The Fennoscandian Postglacial Rebound: Implications for Vertical Reference Frames

Transcription

1 The Fennoscandian Postglacial Rebound: Implications for Vertical Reference Frames Jaakko Mäkinen Finnish Geodetic Institute

2 Contents some thoughts on fixing systems and frames in a changing world EVRF2000 in the Nordic countries dealing with the postglacial rebound: experiences in Fennoscandia current work to update EVRF2000 in the Nordic countries conclusions

3 Very Important to distinguish between the implications of fixing the System, i.e. the acronyms that end with S ETRS89 EVRS2000 this means fixing the principles fixing the Frame, i.e. the acronyms that end with F ETRF ETRF2000 EVRF2000 this means fixing the coordinates

4 Why? because: fixing principles (S) permits the adoption of ever-improving coordinates (F) ==> PROGRESS fixing coordinates (F) stops the progress ==> STAGNATION and PROBLEMS

5 CASE ETRS89 ETRS89 specifies the principles this has permitted the adoption of improved realizations ETRF89...ETRF countries can continually improve their realizations (and have done it) taking advantage of evolving techniques thus there are jumps in time and across borders, but all realizations are consistent enough for most purposes imagine if we would have fixed ETRF89 instead, and required that all coordinates in the future fitted to it

6 CHALLENGE FOR EVRS2000 can the success with ETRS89 be repeated for heights? i.e., ever-improving national/regional realizations of EVRS2000 (not necessarily becoming national height systems) and nevertheless consistent enough across borders obtaining national heights by fit to EVRF2000 is NOT the answer (rather, it is the analogue of fitting 3-D to ETRF89) will be possible post-goce

7 At some time, heights (F) will be specified When will they become inadequate? (a) when new requirements/new measuring technology/new data reveal that the errors in them are too big (b) or when nature changes the reality (to which they supposedly refer) too much event (a) will happen to all of EVRF2000 by 2010 at the 0.1 m level (at least) with GOCE etc. event (b) has already happened to parts of EVRF2000 at the 0.4 m level (Postglacial Rebound PGR)

8 Conflicting requirements to coordinates They should be accurate They should be unchanged

9 Examples of problems in EVRF2000

10 Transformation from National Height Datums in Europe into the EVRF 2000 Datum (NAP) (Sacher et al., 2002) Problem in the Baltic area Finnish N60 system zero (Helsinki tide gauge datum) appears to be 22- ( ) = cm higher than Baltic Height System zero (Kronstadt tide gauge datum) however, it is known from Sea Surface Topography analysis at tide gauges, that N60 zero is about 4 cm lower than Kronstadt zero EVRF2000 has an apparent (-4) = cm misclosure around the Baltic

11 UELN-95/98 Nordic data except Denmark from previous campaigns, epoch No levelling connection round the Gulf of Finland. (Sacher et al., 2002)

12 (a) Errors in any EVRF200x whatever coordinates (= heights) we adopt now, they will be obsolete (= obviously in error) post- GOCE how can we already now plan to update? risk: so much georeferencing will be already be in the obsolete frame that we are stuck with it cf. the difficulty in getting rid of old national systems for the benefit of ETRS89

13 (b) Example of Nature making the EVRF2000 obsolete

14 Uplift mm/yr relative to MSL (Ekman 1996). EVRF2000 based on epoch in Norway, Finland, Sweden Relative height changes of 0.4 m in 40 years

15 Dealing with postglacial rebound The Fennoscandian experience Some selected topics

16 For more information see Mäkinen J., M. Lidberg, K. Schmidt, M. Takalo, M. Lilje, K. Engsager, P.-O. Eriksson, C. Jepsen, P.-A. Olsson, V. Saaranen, R. Svensson, O. Vestøl: Future height systems in the Nordic countries, their relation to the EVRS2000 and to INSPIRE GIS standards. Position paper, Symposium of the IAG Commission X Subcommission for Europe, EUREF Toledo, Spain, 4 7 June p. Submitted to the Proceedings, accessible at the EUREF web page

17 Denmark, Finland, Sweden Country-wide precision levelling campaigns every years: (I) around 1900, (II) around 1950, (III) work-in-progress or just completed updated systems adopted very time already 1950 systems are explicitly time-tagged account for PGR in the processing have associated PGR numbers for maintenance (not for continuous updating of the official heights)

18 Precision levellings and height systems Country First levelling Second levelling Third levelling Years m 0 System Years m 0 System Years m 0 System Denmark DNN GM DNN GI DVR90* DNN GM * Finland NN N60* Norway ( ) NN NN Sweden RH RH70* * Definite epoch attached, land uplift considered

19 Use of PGR numbers Systems are STATIC for the practical user but KINEMATIC behind the curtains, i.e. in maintenance and in advanced applications

20 Other time-tagged systems in the Nordic countries ETRS89 realizations, for the radial component and internal geometry consequently, tailored geoids are in the same epoch gravity all are so far static in use PGR models available

21 Why is the quasi-static approach succesful so far despite absolute vertical motion up to 1 cm/yr? because the user techniques are relative and rely on static coordinates of reference sites (levelling BMs, permanent GPS, etc.) thus only differential motion between the user site and the reference site counts users are not doing long-distance levelling, GPS is at present the issue

22 Relative vertical motion is one order of magnitude less than absolute motion dense set of permanent GPS reference stations maximal PGR difference to nearest permanent site about 2 mm/yr, i.e., 20 mm in 10 years vertical motion can be incorporated in updated tailored geoids

23 Tangential motion: concentrically up to 3 mm/yr away from the uplift center, not accounted for in ETRS89 rules Not treated here see for instance Milne et al. (2001)

24 Updating the EVRF2000 in Nordic countries new levelling data introduced new PGR models screened and compared bridging the Gulf of Finland

25 Plan endorsed by EUREF TWG (Toledo, June 2003) Nordic Geodetic Commission (NKG) and UELN computing center work together: NKG screening their own data, modelling PGR around the Baltic, doing regional adjustments to compare models UELN center doing in parallel European adjustment

26 Current work by Working Group for Height Determination of the NKG new data from third levellings being introduced PGR information from repeated levelling, tide gauges, permanent GPS, and geophysical models of Glacial Isostatic Adjustment (GIA) screened and compared used for reducing the levellings to oceanographic levelling and GPS/geoid ties over the Gulf of Finland, to tie to the UELN-95/98 from the south oceanographic levelling and GPS/geoid ties over the Gulf of Bothnia for cutting across the Baltic loop

27 Some examples of PGR comparisons Different types of observations and different GIA models agree at the 1 mm/yr level

28 Comparison of CGPS results (Johansson et al., 2002) with GIA map redrawn from Lambeck et al. (1998) GIA map relative to MSL GPS not used in the generation of the map GPS radial rates divided by 1.06, and 1.5 mm/yr subtracted to account for geoid rise and rise in MSL

29 Comparisons Curves: uplift relative to MSL from 3 precise levellings (Saaranen and Mäkinen, 2002) Columns: uplift from CGPS minus uplift from levelling Left column: BIFROST processing (Johansson et al., 2002) Right column: FinnRef processing (Mäkinen et al., 2003)

30 Conclusions so far: PGR observations agree between themselves and with different GIA models to approximately 1 mm/yr RMS

31 Oceanographic tie over the Gulf of Finland From Ollikainen et al. (2003)

32 Mean Sea Level (MSL) at tide gauges in Finland, Estonia and Kronstadt in the epoch Standardized to period In Estonia and at Kronstadt in the Kronstadt datum, in Finland in the N60 system. Sea surface topography differences are expected to be zero across the Gulf of Finland. Conclusion: Kronstadt zero is about 3 cm higher than N60 zero.

33 Vertical velocities from GIA model by Milne et al. (2001)

34 Summary essential improvements to heights in the Nordic countries coming soon EVRF would benefit greatly from including them in its next edition PGR changes elevation differences by 0.1 m per decade, is handled rigorously in national systems, how about European systems? PGR changes relative horizontal positions by centimetres per decade, no provision in ETRS89 rules

35 Immediate wishes please do not introduce an EVRF2000 or EVRF200x without taking into account what we are coming up with soon or if you do, provide for a quick update we need to discuss the form of time dependence of EVRF (and ETRF)

36 References Ekman M (1996): A consistent map of the postglacial uplift of Fennoscandia. Terra Nova 8, Johansson, J.M., Davis, J.L., Scherneck, H.-G., Milne, G.A., Vermeer, M., Mitrovica J.X., Bennett, R.A., Ekman, M., Elgered, G., Elósegui, P., Koivula, H., Poutanen, M., Rönnäng, B.O., Shapiro, I.I., Continuous GPS measurements of postglacial adjustment in Fennoscandia, 1. Geodetic results. J. Geophys. Res.,107, No. B8, Lambeck K., Smither C., Ekman, J. (1998): Tests of glacial rebound models for Fennoscandia based on instrumented sea- and lake-level records. Geophys. J. Int. 135, Mäkinen, J., H. Koivula, M. Poutanen and V. Saaranen (2003): Vertical velocities in Finland from permanent GPS networks and from repeated precise levelling. J. Geodynamics 35, Milne, G.A., Davis, J.L., Mitrovica, J.X., Scherneck, H.-G., Johansson, J.M. Vermeer, M.. Koivula, H., Space-Geodetic Constraints on Glacial Isostatic Adjustment in Fennoscandia. Science 291 (23 March 2001), Ollikainen M., Rüdja A., Jürgenson H., Mäkinen J.: Height connection over the Gulf of Finland using oceanographic levelling, and GPS/geoid ties. Symposium of the IAG Commission X Subcommission for Europe, EUREF Toledo, Spain, 4 7 June 2003.