Vertical Reference for Seabed Mapping
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Using Ellipsoid as Vertical Using Ellipsoid as Vertical Reference for Seabed MappingReference for Seabed Mapping
Why?Why?How?How?
Hanne Hodnesdal, Herman Iversen, Birgit K. Lynge, Lars K. Nesheim, Arne E. Ofstad, Stig vstedal
Presented by Noralf Slotsvik,Norwegian Hydrographic Service
(NHS)
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A A definitiondefinition
A specialist is a person far away from home
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Using Ellipsoid as Vertical Using Ellipsoid as Vertical Reference for Seabed MappingReference for Seabed Mapping
GPS antenna
Sea level
Echo sounder
DES
Seabed
Ellipsoide
v
DEllipsoide
H DEllipsoide=H-DES-v
DK0DK0= DMSS - Z0
Chart datum (CD)Z0
Mean sea Surface (MSS)
MSSH
DMSS=MSSH-DEllip.
DMSS
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The other alternative: The other alternative: Seabed mapping with water level dataSeabed mapping with water level data
Chart Datum (CD)
Echo Sounder
Seabed
Sea level
draught
DepthCD
DepthES
Water Level (WL)
DepthCD = DepthES + draught - WLCD
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Water levelWater level at the time of seabed mapping at the time of seabed mapping must be removed from the depth datamust be removed from the depth data
Methods to obtain water level data: Permanent tide gauge possibly combined with
tide zone Temporary tide gauge associated with a
permanent tide gauge to get the relation to mean sea level
Predicted tide (based on tide models) Use GPS to measure water level with ellipsoid
as reference
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Permanent tide gauges Permanent tide gauges operatedoperated by NHSby NHS22 gauges along the coast of Norway og one gauge on Svalbard
Oslo
0
1
2
3
5 10 15 20 26 30
Vard
0
1
2
3
5 10 15 20 26 30
Troms]
0
1
2
3
5 10 15 20 26 30
Rrvik]
0
1
2
3
5 10 15 20 26 30
Stavanger
0
1
23
5 10 15 20 26 30
Bergen
0
1
2
3
5 10 15 20 26 30
OsloOscarsborgVikerHelgeroa
Tregde
Stavanger
Bergen
MlylesundKristiansund
HeimsjTrondheim
Rrvik
Bod
Kabelvg
Andenes
NarvikHarstad
Troms
Hammerfest
HonningsvgVard
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Tidal zones: Almost equal tide within a zoneTidal zones: Almost equal tide within a zone
HAMMERFEST
1,0-25
0,9810
0,9715 0,97
25
0,99-15
0,995
0,99-20
0,99-10
0,99-10
Hammerfest
Troms
0,990
0,99-5
Range factor
Time delay
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Range factor and time delay from Range factor and time delay from HammerfestHammerfest
8o 25
N
30'
oN
30'
72oN
Hammerfes t
(0.92, -15 min)
(0.92, -5 min)
(0.80, -10 m in)
(0.83, -20 min)
(Hgde- og tidskorreksjon fra Hammerfest)(Range factor, time delay)
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What we need for using the ellipsoid as vertical What we need for using the ellipsoid as vertical reference for seabed mapping? (1 of 2)reference for seabed mapping? (1 of 2)
Mandatory: Vertical position of high quality
High quality GPS-receiver Software for post processing of GPS-data (Terrapos) Continuous logging of GPS-data
The position of the GPS-receiver must be known in the coordinate system of the vessel
The motion of the vessel must be known (attitude data: heave, roll, pitch and heading)
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What we need for using the ellipsoid as vertical What we need for using the ellipsoid as vertical reference for seabed mapping? (2 of 2)reference for seabed mapping? (2 of 2)
Not mandatory: If we want to convert to mean sea level we must
know the difference between mean sea level and the ellipsoid. MSS (Mean Sea Surface) models can be used.
If we want to convert to Chart Datum (CD) we must know the difference between Chart Datum and mean sea level . Along the Norwegian coast the Chart Datum is equal to LAT (lowest astronomical tide) with a few exceptions. Tide models can be used to estimate LAT at high sea.
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Accuracy achievable today (95%)Accuracy achievable today (95%)
Vertical position GNSS height: GNSS = 8 cm
Surface models MSS: MSS = 10 cm (high sea) MSS = ? cm (coast) LAT: LAT = 30 cm (high sea) LAT = 10 cm (coast)
Accuracy is improving
In a complete error budget all error sources must be included (echo sounder, sound velocity profile, attitude data, horizontal positioning,...)
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Advantages and disadvantagesAdvantages and disadvantages Advantages:
Water level measurements are not needed
Knowledge of the draught is not needed
Do not have to consider different tidal zones
Less possibility of mismatch in overlapping survey areas by using a consistent reference
GPS measured low frequency waves not measured by the heave sensor
(GPS measures heave better than the heave sensor?)
...
Disadvantages High quality vertical position is
needed The position of the GPS-antenna
relative to the echo sounder must be known
Confusing: The ellipsoidal depth will not represent the true ocean depth
MSS an LAT-models must be known to convert to mean sea level and and Chart Datum
MSS- and LAT-models are continuously improving (version control is necessary)
...
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Seabed mapping at high seas Seabed mapping at high seas and close to the coastand close to the coast
At high seas it is favourable to use the ellipsoid as reference: 1. Difficult to get good water level data at high seas2. There are good MSS-models covering these areas (except
for high latitude and areas with sea ice)3. It does not matter that the LAT-surface is of less quality at
high seas since the requirements for depth accuracy in navigational charts are weak at high seas.
NHS can still not use the ellipsoid as reference close to the coast:1. MSS-models are still of poor quality2. The requirements for depth accuracy in navigational charts
are stringent
Still it is attractive in the future to use the ellipsoid close to the coast as well
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Using ellipsoid as vertical referenceUsing ellipsoid as vertical referenceHow it has been done at NHS so farHow it has been done at NHS so far
Existing production line has been used Water level (based on permanent tide gauge in
Hammerfest) has been delivered continuously to the vessel
GPS has been logged continuously (1 Hz) and post processed with Terrapos HGPS
GPS-height has been used to make a file with water level data. WLGPS = HGPS - heave - MSS ( Chart Datum)
GPS-water level (WLGPS ) has replaced former delivered water level before depth processing and quality control
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GPSGPS--water levelwater level Calculating GPS-water level
WLGPS = HGPS - heave - MSS ( Chart Datum)
The calculation is reversible. We store MSS and Chart Datum values
GPS-water level contain rests of heave probably because the heave-sensor is not perfect (we want to carry out a test)
Test 1: GPS-water level has been compared with traditional water level (Svalbard and Mareano project)
Test 2: Depth-data has been processed with both GPS-water level and water level from tide gauge and then compared
Test 3: Accuracy of HGPS has been tested
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Seabed mapping on Svalbard July 2005Seabed mapping on Svalbard July 2005 Comparing
traditional method with seabed mapping with ellipsoid as reference
Analysing: 20-21 July-05
Ellipsoid as reference : KMS04 from Danish National
Space Centre
Traditional method : Permanent tide gauge in Ny-
lesund Temporary tide gauge in
Hornsund
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00:00 06:00 12:00 18:00 00:0034
34.5
35
35.5
36
36.5
37
Time
[
m
]
Measured GPS height, 20 July 2005
GPS height with heaveGPS height with heave removed
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03:07 03:08 03:0934
34.5
35
35.5
36
36.5
37
Time
[
m
]
Measured GPS height, 20 July 2005
GPS height with heaveGPS height with heave removed
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GPSGPS--water level and water level from water level and water level from temporary tide gaugetemporary tide gauge
00:00 06:00 12:00 18:00 00:0034
35
36
37
[
m
]
Measured GPS height, 20 July 2005
GPS heightMSS
00:00 06:00 12:00 18:00 00:00-1
0
1
2
Time
[
m
]
GPS height rel. MSSWater level, Hornsund
Mean of difference is 12.5 cm
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GPSGPS--water level and water level from water level and water level from temporary tide gaugetemporary tide gauge
00:00 06:00 12:00 18:00 00:0033
34
35
36
[
m
]
Measured GPS height, 21 July 2005
GPS heightMSS
00:00 06:00 12:00 18:00 00:00-1
0
1
2
Time
[
m
]
GPS height rel. MSSWater level, Hornsund
Mean of difference is 15.5 cm
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Range factor and time delay from Range factor and time delay from HammerfestHammerfest
8o 25
N
30'
oN
30'
72oN
Hammerfes t
(0.92, -15 min)
(0.92, -5 min)
(0.80, -10 m in)
(0.83, -20 min)
(Hgde- og tidskorreksjon fra Hammerfest)(Range factor, time delay)
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Water level relative mean sea levelWater level relative mean sea level
05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00
-1.0
0.0
1.0
2.0Hammerfest (no correction)Hammerfest (average secondary harbour corrections)Hammerfest (interpolated secondary harbour corrections)Water level from GPS
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Water level relative mean sea levelWater level relative mean sea level
05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00
-1.0
0.0
1.0
2.0Hammerfest (no correction)Hammerfest (average secondary harbour corrections)Hammerfest (interpolated secondary harbour corrections)Water level from GPS Filtered water level from GPS
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05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00Time [s]
-1.0
0.0
1.0
2.0
H
e
i
g
h
t
[
m
]
Hammerfest (no correction)
-
05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00Time [s]
-1.0
0.0
1.0
2.0
H
e
i
g
h
t
[
m
]
Hammerfest (no correction)Hammerfest (average secondary harbour corrections)
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05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00Time [s]
-1.0
0.0
1.0
2.0
H
e
i
g
h
t
[
m
]
Hammerfest (no correction)Hammerfest (average secondary harbour corrections)Hammerfest (interpolated secondary harbour corrections)
-
05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00Time [s]
-1.0
0.0
1.0
2.0
H
e
i
g
h
t
[
m
]
Hammerfest (no correction)Hammerfest (average secondary harbour corrections)Hammerfest (interpolated secondary harbour corrections)Water level from GPS
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05/11/06 0:00 05/11/06 12:00 05/11/07 0:00 05/11/07 12:00 05/11/08 0:00Time [s]
-1.0
0.0
1.0
2.0
H
e
i
g
h
t
[
m
]
Hammerfest (no correction)Hammerfest (average secondary harbour corrections)Hammerfest (interpolated secondary harbour corrections)Water level from GPS Filtered water level from GPS
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Tide gauge GPS - heights
5-10 m depth, EM3000D
Ca 50 m
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Tide gauge GPS - heights
40 m depth, EM1002
Ca 100 m
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Tide gauge GPS - heights
40 m depth, EM1002
Ca 50 m
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Further work at NHSFurther work at NHS Document the accuracy of GPS-height
Test has been carried out
Find out how to handle heave Can we use GPS-height to measure heave? Test?
Document error sources in connection with traditional water level measurements
Compare GPS-water level and traditional water level
Consider and possibly implement changes in the production line Should depth data be stored relative to the ellipsoid, mean
sea level or chart datum?
Provide MSS and Chart Datum surfaces as well as a system for updating and version control
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ConclusionsConclusions To use the ellipsoid as vertical reference for
seabed mapping is today a relevant method since the GPS-height has become more accurate.
At high seas this method is especially favourable since it is difficult to provide reliable water level data
There are MSS-models covering high seas. These can be used to convert from ellipsoid to mean sea level
Tide models can be used to make Chart Datum surfaces. These can be used to convert from mean sea level to Chart Datum.
GPS-height contain heave. When we subtract heave measured by the heave sensor we still observe rests of heave.