons of cargo annually. (Simmons, 2010). All vessels engaged in foreign trade are required to employ a Pilot when transiting the Columbia River. Figure 2: Project Area of H11859 The 7.74 square nautical miles of the survey area extend from the Columbia River Mile 101 to 110; and includes Willamette River Mile 0-17, Multnomah Channel and North Portland Harbor. The navigation charts in the area of interest are generally quite large scale ranging from 1:5,000 insets to 1:40,000. The vertical datum is Columbia River Datum, an adopted low- water gradient datum relative to North American Vertical Datum of 1988 (NAVD88). (Stolz, 2005) The workflow from ping to product is divided between the field, the processing branch and the Marine Chart Division (MCD). There is meticulous quality control within each separate step but a holistic examination of the process has not been conducted in several years. The purpose of this evaluation was to review processes across all phases of the ping to product workflow regardless of where it took place. The iterations of the data through this lengthy process haven t been evaluated in great detail since the processing branches moved from paper- based deliverables (H- Drawing) to a digital vector deliverable in S57 known as the HCell. (Barry, 2005) 2

3 Figure 3: Process Workflow Overview Field Acquisition and Processing: Bathymetry was acquired using Reson 8101 and 7125 systems, positioned with Applanix POS/MV 320 v4 and processed in CARIS Hydrographic Information Processing System (HIPS). The survey was collected using the requirements defined in the 2007 National Ocean Service (NOS) Hydrographic Surveys Specifications and Deliverables (HSSD). The customary key contributors to the correction of bathymetry are tide, vessel attitude (heave, pitch and roll), vessel offsets and sound velocity. Since this survey was reduced to chart datum using ERS techniques, the larger correctors like tide and vessel draft were not necessary. Dynamic draft (settlement and squat) values were calculated through the use of RTK GPS observations, but were not included in the vessel file or applied during processing due to the use of GPS water levels. One of the many benefits of GPS water levels is that dynamic draft is incorporated in the GPS height measurements that are recorded at the survey vessel. (H11059 DAPR, 2009) Figure 4: Columbia River Datum (Chart Datum) A critical component for any hydrographic survey using GPS heights is a separation model from the reference ellipsoid to the chart datum. For this survey the contractor developed a high resolution separation model of the relationship between NAD83 (GRS- 80 ellipsoid) and Columbia River Datum (CRD). The resulting model files were used during both acquisition and processing to reduce GPS heights relative to the ellipsoid to GPS water levels relative to CRD. (H11059 DAPR, 2009) Bathymetry Process and Corrector Workflow: Bathymetric Data is Acquired o Raw soundings, attitude, heading, height, and position data were recorded in XTF format 3

4 o POS/MV and inertial reference systems were used to measure attitude, heading, height, and position Convert Raw XTF Data into CARIS HDCS Format Apply True Heave Apply Sound Speed Profiles o Nearest in time, within two hours Load post- processed attitude, navigation, and heading Merge vessel offsets, apply GPS tides Compute Total Propagated Uncertainty (TPU) Reject TPU greater than the horizontal and vertical error limits specified in the NOS Hydrographic Surveys Specifications and Deliverables (April 2007). Designate Soundings Apply HDCS Data to Grid Surface o Half meter Combined Uncertainty and Bathymetric Estimator (CUBE) weighted surface IHO S- 44 Order 1 Density & Local Disambiguation method Shallow Advanced settings o Half meter finalized surface o One meter combined surface Identify Features and Convert from Designated Sounding To S File The calculated uncertainty values of all nodes within the Original (not finalized or combined) CUBE surfaces ranged from to meters. The survey fulfilled International Hydrographic Organization (IHO) Order 1 specifications for depth accuracy. (H11059 DR, 2009) Object detection requirements were exceeded throughout the survey. The 2007 HSSD described object detection coverage characteristics as: The multi- beam shall be operated and the grid resolutions shall be appropriate, such that a 1 meter cube can be detected to 20 meter depths (for depths greater than 20 meters, the minimum size of detectable targets shall be 5% of the depth.) Grid resolution of 2% to 4% of the depth, to a minimum resolution of 0.5 meters. Maximum propagation distance of soundings to node of 4% of the depth or one grid resolution, whichever is greater At least 99% of all nodes on the surface must be populated Maximum surface uncertainty is IHO Order 1 for depths less than 100 meters (IHO Order 2 for depths greater than 100 meters) No holiday larger than 3 nodes across Hydrographic Processing Branch: 4

5 The survey data package is submitted from field unit to NOAA s Atlantic (Norfolk, VA) or Pacific (Seattle, WA) Hydrographic Branch. The role of the hydrographic branch is to verify data quality and reduce the bathymetric surfaces and features to a chart scale product (the HCell) in preparation for MCD where the entire chart product is assembled and produced. 5

6 Survey Acceptance Review: The survey was delivered to the Pacific Hydrographic Branch (PHB) in May, 2010 where it went through a rigorous Survey Acceptance Review (SAR). During the SAR a NOAA Physical Scientist using CARIS Hydrographic Information Processing System (HIPS) and CARIS Bathy DataBASE (BDB), evaluated key data integrity components of the survey including surfaces, features, correctors and reports relative to the requirements detailed in the 2007 HSSD. The outcome of the SAR was a combined one- meter surface and a set of verified survey scale features ready for compilation to the HCell. In addition, the field unit was provided a detailed description of the review process, items of note and feedback for improvement. The complexity of this survey is evident in the lengthy processing time as well as the number of Dangers to Navigation (DToN) and navigationally significant features. While ultimately one sounding or feature is depicted on the chart product, it s overly simplistic to suppose a single point is truly what s represented. Numerous soundings contribute to the CUBE grid node of the original surface. The integration of CUBE into NOAA s bathymetric processing is the subject of numerous academic papers and studies. Figure 5: Bathymetric soundings contributing to CUBE Grid Nodes Four Examples for this Review: The bathymetric representation of the four examples chosen for this study are shown below with corresponding vertical measurements, position, overview of the original, finalized and combined grids and a 3D overview of the bathymetry. In three of the four examples, a sounding was designated to insure the grid honored the least depth of the feature. The designated sounding is honored in the finalized and combined grids. 6

7 Sounding: The sounding chosen for this evaluation had no special significance and unlike the other three examples in this analysis, was not designated. Sounding (not designated) Meters Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total n/a n/a Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W Figure 6: Original (0.5m), Finalized (0.5m) and (1m) Grid Nodes Figure 7: 3D Overview of Seafloor for Sounding Example 7

8 Obstruction: Obstruction (designated) Meters Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total n/a n/a Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W Figure 8: Original (0.5m), Finalized (0.5m) and (1m) Grid Nodes Figure 9: 3D Overview of Seafloor for Obstruction Example (pile) 8

9 Wreck: Wreck (designated) Meters Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total n/a n/a Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W Figure 10: Original (0.5m), Finalized (0.5m) and (1m) Grid Nodes Figure 11: 3D Overview of Seafloor for Wreck Example 9

10 Rock: Rock (designated sounding) Meters Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total n/a n/a Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W Figure 12: Original (0.5m), Finalized (0.5m) and (1m) Grid Nodes Figure 13: 3D Overview of Seafloor for Rock (sounding) Example 10

11 HCell Compilation: Once the Survey Acceptance Review (SAR) is completed, cartographic compilation begins. The SAR reviewer provided the cartographer two data deliverables for compilation: a single resolution combined surface and the Final Features File (FFF). In this example, the 1m single resolution surface was derived from higher resolution 0.5m collection of finalized surfaces. The Final Features File contained all features addressed by the hydrographic survey with full S- 57 attribution, and any additional remarks and recommendations. These files were used to compile data relative to the largest scale chart. Using a tool to auto- generate soundings in CARIS Bathy Database, a high density set of soundings was created on the shoalest points from the 1m combined surface. Density of the sounding set is subjective, but generally is 5 to 10 times greater than the density of soundings on the existing largest scale chart in the vicinity. Soundings were then selected for charting either manually or automatically to best emulate the existing density of charted soundings. During the sounding selection process, features from the FFF are also selected based on chart scale and navigational significance. The four points in this case study were derived from the Bathymetry Associated with Statistical Error (BASE) surface. Occasionally features with a depth from the FFF are used in compilation. This may occur when a feature s least depth cannot be achieved by sonar due to dangerous proximity of near shore hazards that impede safe navigation for the survey vessel. Other cases may arise where the hydrographer is not confident the least depth on a feature was captured, such as the mast of a ship wreck. Examples are least depths acquired by leadline, leveling, laser range finder, divers least depth gauge, or any other non- sonar acquired depth. During compilation, the cartographer may change a point s feature class. For example, the rock as identified by the field was judged to be more appropriately charted as a sounding. Depth and position are not altered during the change of feature class. All soundings and features selected for charting were delivered from the processing branches to MCD in millimeter precision in the HCell for H The HCell was delivered to the MCD in February, Marine Chart Division MCD, located in Silver Spring, MD, receives the HCell from the Processing Branch and compiles it together with other layers of source data, such as buoys, bridges and aerial photography. MCD s primary navigation products are the Raster Nautical Chart (RNC) and Electronic Nautical Chart (ENC). 11

12 Chart Compilation: Raster Nautical Charts: The HCell was applied to the appropriate nautical chart products in June, Upon receipt at MCD, the HCell was converted from S to shape file format. All points with a depth (VALSOU) were converted to raster chart units and NOAA rounding was applied. Unlike standard arithmetic rounding based on 0.5, NOAA rounding is based on For example, the rock is 1.444m, which equals 4.738ft. Under NOAA rounding the new depth of the rock becomes 4ft because is less than NOAA rounding provides a slight conservative buffer against uncertainty and shifting depths of the sea floor. Depending on region, NOAA charts have soundings in three different units. Most deep water charts and almost all Alaskan and Pacific charts are in fathoms and feet. Most charts in the Atlantic and Gulf of Mexico are in feet. There are few charts near the US Canadian border on the East Coast that are in meters. For charts in fathoms and feet, NOAA rounding takes place on the subscript foot, and in that way is no different from NOAA rounding applied to soundings on charts in feet. However, on charts with fathoms and feet, once depths exceed 11 fathoms, all soundings are in whole fathoms and NOAA rounding is applied to those whole fathoms. There are some charts in fathoms and fractions, though they are being phased out. For NOAA s few metric charts, soundings are in meters and tenths up to 20 meters deep, then half meter precision from 20 to 30 meters, and whole meters for soundings 30 meters and deeper. Maximum artificial depth differences due to application of NOAA rounding on RNCs are shown below. Fathom RNCs: 0.457m Feet RNCs: 0.076m Metric RNCs: 0.25m Change in position of points from HCell to RNC in H11859 ranged approximately from 1 to 10m. As expected the greatest shifts in position typically occur on smaller scale charts. NOAA s Nautical Chart Manual (NCM) Vol. I Section states: Soundings shall be charted in their exact geographic positions. The visual center of the whole number (one or more digits) is always considered the geographic position of the charted sounding, including a whole number with a fractional component and a whole number with a subscript component. The visual center of the bar between numerator and denominator is always considered the position of the charted fractional soundings without a whole number component. Soundings are rarely moved from their geographic positions. When it is necessary, the maximum position displacement from its exact geographic position is 1/2 the height of the charted whole number not including fractional or subscript components. After evaluation of the four sample points, the differences in positions of HCell points and RNCs did not exceed half the height of the charted whole number; therefore, acceptable under NCM specifications. Test points varied as shown in the right column of the table below. In the Figure 14, the center of the grey box is perceived to be middle of the point on the RNC. 12

13 Vertical and Horizontal Changes from HCell to RNC Sounding 9.687m = ft 32 ft NOAA rounded 4.2m ESE of original Obstruction 5.909m = ft 19 ft NOAA rounded 2m ENE of original Wreck 6.710m = ft 22 ft NOAA rounded 8.5m S of original Sounding (Rock) 1.444m = 4.738ft 4 ft NOAA rounded 9.1m SSE of original Figure 14: Horizontal displacement between HCell and RNC Chart Compilation: Electronic Navigation Charts Under MCD s current workflow, ENCs are made directly from RNCs. Source data such as aerial photography to update shoreline from NOAA s Remote Sensing Division (RSD), or other sources of chart data aside from the HCell, may be applied to ENCs before RNCs. MCD is in the process of harmonizing RNC and ENC charts as closely as possible. Point, line, and area files from RNC production are used to create and update ENCs, so no digitization is done in this stage, however any generalization or horizontal adjustments performed during RNC compilation to prevent overprint or to accommodate scale are transposed onto ENCs. Depth values are converted from RNC chart units (feet or fathoms and feet) back to meters. The metric values are then truncated to one decimeter, which causes artificially shoaler depths by 0.1m for some points like the sounding and obstruction as shown in the table below. The red ENC points in Figure 15 indicate minor horizontal shifts between RNC and ENC positions. The displacement is speculated to be caused by different center points of font display in MCD s RNC and ENC production software. Vertical and Horizontal Changes from RNC to ENC Sounding 32ft = 9.753m 9.7m truncation 4m W of RNC position Obstruction 19ft = 5.791m 5.7m truncation 3m W of RNC position Wreck 22ft = 6.706m 6.7m truncation 8m N of RNC position Sounding (Rock) 4ft = 1.219m 1.2m truncation 6m N of RNC position 13

14 Figure 15: Horizontal displacement between RNC and ENC RNC & ENC Display Three independent ENC viewers were used to inspect the exact depths and positions of the four example points: CARIS EasyView, Navi- Sailor 4000 ECDIS and Coastal Explorer Positions were exactly the same in all displays, although Coastal Explorer displays some feature depths 0.1m shoaler. CARIS EasyView ENC Navi- Sailor 4000 ECDIS Coastal Explorer 2011 Sounding Obstruction Wreck Sounding (Rock)

15 There are numerous nautical chart display softwares available to the mariner and with each one; the display of RNCs and ENCs are varied. Figure 16 shows four views of the sample Obstruction on ENC US5OR16M and RNC Figure 16: Sample obstruction viewed in different display and navigation software 15

16 Conclusion: There are nine significant steps (or transformations) from the raw sounding to what is eventually displayed on the navigation products. These steps take place in the field, at the Processing Branch and at the Marine Chart Division. 1 Raw Depth 2 Corrected Depth 3 Original Grid Depth 4 Finalized Grid Depth 5 Combined Grid Depth 6 HCell Sounding 7 Converted from Meters to Feet 8 Converted/NOAA Rounded RNC 9 Converted Feet to Meters ENC Figure 17: Nine Steps from Ping to Product Field Processing Branch Marine Chart Division Application of soundings and features to the navigation product is a series of evolving processes. OCS continues to improve practices to streamline the most accurate processing of hydrographic data for its customers. The accuracy of charted features and soundings is tied to technological capabilities and policy. Aboard the survey vessel, from the raw depth to the corrected depth, improvements by way of sound velocity, GPS tides, etc are an ongoing scientific pursuit that is well- modeled and objective in practice. Cartography relies on the best quality bathymetry to be the foundation of depth and horizontal positional accuracies on the RNC and ENC. This quality is mandated internally with OCS Hydrographic Survey Specifications and Deliverables with fundamental guidance from the IHO Standards. A bathymetric sounding or feature s final depiction on the navigation product is where policy dictates accuracy. The evaluation of transformations of a sounding from ping to product is a good quality check of OCS processes. It is recommended this evaluation be done routinely with more examples from more surveys to provide a better statistical understanding of the analysis. Specifically, more varied depth ranges should be evaluated to determine horizontal and vertical shifts in coarser grid resolutions and subsequent smaller scale chart products. It would be expected these shifts would increase as grid resolution decreased. Discrepancies from the bathymetric data that charted soundings and features originated from are navigationally insignificant and err on the side of caution by way of shoaler depictions through BASE surfaces, application of NOAA rounding, and truncation. For purposes of real world navigation, NOAA charts are excellent when used at correct scale. Below are the tabular results of this evaluation. 16

17 Results: Sounding (not designated) Meters Feet Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W HCell N W Converted from meters to feet n/a n/a Converted/NOAA rounded RNC N W Converted feet to meters ENC US5OR15M N W ENC display on commercial software 9.7 or 9.6* N W *9.6 Displayed only in Coastal Explorer, not accounted for in total vertical range. Vertical Range (Corrected Depth to ENC Depth) Horizontal Range (Corrected Depth to ENC Depth) 0.056m 0.531m Obstruction (designated) Meters Feet Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W HCell N W Converted from meters to feet n/a n/a Converted/NOAA rounded RNC N W Converted feet to meters ENC US5OR16M N W ENC display on commercial software 5.7 or 5.6* N W *5.6 Displayed only in Coastal Explorer, not accounted for in total vertical range. Vertical Range (Corrected Depth to ENC Depth) Horizontal Range (Corrected Depth to ENC Depth) 0.209m 0.531m 17

18 Wreck (designated) Meters Feet Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W HCell N W Converted from meters to feet n/a n/a Converted/NOAA rounded RNC N W Converted feet to meters ENC US5OR17M N W ENC display on commercial software 6.7 or 6.6* N W *6.6 Displayed only in Coastal Explorer, not accounted for in total vertical range. Vertical Range (Corrected Depth to ENC Depth) Horizontal Range (Corrected Depth to ENC Depth) 0.01m 0.309m Rock (designated sounding) Meters Feet Latitude Longitude Raw Depth n/a n/a Corrected Depth N W Corrector Total Original Grid Depth N W Finalized Grid Depth N W Combined Grid Depth N W HCell (Rock changed to sounding) N W Converted from meters to feet n/a n/a NOAA rounded RNC N W Converted feet to meters ENC US5OR15M N W ENC display on commercial software N W Vertical Range (Corrected Depth to ENC Depth) Horizontal Range (Corrected Depth to ENC Depth) 0.244m 2.161m Total Average Shift of Test Points Average Vertical Shift Average Horizontal Shift 0.130m 0.883m 18

19 References Barry, C., Legeer, S., Parker, E., VanSant, K. (2005). U.S. Office of Coast Survey's Re- Engineered Process for Application of Hydrographic Survey Data to NOAA Charts. CARIS 2005 Conference, Halifax, Nova Scotia, Canada. David Evans and Associates, Inc. (2009). Descriptive Report (DR), H David Evans and Associates, Inc. (2009). Data Acquisition and Processing Report (DAPR), OPR- N338- KR- 08. NOAA (2007). NOS Hydrographic Surveys Specifications and Deliverables, April NOAA (2013). Nautical Chart Manual Volume 1, Version January Simmons, S. (2010). Historical Waterborne Commerce on the Columbia- Snake River System: Commodity Movements Up and Down River, Pullman, WA: Freight Policy Transportation Institute. Stolz A., Martin C., Wong C. (2005). Vertical Control in a Tidally Influenced Complex River System With a Fixed Low Water Datum, THSOA HYDRO005, Paper 09.1, NOAA/NOS Center for Operational Oceanographic Products and Services. Any mention of a commercial product is for informational purposes and does not constitute an endorsement by the U.S. Government or any of its employees or contractors. 19