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Satellite derived reference surfaces for surveying (VORF, BLAST and LAT)
Dr Ole B. Andersen,
DTU Space,
Copenhagen, Denmark,
2 DTU Space, Technical University of Denmark
Who am I.
• Author of KMS02/DNSC08/DTU10 high resolution
• marine geophysical/geodetic maps.
– 1 minute Global altimetric gravity field map
– 1 minute Mean Sea Surface height
– 1 minute Altimetry improved bathymetry
– Global ocean tide models.
– Global/regional/Arctic Sea level change(altimetric era).
Sea Level Change
Bathymetry
Gravity
3 DTU Space, Technical University of Denmark
Overview.
Introduction (reference surfaces).
Surveys techniques – Satellite support
Satellite Altimetry and the Mean sea surface.
State of the Art Global Ocean tide models.
LAT and MSS determination
Accuracies / Example – Crossing the Gulf Stream
BLAST and transformation to local VRF……
Summary - outlook
4 DTU Space, Technical University of Denmark
BLAST EU-7th Interreg FW project
5 DTU Space, Technical University of Denmark
BLAST (EU 7th Interreg FW project)
•The BLAST project has a primary focus on
–"Bringing Land and Sea Together“ (BLAST),
–by harmonizing and integrating on+offshore data.
– DTU major role is on
– Transform parameters between different vertical reference systems (VRF)
– Develop a software tool to implement the transformation between the different VRF’s.
– BLAST IS ONLY DEFINED IN THE NORTH SEA.
– BLAST METHODS IS APPLICABLE GLOBALLY AND IN THE ARCTIC
– BLAST IS A DEMONSTRATOR PROJECT.
6 DTU Space, Technical University of Denmark
BLAST AND VORF
•VORF = Vertical Offshore Reference Frame
•A set of mathematical models of the major surfaces used in the current and “future charting”
•A suite of software utilities allowing the transformation of mapping and positioning data between the VORF surfaces
•UKHO established first demonstrator UK-VORF for Britain in 2007
•BLAST established North Sea VORF in 2011
7 DTU Space, Technical University of Denmark
Some usages / reasons for VORF?
• Continuing developments in GPS/GALILEO/GNSS
• LIDAR and multibeam technology surveyed using GNSS.
• Analogy with various height reference systems on land
• International / independent of national Datums (chart)
• Deal with the increased use of
GPS/GNSS-based hydrographic
surveys.
• Applicable to Navigation
and ship safety Lowest
8 DTU Space, Technical University of Denmark
Surfaces and names:
GRS80 Ellipsoid
Chart Datum
(local)
LAT
MLWS
MSL /MSS
9 DTU Space, Technical University of Denmark
LAT
Charted depth
depth
measurement
(and time
of observation)
survey vessel Tide gauge
observed tide
(and time) Tidal correction
derived from
Use co-tidal
chart
Current practice for bathymetric data processing
Issues: Complexity – onshore and
offshore operations, time Latency – the process takes
time and hence is expensive Accuracy issues – co-tidal charts have limited resolution and are
derived from limited data; seabed gauges are expensive
Inconsistency – practices using Chart Datum are sometimes poorly defined
Requires transformation into local (oountry) datum of the tide gauge
ERRORS in co-tidal charts, transformation etc.
… or use seabed gauge
Sea Surface
10 DTU Space, Technical University of Denmark
LAT
Charted depth
depth
measurement
Tidal correction
Bathymetric data processing with BLAST/VORF and GPS
GRS80 Ellipsoid
survey vessel (+GPS)
he
Tidal correction =
he – LAT
Charted depth =
Depth measurement – tidal correction
- accessible everywhere
Sea Surface
11 DTU Space, Technical University of Denmark
Blast/VORF computation method
Tide gauge
geoid
MSS MSS (altimetry) 5km interpolate
Altimetric Tidal modelling
Altimetric LAT= VORF
BLAST approach is applicable to the Arctic.
We do have an accurate altimetric MSS (or Mean sea level)
We do have accurate Ocean tide model from Altimetry
So we can derive an altimetric LAT to be used for surveying
This LAT will be in Ellipsoidal reference frame (GPS)
12 DTU Space, Technical University of Denmark
How to derive the MSS Satellite Altimetry
Ground Tracks
ENVISAT
TOPEX
JASON
GFO
ERS1/2
13 DTU Space, Technical University of Denmark
DTU10 Mean Sea Surface (18 years)
Height in ”GRS80”
14 DTU Space, Technical University of Denmark
Arctic -> Ice coverage
Decrease in total sea ice extent:
September: 11.1 % per decade
March: 2.8 % per decade
Richter-Menge et al, 2008
March
September
15 DTU Space, Technical University of Denmark
Problems
Measurement Periods do not match
ICESat covers selected periods 2002-2006
CryoSat-2 was launched in 2009
The art is to fit surfaces on each other
1) Reference E1/E2/ENVISAT to TP/J1/J2
2) Reference ICESat to Envisat (same time)
3) Reference CryoSat-2 to ICESat+ENVISAT
The Arctic Ocean – ”Problems”
DTU 10 MSS (height in meters)
16 DTU Space, Technical University of Denmark
GPS vs MSS
320 GPS measured Tide Gauges
Around Britain.
TG – MSS (extrapolated)
Mean = 1.24 cm (DNSC08)
Std = 6.8 cm
Upgrading MSS to DTU10
Mean = 0.11 cm (DTU10)
Comparison curtesy by
Marek Ziebart, UCL London,
Extrapolation towards the coast is
Required for the MSS…
How accurate is this ??
17 DTU Space, Technical University of Denmark
Getting a LAT from Ocean Tides – M2 loop
18 DTU Space, Technical University of Denmark
New DTU10 ocean tide model
1. Empirical model based on FES2004.
2. Response method – Similar to GOT 4.7 global ocean tide.
3. New satellite data:
18 years of joint TOPEX/Poseidon-Jason-1-Jason-2 mission
4 years of TOPEX-Jason-1 interleaved mission
10 years GFO up to ±72 °
15 Years ERS-2-Envisat
Models 28 tidal constituents
including largest shallow water M4
19 DTU Space, Technical University of Denmark
20 DTU Space, Technical University of Denmark
Run the tide model for 19 years to find minimum (incl Arctic)
21 DTU Space, Technical University of Denmark
Ellipsoidal LAT
Datum
EXAMPLE NORTH SEA
(validated)
BUT ITS GLOBAL
Ellipsoidal LAT surface
Based on:
DTU10MSS- DTU10OT
Relative to
WGS84/GRS90
GPS consistent
(also Tide system
consistent)
22 DTU Space, Technical University of Denmark
Accuracy Discussion
Errors in altimetric MSS + Tides.
Errors in near-coastal extrapolation
MSS error ~ 5 cm open ocean
Tide error ~ 5 cm open ocean
Coastal
UK VORF met specs (10 – 15 cm 1s) across
~80% of inshore and~100% of offshore
Increased complexity of tides
Shallow water tides.
Lack of Altimetric Data (fjords).
Worst case scenario could be 1 m.
Arctic Issues:
ICE – Disturbs radar….
MSS might be too high by 10-20
Cm due to sea ice returns.
23 DTU Space, Technical University of Denmark
Important Shallow water constituents -> Chart Datum
24 DTU Space, Technical University of Denmark
Way forward
VORF / BLAST
UK VORF demonstrated approach
Using 320 UK GPS measured Tide Gauges
2010: UKHO/NAVY initiated global
VORF extention using DTU10MSS.
2011: PSMSL initiated global GPS
Tide Gauge availability/DTU10Integration
We now have CryoSat-2 SAR satellites
Mearusing closer /accurate to the coast
Extrapolation last 5 km towards the coast
What about ”deep” fjords.
25 DTU Space, Technical University of Denmark
Latitude (degrees)
Sea S
urf
ace H
eig
ht
(mete
rs)
28,5 30 31,5 33 34,5 36 37,5 39 40,5 42-1,2
-0,8
-0,4
0
0,4
0,8
1,2
1,6
2SSH - Tides not removedSSH - Tides RemovedDNSC08 MDT - no IB
Hantenna(t) = HLaser (t) + Hoffset
Hlaser(t)=Hpitch+Hspeed+Hpoint+Hweight+Hbow-wave
Example
DTU10MSS as reference
Crossing the Gulf Stream
The variability of currents
Getting SSH with laser….
The RMS between LASER and MSS = 17 cm
26 DTU Space, Technical University of Denmark
Looking back…… Integrating with existing reference surfaces (The BLAST project)
• Data might be in various systems
• These can be transferred into the same system using BLAST
country VD Offset
(cm)
B DNG
(pure leveled heights)
-232
D DHHN92
(normal heights)
1
F NGF-IGN69
(normal heights)
-47
DK DVR90
(orthometric heights)
0
N NN1954
(orthometric heights)
-1
NL NL_AMST / UNCOR
(pure leveled heights)
+2
UK Newlyn (ODN)
(ortometric heights)
+5
27 DTU Space, Technical University of Denmark
Offshore framework
survey vessel (+GPS)
NO*
FR*
UK
BE (via MSL)
DE*
FR*
NL (via MSL)
UK
MSL
BE FR*
NL
UK
BE, NL, DK*, NO*
geoid
LAT
CD
ellipsoid
* = data partially covers North Sea area of this MS
28 DTU Space, Technical University of Denmark
country LAT CD
B Grid
No
D Grid (part of NS territorial
region)
No
F Available but Outside Available
but outside
DK Defined by 8 points
Recommend GOT4.7 LAT
No
N Defined by 3 points
Uses GOT4.7 LAT
No
NL Grid No
UK Grid Grid
MARINE:
VRF program includes the following
National LAT / CD datums
All implemented on to common highresBLAST grid
Associated coverage file associated with each grid
Informs on ”reliable coverage”
29 DTU Space, Technical University of Denmark
Blast transformation program
Created a ”command line”
Interface program.
Development of User
interface baced
On response from users
30 DTU Space, Technical University of Denmark
Outlook
Satellite derived MSS and Ocean Tide LAT applicable.
Could be established and quality controlled for the Arctic.
UKHO – UK Navy currently extending VORF with global DTU10MSS
UK VORF + BLAST paved the road
Must be integrated with local data to enhance local modelleing.
Implementing ”error handling”
31 DTU Space, Technical University of Denmark
Backup slides -
32 DTU Space, Technical University of Denmark
Arctic Ocean TPX 7.2 vs GOT 4.7 DTU10 vs GOT 4.7
33 DTU Space, Technical University of Denmark
BLAST Reference
surface
EGG02008 Geoid (N)
Numbers are
GPS levelling height
Hellip-Hlevel-N
Clear tilt in the UK
Levelling data.
34 DTU Space, Technical University of Denmark
EVRF-EGG Transformation
The EVRF2007 based surfaces (land) will be
Tranformed via the the EGG08 geoid.
The surface “EVRF-DIF” has been modelled to handle this.
And this is implemented in the BLAST program.
35 DTU Space, Technical University of Denmark
BLAST LAT surface
Based on DTU10
Ocean tide model
Identical to within 5 cm of
GOT4.7 ocean tide model
36 DTU Space, Technical University of Denmark
Blast transformation program Input / Output
• Input specify which country and ”system” you are in
#The data is as follows:
#ID lat lon h country system
pt1 55.1 8.4 2 1 1
2 56.3 9.8 3.7 2 1
33 57.8 10.34 4 2 1
hgt 57.8 10.321 43.5 8 5
37 DTU Space, Technical University of Denmark
MSS from Satellite Altimetry
38 DTU Space, Technical University of Denmark
The DNSC08 Mean Sea Surface
– First purely Geometrical MSS – Mean 1993-2004 period
– Derived from T/P, T/P TDM, ERS1 ERM+GM, ERS2 ERM, ENVISAT, Geosat GM, GFO
– Total 12 years of data using T/P + Jason-1 as reference
– The MSS has been derived in the Mean Tide System
– NEW!!!! DTU10 Available – Mean 1993-2009
– Vertical Accuracy 4-6 cm deep ocean best - less accurate close to coast
Want complete coverage in space and time”
”Get the best out of ERM (Variability averaged out) and GM (high spatial resolution)”
MSS = MDT + Geoid
Model (Name)
T/P data
Years
Res
KMS04
CLS01
GSFC00/00.1.
KMS01
NCU01
GSFC98
CLS-SHOM 98,
KMS98
CSR95
OSU95
9 (93-01)
7 (93-99
7 (93-99)
7 (93-00)
6 (93-98)
3 (93-95)
3 (93-95)
3 (93-95)
2 (93-94)
1 (93-93)
2
2
2
2
2
2
2
3.75
3.75
5
39 DTU Space, Technical University of Denmark
MDT (DNSC08MSS-EGM2008 = DNSC08MDT)
40 DTU Space, Technical University of Denmark
Latitude (degrees)
Sea S
urf
ace H
eig
ht
(mete
rs)
28,5 30 31,5 33 34,5 36 37,5 39 40,5 42-1,2
-0,8
-0,4
0
0,4
0,8
1,2
1,6
2SSH - Tides not removedSSH - Tides RemovedDNSC08 MDT - no IB
Hantenna(t) = HLaser (t) + Hoffset
Hlaser(t)=Hpitch+Hspeed+Hpoint+Hweight+Hbow-wave
GPS PPP processing
TIDE SYSTEMS
GPS TIDE FREE!!!!!!!
41 DTU Space, Technical University of Denmark
A good MSS and Geoid -> Good MDT
MDT = DNSC08MSS – NAT04G
42 DTU Space, Technical University of Denmark
DNSC08MSS-LAT47 (Ref to GRS80)