Keys to Merging Digital Bathymetric and Topographic Data:

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Keys to Merging Digital Bathymetric and
Topographic Data Development of a Seamless
Multi-source Topographic/Bathymetric Elevation
Model
Dean Gesch
USGS/EROS Data Center
gesch_at_usgs.gov
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NOAA Collaborators
Bruce Parker Robert Wilson
Kurt Hess
Coast Survey Development Lab.
NOAA/National Ocean Service
Dennis Milbert National
Geodetic Survey NOAA/National Ocean Service
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Background

• The elevation component of The National Map is
  specified as having both topographic and
  bathymetric information
• Geospatial One-Stop elevation framework data will
  include seamless land elevations and near-shore
  bathymetry
• A seamless model across the land/water interface
  fits NOAAs vision for a Digital Coast

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Background

• Merges of topographic and bathymetric data have
  been done before, mostly for cartographic
  purposes, and datum differences were not handled
  in a rigorous manner
• Users have identified consistency of coastal
  datasets as the most important characteristic
• USGS experience with merging Lake Tahoe
  bathymetry and surrounding elevation data a
  much simpler case!

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The Tampa Bay Demonstration Project

• Many applications of geospatial data in coastal
  environments require knowledge of the near-shore
  topography and bathymetry

• Existing topographic and bathymetric data were
  collected independently for different purposes,
  and thus are difficult to use together because of
  datum differences

• USGS and NOAA have collaborated to develop tools
  and techniques to facilitate integration of the
  best available USGS topographic data and NOAA
  hydrographic survey data

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Best available NOAA hydrographic data
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Best available NOAA hydrographic data

• 800,000 soundings from 47 surveys were extracted
  from GEODAS
• Index polygons were created for each survey
• Indexes were sorted, merged, and clipped to form
  15 new master spatial-temporal polygons
• Master polygons used as filter to select most
  current soundings

Variable density of soundings in Tampa Bay
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Best available NOAA hydrographic data
- Soundings in Tampa Bay are referenced to tidal
datums mean low water (MLW) and mean lower low
water (MLLW)
Mean Low Water
Mean Lower Low Water
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Tidal Datums
Tidal datums are derived from long term tide
records at tide gauges
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Vertical Datum Transformation

• Prior to merging, bathymetric and topographic
  data must be transformed to a common vertical
  reference frame
• Vertical datum for soundings MLW and MLLW
• Vertical datum for topographic data NAVD 88 or
  NGVD 29 (orthometric, or mean sea level datums)
• Common vertical datum NAD 83 (86) ellipsoid
• VDatum, a new tool developed by the National
  Geodetic Survey, performs transformation among 26
  vertical datums

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The Datum Transformation Constellation
WGS 84 (G873)
MLLW
ITRF88
LMSL
ITRF96
NEOS 90
ITRF93
MTL
ITRF97
ITRF91
WGS 84 (G730)
SIO/MIT 92
NAVD 88
DTL
MLW
MHHW
ITRF94
ITRF92
WGS 84 (orig.)
PNEOS 90
ITRF89
NAD 83 (86)
NGVD 29
ITRF90
MHW
How many paths?
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The Datum Transformation Roadmap
WGS 84 (G873)
MHHW
WGS 84 (G730)
WGS 84 (orig.)
ITRF97
MHW
NGVD 29
ITRF96
ITRF94
MTL
ITRF93
NAD 83 (86)
NAVD 88
LMSL
ITRF92
ITRF91
DTL
ITRF90
ITRF89
MLW
ITRF88
SIO/MIT 92
NEOS 90
MLLW
PNEOS 90
Each line is a transformation
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Ellipsoid - Geoid Relationship
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Range in CONUS -50.97 to 3.23 meters Geoid
undulation separation from NAD 83 ellipsoid
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Orthometric (Sea Level) Datums
NGVD 29 21 U.S. and 5 Canada fixed tide gauges
NAVD 88 1 fixed tide gauge
The adopted control points and constants defining
the heights cause NGVD 29 and NAVD 88 to differ
by up to 2.2 meters
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NGS tool VERTCON performs conversions
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The Datum Transformation Roadmap
WGS 84 (G873)
MHHW
WGS 84 (G730)
WGS 84 (orig.)
ITRF97
MHW
NGVD 29
ITRF96
ITRF94
MTL
ITRF93
NAD 83 (86)
NAVD 88
LMSL
ITRF92
ITRF91
DTL
ITRF90
ITRF89
MLW
ITRF88
SIO/MIT 92
WGS 84 (G873) 86.39 feet
NEOS 90
MLLW
NAD 83 (86) 81.33
PNEOS 90
NAVD 88 73.1061
NAVD 88 0.0
LMSL -0.53
MLLW -1.92
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Vertical Datum Transformation
- VDatum uses results from a calibrated and
verified hydrodynamic model of Tampa Bay to
transform among tidal datums
Relationship of vertical datums for Tampa Bay
WGS 84 (G873)
86.39 ft
26.33 m
NAD 83 (86)
81.33 ft
24.79 m
MHHW
0.792 ft
0.241 m
MHW
0.409 ft
0.125 m
NAVD 88
0.0 ft
0.0 m
LMSL
-0.535 ft
-0.163 m
NGVD 29
-0.850 ft
-0.259 m
MLW
-1.495 ft
-0.456 m
MLLW
-1.919 ft
-0.585 m
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Best available USGS topographic data
The National Elevation Dataset (NED)
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Bathymetry / topography merge procedure
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Bathymetry / topography merge procedure

• - Extract area from NED
• Select land area (elevations greater than zero)
• Convert elevations to ellipsoid reference with
  NGS tools (VERTCON and GEOID99)

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Bathymetry / topography merge procedure

• - Extract area from NED
• Select land area (elevations greater than zero)
• Convert elevations to ellipsoid reference with
  NGS tools (VERTCON and GEOID99)
• Convert land elevations within 600 meters of
  shoreline to points
• Select bathymetry points in water (as defined
  by DEM shoreline)

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Bathymetry / topography merge procedure

• - Produce grid of land points and bathymetry
  points with ANUDEM (thin plate spline
  interpolation)
• Clip output grid to include land elevations
  within 300 meters of shoreline
• Mosaic land elevation grid and bathymetry grid
  with blending (weighted averaging) in overlap area

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Bathymetry / topography merge results

• Merged topographic/bathymetric digital elevation
  model (1-arc-second grid spacing)
• Accuracy check of bathymetric grid vs. recent
  high-accuracy transects

N 5,928 Min/Max -11.9 15.6 ft. Mean -0.4
ft. Std. Dev. 1.3 ft. RMSE 1.4 ft.
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Bathymetry / topography merge results
Bathymetry grid makes use of all available
soundings, providing significantly more detail
than charts
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Bathymetry / Topography Merge Processing
Issues and Challenges
- Where is the shoreline? Wheres the topo?
Wheres the bathy? Which one takes
precedence in overlap areas?
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Bathymetry / Topography Merge Processing
Issues and Challenges
A
- Resolution and accuracy of topo and bathy data
(evident in interpolated grid)
A
B
B
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Bathymetry / Topography Merge Processing
Issues and Challenges
- Need a spatial display of data quality
a data reliability map
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Bathymetry / Topography Merge Processing
Issues and Challenges

• Resolution of final merged grid?

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Bathymetry / Topography Merge Processing
Issues and Challenges
- Integration of 3rd party high resolution data
(including bathymetric LIDAR data)
Univ. of FL LIDAR
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Bathymetry / Topography Merge Processing
- SHOALS bathymetric/topographic LIDAR data along
the land/water interface would provide
up-to-date, high resolution data in the critical
intertidal zone
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Application Coastal storm surge modeling
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Application Custom shoreline delineation
Intersecting a vertical reference plane through
the seamless merged elevation model results in
changes in the shape and location of the shoreline
MSL
MLLW
N
N
3
15
meters
meters
Mean Lower Low Water
Mean Sea Level
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Application Coastal basin delineation
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Key Points

• Vertical datum transformation is the key to
  successfully producing a seamless merge of
  topographic and bathymetric data at the
  land/water interface
• Standard vector and raster GIS tools work well
  for pre-processing and merging topographic and
  bathymetric data
• Recent, high-resolution, high-accuracy data
  significantly increase the value of the merged
  product

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Merge Results Southern Louisiana
Port Fourchon to Grand Isle
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Current Activities

• NOAAs Coast Survey Development Lab is extending
  coverage of VDatum, working toward a National
  VDatum
• USGS is developing merged models for Puget Sound,
  Hawaii, and Cape Cod sites