Satellite derived reference surfaces for surveying

transcript

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

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

BLAST EU-7th Interreg FW project

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.

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

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

Surfaces and names:

GRS80 Ellipsoid

Chart Datum

(local)

MSL /MSS

Charted depth

measurement

(and time

of observation)

survey vessel Tide gauge

observed tide

(and time) Tidal correction

derived from

Use co-tidal

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

Charted depth

measurement

Tidal correction

Bathymetric data processing with BLAST/VORF and GPS

GRS80 Ellipsoid

survey vessel (+GPS)

Tidal correction =

he – LAT

Charted depth =

Depth measurement – tidal correction

- accessible everywhere

Sea Surface

Blast/VORF computation method

Tide gauge

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)

How to derive the MSS Satellite Altimetry

Ground Tracks

ENVISAT

ERS1/2

DTU10 Mean Sea Surface (18 years)

Height in ”GRS80”

Arctic -> Ice coverage

Decrease in total sea ice extent:

September: 11.1 % per decade

March: 2.8 % per decade

Richter-Menge et al, 2008

September

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)

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 ??

Getting a LAT from Ocean Tides – M2 loop

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

Run the tide model for 19 years to find minimum (incl Arctic)

Ellipsoidal LAT

EXAMPLE NORTH SEA

(validated)

BUT ITS GLOBAL

Ellipsoidal LAT surface

Based on:

DTU10MSS- DTU10OT

Relative to

WGS84/GRS90

GPS consistent

(also Tide system

consistent)

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.

Important Shallow water constituents -> Chart Datum

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.

Latitude (degrees)

28,5 30 31,5 33 34,5 36 37,5 39 40,5 42-1,2

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

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

(pure leveled heights)

D DHHN92

(normal heights)

F NGF-IGN69

(normal heights)

DK DVR90

(orthometric heights)

N NN1954

(orthometric heights)

NL NL_AMST / UNCOR

(pure leveled heights)

UK Newlyn (ODN)

(ortometric heights)

Offshore framework

survey vessel (+GPS)

BE (via MSL)

NL (via MSL)

BE FR*

BE, NL, DK*, NO*

ellipsoid

* = data partially covers North Sea area of this MS

country LAT CD

B Grid

D Grid (part of NS territorial

region)

F Available but Outside Available

but outside

DK Defined by 8 points

Recommend GOT4.7 LAT

N Defined by 3 points

Uses GOT4.7 LAT

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”

Blast transformation program

Created a ”command line”

Interface program.

Development of User

interface baced

On response from users

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”

Backup slides -

Arctic Ocean TPX 7.2 vs GOT 4.7 DTU10 vs GOT 4.7

BLAST Reference

surface

EGG02008 Geoid (N)

Numbers are

GPS levelling height

Hellip-Hlevel-N

Clear tilt in the UK

Levelling data.

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.

BLAST LAT surface

Based on DTU10

Ocean tide model

Identical to within 5 cm of

GOT4.7 ocean tide model

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

MSS from Satellite Altimetry

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

GSFC00/00.1.

GSFC98

CLS-SHOM 98,

9 (93-01)

7 (93-99

7 (93-99)

7 (93-00)

6 (93-98)

3 (93-95)

2 (93-94)

1 (93-93)

MDT (DNSC08MSS-EGM2008 = DNSC08MDT)

Latitude (degrees)

28,5 30 31,5 33 34,5 36 37,5 39 40,5 42-1,2

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!!!!!!!

A good MSS and Geoid -> Good MDT

MDT = DNSC08MSS – NAT04G

DNSC08MSS-LAT47 (Ref to GRS80)

Satellite derived reference surfaces for surveying

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