Uchenna Nwankwo, Stephan Howden and Dave Wells Further Investigations of VDatum to NAD83 Vertical Separations Using United States Geological Service (USGS) Coastal Water Levels gage and Hydrolevel Buoy. Uchenna Nwankwo, Stephan Howden and Dave Wells The University of Southern Mississippi

Separation model infrastructure Goal - reliable consistent (seamless) transformation among various vertical spatial referencing systems: (a) with respect to gravity = geoid (b) with respect to geometry = ellipsoid (c) with respect to water level statistics = chart datum Requirements - two sets of “separation models”: geoid - ellipsoid separation model (classical geodesy problem) chart datum - ellipsoid separation model (our concern here) Tools to access separation model values – e.g. VDATUM #ushydro2019

VDATUM uncertainty in Gulf of Mexico Published Vdatum uncertainties for MHHW, MLLW, DTL and Gt is 0.17 m while for NAVD88 is 0.08 m “Important: Transformation Uncertainties in the ‘Louisiana / Mississippi - Eastern Louisiana to Mississippi Sound’ Regional Model, have been found to range from 20 to 50 cm in particular locations from the Mississippi River Delta north to Lake Pontchartrain. These issues most likely can be attributed to subsidence, newly established datums, and changes to the understanding of NAVD88 based on new versions of the GEOID. The VDatum Team is currently looking at resolving these uncertainties.” From https://vdatum.noaa.gov/ #ushydro2019

Previous results at CSXRR and the Rigolets Besides NAVD88 at the Rigolets, the other datums at both stations were within VDatum published uncertainty All values in m Rigolets CSX Datum VDATUM minus USM MHHW 0.103 ± 0.171 0.089 ± 0.171 DTL 0.091 ± 0.170 0.090 ± 0.170 NAVD88 (GEOID12B) -0.085 ± 0.078 0.018 ± 0.078 MLLW 0.082 ± 0.172 0.080 ± 0.172 4/20/2019 #ushydro2019

Verifying USGS provided offsets at Delacroix station NAD83 Ellipsoid Gage datum NAVD88 Reference Mark GNSS antenna a e b c  id Parameters Source Delacroix (m) a GNSS Antenna Ellipsoidal height NAD83 GrafNav8.7 -18.67 +/- 0.03 b Antenna above reference mark USM data 3.69 +/- 0.01 c Reference mark above gage datum USGS 4.29 +/- 0.01 d NAVD88 seperation (GEOID 12B) GEOID12B -25.23 +/- 0.03 e Gage datum minus NAVD88 -1.28 +/- 0.01 f Gage datum ellipsoid height method A a - b – c -26.65 +/- 0.03 g Gage datum ellipsoid height method B d + e -26.51 +/- 0.03 h Gage datum ellipsoid height deviation f - g -0.14 +/- 0.03 d Method B Method A 4/20/2019 #ushydro2019

Datum transfer for Delacroix station Primary gage used Shell Beach NOAA gage Datums are with respect to gage datum Comparison of datum transfer results Datum USM 2017 (m) NOAA 2017 (m) NOAA 2016 (m) DTL 1.49+/-0.06 1.45 1.43 MLLW 1.37+/-0.06 1.32 1.33 MHHW 1.61+/-0.06 1.58 1.54 Gt 0.23+/-0.06 0.26 0.21 MSL 1.44 Three months time series JOA Surveys LLC, Online Tidal Datum Tool, www.tidaldatumtool.com 4/20/2019 #ushydro2019

Datums Comparison Delacroix-VDatum Method A Using method A DTL and MLLW values agree to within the published uncertainty of VDatum. Using method B DTL, MLLW and MHHW values agree to within published uncertainty of Vdatum. From the two methods, we deduced that the VDATUM overestimates the Great Diurnal Range. Datum Delacroix m NOAA Vdatum m Delacroix – Vdatum m DTL -25.16+/-0.06 -25.05+/-0.17 -0.11+/-0.18 MLLW -25.28+/-0.06 -25.28+/-0.17 -0.00+/-0.18 MHHW -25.04+/-0.06 -24.82+/-0.17 -0.22+/-0.18 GT 0.23+/-0.06 0.46+/-0.17 -0.23+/-0.18 Method B Datum Delacroix m NOAA Vdatum m Delacroix – Vdatum m DTL -25.02+/-0.06 -25.05+/-0.17 -0.03+/-0.18 MLLW -25.14+/-0.06 -25.28+/-0.17 -0.14+/-0.18 MHHW -24.90+/-0.06 -24.82+/-0.17 -0.08+/-0.18 GT 0.23+/-0.06 0.46+/-0.17 -0.23+/-0.18 4/20/2019 #ushydro2019

Datum transfer for Chandeleur East Hydrolevel buoy Primary gage used is Dauphin Island NOAA gage No datum transfer results from online NOAA datum transfer calculator 4/20/2019 #ushydro2019

Datums Comparison Buoy-VDatum Great Diurnal Range  Station Shell Beach NOAA Pilot Station East NOAA Pilot Town NOAA Dauphin Island NOAA   Vdatum SBCH VDatum PSE PT DI Values 0.46 0.45 0.37 0.36 0.31 0.26 Difference 0.01 0.05 0.00 The agreement in Gt between the buoy and Pilot Station East and Dauphin Island makes our results reasonable All the datums besides Gt do not agree to within the published Vdatum uncertainty Underestimation of DTL by VDatum was observed to be responsible Datum Hydrolevel m NOAA Vdatum m Hydrolevel-Vdatum m DTL -23.63+/-0.06 -24.03+/-0.17 0.40+/-0.18 MLLW -23.81+/-0.06 -24.22+/-0.17 0.41+/-0.18 MHHW -23.45+/-0.06 -23.83+/-0.17 0.38+/-0.18 GT 0.36+/-0.06 0.39+/-0.17 -0.03+/-0.18 4/20/2019 #ushydro2019

Conclusions We suspect that the 14 cm difference in gage zero ellipsoid elevation observed at the Delacroix station was due to subsidence VDatum Gt compared with those from tidal datum transfers were within 0.04 m everywhere except at Delacroix where the difference was 0.23 m. Examining Gt at the USGS gages, the buoy, and the regional NOAA tide gages, we hypothesize that the overestimation of Gt at Delacroix by Vdatum maybe due to the underlying tidal model having too much hydrodynamic connectivity to Shell Beach. 4/20/2019 #ushydro2019

Conclusions Ellipsoid to DTL agrees with Vdatum within 0.12 m, or better, everywhere but Chandeleur East HydroLevel buoy (DTL differs by ~ 0.4 m from Vdatum) The DTL discrepancy at the buoy is preliminary. The record was only 3 weeks and was broken into 2 segments due to vandalism. 4/20/2019 #ushydro2019

Future Work Install permanent tide gage with GNSS receiver in Gulfport, MS Deployment of buoy at the center of area of concern if funding is available. 4/20/2019 #ushydro2019

Acknowledgment Thanks to Mike Runner at the USGS for providing information about the USGS coastal gages and allowing us to perform GNSS surveys at the gages. Funding is provided by NOAA through a grant to the USM Mapping Center of the USM Hydrographic Science Research Center. A special thanks to Late RADM (ret.) Ken Barbor, Director of the USM Hydrographic Science Research Center We appreciate the contributions of Marvin Story, Kevin Martin and Bryan Parker of The University of Southern Mississippi and Venice Tang of Waypoint Support, NovAtel Inc. Nathan Wardwell of JOA surveys LLC 4/20/2019 #ushydro2019

Questions? 4/20/2019 #ushydro2019