Vertical Control Challenges: Case Study of a 2008 Multibeam Survey in the Sparsely Referenced Environment of Bristol Bay, Alaska

8/9/2019 Vertical Control Challenges: Case Study of a 2008 Multibeam Survey in the Sparsely Referenced Environment of Bri

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Vertical Control Challenges: Case Study of a 2008 Multibeam Survey in the

Sparsely Referenced Environment of Bristol Bay, Alaska

LTJG Meghan McGovern, Weston Renoud, Dr. Robert A. McConnaughey

National Oceanographic and Atmospheric Administration, National Marine Fisheries

Service & National Ocean ServiceApril 2009

The Bristol Bay region is a prime example of an area with vast economic and

environmental significance but with minimal modern bathymetric data and poor spatial

coverage of vertical reference stations. This significantly limits the accuracy of data

essential to commerce, ecosystems, recreation, and research.

This case study used a 2008 multibeam trackline survey of Bristol Bay to make a

qualitative evaluation of Post Processed Virtual Reference Stations (PPVRS), PrecisePoint Positioning (PPP), single station tides, and discrete tidal zoning as methods for

vertical transformation. Having few measured separations between the Mean Lower LowWater (MLLW) tidal datum, the International Terrestrial Reference Frame 2005(ITRF05), and the GEOID06 model in Bristol Bay, the data were processed to compare

mean-difference trends rather than quantitave values, in order to highlight quality and

isolate errors within each process.

Limitations were found in all processes, but PPP introduced the least error and is

presented as the preferable choice to survey to ITRF05 in this region of Alaska. Since

the method relies exclusively on precise satellite correctors, it does not introduce theinherent limitations of using the distant references that all other methods will use.

However, since tides are not locally observed in most offshore surveys, and since land-

based tidal observations and zoning are limited, a MLLW to ellipsoid or geoid separationis not known. This means that PPP data is accurate and robust, yet impractical for

charting unless tidal observation is recorded onsite. As survey priorities begin to includearctic waters, where offshore water levels are not necessarily deep, PPP should be

considered for use in conjunction with offshore water level gauges to measure the

MLLW to ellipsoid and geoid separation.


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Introduction

Alaska represents 64 percent of the Nations coastal waters (NEMW 2000) and is one of

the most productive marine ecosystems in the world (NRC 1996). The economic value

of the Bering Sea and Bristol Bay is, in part, supported by approximately 25

commercially important species, providing 2-5 percent of the worlds fishery productionand 56 percent of the fishery production of the United States (NRS 2006). In addition to

the economic and ecological vitality of the region, it is an important area for indigenoussubsistence, research, commerce, commercial development, and recreation. The

commonality among these activities is that they all use and benefit from precisely-

positioned sounding data.

Unfortunately, although this region consists of over 300,000 sq. nm of navigationally

significant waters (OCS 2008), the region has few of the vertical control reference

stations (NOS 2008, USCG 2002) that are traditionally used to vertically transformsoundings. Emerging technologies offer the possibility to increase accuracy and

precision in vertical transformation, but questions remain with respect to datumseparations, long-range trends between these datums, and quality of methods for verticaltransformation in this sparsely-referenced region.

This case study uses four methods of vertical transformation to examine trends betweendatums and to observe their qualitative efficacy for future surveys in Alaska.

Method

Multibeam data were acquired along a 140 nautical mile (nm) transect using a Reson

Seabat 8111 (Reson A/S, Slangerup, DK)1

system over a 23-hour period through 8-9

August 2008. The transect crosses from the Bering Sea into Bristol Bay (Figure 1) withdepths ranging from ~70 to ~150 meters.

Traditionally, nearshore surveys in Alaska use tide gauges to transform survey soundingsto Mean Lower Low Water (MLLW). These gauges are then tied to benchmarks which

locally link MLLW to the North American Datum of 1983 (NAD83) Ellipsoid, providing

the ability to apply modern water level updates to older surveys without re-surveying.Tidal models are extrapolated from these data to produce discrete tide zones to

encompass part or all of a survey area. In the last few years, Post-processed Virtual

Reference Station (PPVRS), a type of Post-Processed Kinematics (PPK), has been usedto survey to the ellipsoid in places where there is a measured ellipsoid to MLLW

separation.

This transect study uses each of these traditional and modern methods, and is located

over 50 nautical miles from the nearest reference station.

1Mention of a commercial company or product does not constitute an endorsement by NOAA, National Marine

Fisheries Service, or National Ocean Service. Use of information from this publication concerning proprietary

products or the tests of such products for publicity or advertising purposes is prohibited.


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Figure 1: Transect located in Bristol Bay overlaid on NOAA Chart 513.

The transect was segmented into three 16 nm x 16 nm regions which contain six surveylines each. Segments include multibeam data from the northeast (NE) and southwest

(SW) extents as well as the middle (M) (Figure 2).

Figure 2: Study areas located at the middle (M), and northeast (NE) and southwest (SW)

extents of the transect.


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Each of the three segments was processed using four vertical transformation methods:

1. Discrete tide zone method using tide station Unalaska (9462620)

2. Post-processed Virtual Reference Station (PPVRS) method for two models:

a. ITRF05 model

b.

GEOID06 model3. Precise Point Positioning (PPP) method for two models:

a. ITRF05 model

b. GEOID06 model4. Single stationmethod using tide station Unalaska (9462620)

Discrete Tide Zone Method:

Discrete zoning provides time and range correctors to sounding data in order to

compensate for distance from a tide gauge. Data within the transect segments werevertically transformed to the MLLW datum using a modified zone file provided by

NOAAs Center for Operational Oceanographic Products and Services (CO-OPS) (redand blue polygons, Figure 3). The northeastern-most zone (blue polygon, +120 minute

time delay) was enlarged to encompass data initially left out of the zoning. Thismodification was not considered significant for the comparisons, as no vertical artifacts

were seen in this part of the data after the zone was modified.

Figure 3: Modified (blue polygon) CO-OPS tidal zoning (red polygons) based on

Unalaska station 9462620.

Soundings were transformed in CARIS (CARIS, Fredericton, NB) using a rigid bodytransform to account for offsets between sensors, a vertical tidal datum transform with

discrete tide zones, and sound velocity profiles to account for refraction.


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PPVRS Method:

The Hydrographic Surveys Specifications and Deliverables Manual (HSSD 2008)

specifies the NAD 83 datum as the final horizontal datum. However, PPVRS is

computed in the ITRF05 frame. Currently, there is no utility for the Smoothed Best

Estimate of Trajectory (SBET) file format to convert between ITRF05 and NAD83. Thedifference in vertical offset between the two reference frames across the survey area is

0.065m and is considered insignificant. All ellipsoidal altitudes were left in the ITRF05

frame and will be referred to as the Ellipsoid.

Ellipsoid Model:

The tightly integrated inertially-aided PPVRS technique (Cantor et al 2008) was used topost-process ship navigation and attitude data utilizing POSPac software (Applanix,

Richmond Hill, ON).A network of seven reference stations: four University NAVSTAR

Consortium (UNAVCO) stations (Figure 4) and three National Geodetic Survey (NGS)Continually Operated Reference Station (CORS) stations (Figure 4) were used to

calculate the virtual reference station observations. Figure 4 also shows the Unalaskatide station. All stations, elevations, and approximate ranges to the transect are listed

below in Table 1.

SBET correctors were exported from the PPVRS solution. Only GPS height from the

SBET was applied to data in CARIS using the Load Navigation/Attitude Data function.As with the discrete tide zone method, soundings were transformed in CARIS using a

rigid body transform to account for offsets between sensors, and sound velocity profiles

were applied to account for refraction. GPS heights were combined with remote heave tocompute CARIS GPS Tides. GPS Tides were applied to transform the soundings to the

Ellipsoid in CARIS.

Geoid Model:

For this model, no additional correctors were computed. GPS heights were combindedwith NGS GEOID06 heights and remote heave to compute CARIS GPS Tides. GPS

Tides were applied to transform the soundings to the Geoid in CARIS.


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Stations used for PPVRS

Station IDApproximate Range

to transect extents (nm)

Elevation

(meters)

AB01 300-450 24.577

AB06 110-120 499.755

AB12 150-300 586.486

AV07 65-170 520.357

AV09 70-200 104.957

BAY5 100-150 50.052

BET1 250-370 51.100

Table 1: UNAVCO and NGS CORS stations used with the PPVRS method for the

Ellipsoid and Geoid models, with elevations and approximate ranges to transect extents.

Figure 4: Showing transect in blue, UNAVCO stations in green, NGS CORS in red, and

Unalaska tide station in yellow. Positions and distances are jittered slightly due to

station overlap.


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PPP Method:

As stated in the PPVRS method, PPP was computed in the ITRF05 frame but not

converted to the NAD83 datum. Once again, the vertical offset between the two

reference frames across the survey area were insignificant, and ellipsoidal altitudes are

referred to as the Ellipsoid.

Ellipsoid Model:

Precise Point Positioning (PPP) was computed with POSGNSS software (Applanix,Richmond Hill, ON) using NASA Crustal Dynamics Data Information System (CDDIS)

precise satellite orbits and clock files. Again, POSPac was used to process the ships PPP

navigation and attitude.

SBET correctors were exported from the PPP solution. Only GPS height from the SBET

was applied to data in CARIS using the Load Navigation/Attitude Data function. Aswith the discrete tide zone method, soundings were transformed in CARIS using a rigid

body transform to account for offsets between sensors, and sound velocity profiles wereapplied to account for refraction. GPS heights were combined with remote heave to

compute CARIS GPS Tides. GPS Tides were applied to transform the soundings to theEllipsoid in CARIS.

Geoid Model:

For this model, no additional correctors were computed. GPS heights were combinded

with NGS GEOID06 heights and remote heave to compute CARIS GPS Tides. GPS

Tides were applied to transform the soundings to the Geoid in CARIS.

Single Station Method:

Soundings were vertically transformed in CARIS using a single tide file from the

Unalaska station (9462620), encompassing the times and dates of the survey. As withdiscrete zoning, soundings were also transformed in CARIS using a rigid body transform

to account for offsets between sensors and sound velocity profiles were applied to

account for refraction, but in this case the vertical transform was made to the MLLW

tidal datum using a single tide file. This file does not contain time delay or rangecorrectors as with discrete zoning, and instead simply applies the local station

observations to the entire data set.


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Trend Analysis:

There are few vertical datum separations measured in Alaska. Because of this, the study

calculated separations, but examined trends in separation rather than the numerical value

of the separation. For each line segment and each method, 8-meter resolution CARIS

Combined Uncertainty and Bathymetric Estimator (CUBE) surfaces were created andthen imported to IVS Fledermaus (Interactive Visualization Systems, Fredericton NB).

Using IVS Fledermaus, surface separations or differences were computed between the

following data sets:

1. ITRF05 Ellipsoid vs. Single Tide Station

2. ITRF05 Ellipsoid vs. Discrete Tidal Zoning3. ITRF05 Ellipsoid vs. GEOID06

4. GEOID06 vs. Discrete Tidal Zoning

5. Discrete Tidal Zoning vs. Single Tide Station

The single tide station MLLW transformation was only used as quality control for theITRF05 Ellipsoid and discrete tidal zoning transformations, and was therefore not

compared with the GEOID06 model.


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Results

Overall, each datum separation increased, or diverged, from southwest to northeast along

the transect. The largest divergence was seen between the Geoid and discrete tidal

zoning, and the least divergence was seen between the Ellipsoid and discrete tidal zoning,

where there was very little divergence.

In carrying out the methods for this study, it became apparent that although both thePPVRS and PPP methods were used to transform soundings to the Ellipsoid and Geoid

datums, the PPVRS method showed limitations.

The PPVRS method was significantly limited by altitude error (Table 2). This methodproduced height variations greater than the tide range (Figure 5) in as little as 30 minutes,

and was considered not valuable for transforming soundings to the Ellipsoid or Geoid

datums. This error is most likely attributed to poor geometry of the GPS network in thisarea, coupled with long baselines (Table 1 & Figure 4). Because of this, PPP was the

sole method used to transform soundings to the Ellipsoid and Geoid datums for studycomparisons.

Root mean square (RMS) altitude position error (m)

SW M NE

PPVRS (Min/Max) 0.15/0.40 0.08/0.30 0.08/0.15

PPP (Max) 0.08 0.08 0.08

Table 2: Altitude position error RMS of PPVRS and PPP

Figure 5: Tidal range encompassing time period of data collection.


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Datum separations were observed within each transect segment from SW to NE (Table3). These data were then plotted to show trend in separation between datums (Figure 6).The plotted data are normalized to 8 meters, based on the average of the three segment-

means for each comparison. This was done to facilitate qualitative comparisons.

Mean Difference (m) (Std Dev)Data SetSW M NE

ITRF05 Ellipsoid vs. Single Tide Station 13.857

(0.074)

14.047

(0.255)

14.331

(0.164)

ITRF05 Ellipsoid vs. Discrete Tide Zone 13.870(0.123)

13.821(0.117)

13.800(0.053)

ITRF05 Ellipsoid vs. GEOID06 11.219(0.083)

11.639(0.180)

12.260(0.072)

GEOID06 vs. Discrete Zone 2.651(0.129)

2.1822(0.165)

1.5394(0.077)

Discrete Tide Zone vs. Single Tide Station -0.015

(0.132)0.226

(0.207)0.531

(0.151)

Table 3: Results of mean differences between datums.

Normalized Mean Datum Separations from SW to NE

6.8

7

7.2

7.4

7.6

7.8

8

8.2

8.4

8.6

8.8

Southwest Middle Northeast

Meters

Ellipsoid vs. Single Stn

Ellipsoid vs. Discrete Zoning

Ellipsoid vs. Geoid06

Geoid06 vs. Discrete Zoning

Discrete Zoning vs. Single Stn

Figure 6: Normalized mean datum separations showing trends from SW to NE.


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Single Tide Station vs. Discrete Zoning and Ellipsoid:

This comparison was done as quality control for the Ellipsoid and discrete tidal zoning

MLLW datums. Both the Ellipsoid and discrete tidal zoning MLLW datums diverged

from the single station MLLW datum (Figure 6). It can be reasonably expected that tidal

inaccuracies of the single station increase as distance from the station increases, causingdivergence from both the Ellipsoid and the discrete zoning. However, since any

inaccuracy may unexpectedly parallel or approach a more accurate model rather than

diverging from it, the logical relationship is not necessarily an absolute relationship.

Discrete Zoning vs. Geoid 06 and Ellipsoid:

The discrete tidal zone MLLW datum diverged from the Geoid from SW to NE but was

almost completely parallel with the Ellipsoid from SW to NE (Figure 6). This is

counterintuitive, given the irregularity of the Geoid as compared to the mathematicallyidealized representation of the Ellipsoid. This brings the quality of the discrete tidal

zoning into question. It is not known if the orientation of the zones matching thedirection of the line would cause this, or if the model simply extrapolates beyond

reasonable distance from Unalaska tide station.

It is also possible that this observation may be attributed to sea surface or geoidal

undulation, similar to the separations seen in Fugros (Fugro Pelagos Inc., San Diego)2004 Sitka survey where ellipsoidal to MLLW separation exhibited inconsistencies with

the Geoid99 model (Lockhart et al 2005). There are inadequate data to draw a

conclusion.

Geoid06 vs. Ellipsoid:

The Geoid diverged from the Ellipsoid from SW to NE (Figure 6). It is expected that the

separation between the Geoid and Ellipsoid will fluctuate, since the Geoid is based on agravimetric model and the Ellipsoid is satellite-based. While the result here seems to

make sense, the relationship between the Geoid and the Ellipsoid is an expansive one,

and would be better modeled over a much larger area.


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Discussion

Bristol Bay and the Bering Sea remain a sparsely referenced area of the United States.

Given this areas economic and environmental importance, the limitations of current

reference stations suggest that a non-traditional method should be used for vertical

control in these areas. While nearshore surveys have the opportunity to install (whenpersonnel and equipment are available) or use an existing land-based reference system,

offshore areas that may need precise vertical measurement to meet or exceed end-userneeds should consider measurement alternatives.

This study showed that trends in the relationship between traditional tidal zoning and

other methods appear to be inconsistent, largely due to reliance on sparse historical data.This condition continues to limit survey capabilities and poses a significant challenge to

future missions.

While PPP stands out as a robust measurement technique for surveying to the Ellipsoid in

Alaska, its reliability and accuracy is currently meaningless without a MLLW toEllipsoid separation measurement. This reinforces the need for water level measurementdevices such as tidal buoys or a moored bottom pressure recorder (BPR) that could be

used in conjunction with PPP to measure this separation, but would theoretically need

the required minimum deployment time (30 days) to extrapolate MLLW in an areaknown for its extreme conditions.

One remaining problem is deployment and retrieval of a water-level recording device that

is offshore, and whose necessary placement time may exceed the time of sonar dataacquisition. Transit time and cost will be a significant factor in deciding whether a PPP

and buoy or BPR combination is a reasonable option, given the survey accuracy needs of

the end-user.

As survey priorities change to adapt to climate dynamics in the arctic, and as national and

international interests shift, offshore survey areas may begin to include depths wheremore accurate vertical measurement becomes a necessity. It is recommended that, in

these cases, the combination of PPP with 30-day tide buoy or BPR deployment is

considered.


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References

Cantor, P., R. Brennan, E. Van Den Ameele. 2008 Tightly Integrated Inertially-Aided

Post Processed Virtual Reference Station Technique for Marine Hydrography

Proceedings of the Canadian Hydrographic Conference and National Surveyors

Conference 2008.

John Oswald and Associates, LLC. 2006. Survey Data Sheet for 9462620 Tidal 19observed June 2006 and delivered to National Geospatial Service (NGS),

(http://beta.ngs.noaa.gov/CORS-

Proxy/oraOpusDbWeb/getDatasheet.jsp?PID=BBBB51).

Lockhart, D., A. Orthman. 2005. MLLW and the NAD83 Ellipsoid: An Investigation of

Local Offsets and Trends using PPK and Gauge Derived Water Surfaces. Proceedings of

the United States Hydrographic Conference 2005.

NEMW (Northeast Midwest Institute). 2000. U.S. Land and Water Area by State inSquare Miles, 2000, (http://www.nemw.org/waterland.htm).

NOS (National Ocean Service) Technical Memorandum NOS CO-OPS 0048. 2008. A

Network Gaps Analysis for the National Water Level Observation Network.

NOAA Office of Coast Survey (OCS). 2008. NOAA Hydrographic Survey Priorities.

NOAA Office of Coast Survey (OCS). 2008. NOAA Hydrographic SurveysSpecifications and Deliverables.

United States Coast Guard (USCG). 2002. DGPS Site Coverage Alaska,(http://www.navcen.uscg.gov/dgps/coverage/Alaska.htm).

Documents

Vertical Control Challenges: Case Study of a 2008 Multibeam Survey in the Sparsely Referenced Environment of Bristol Bay, Alaska