Dear Colleague,
Background
You may know that in the last few years there have been two important workshops at which the use of GPS (and possibly other advanced geodetic techniques) at tide gauge stations was discussed, with the aims of:
(a) first, determining the geocentric coordinates of the tide gauge bench marks (TGBM's) at a number of stations worldwide,
and (b) longer term, the construction of time series of horizontal and vertical velocities of the TGBM's ('land movements').
The conclusions of the workshops have been published as:
Carter, W.E., Aubrey, D.G., Baker, T.F., Boucher, C., Le Provost, C., Pugh, D.T., Peltier, W.R., Zumberge, M., Rapp, R.H., Schutz, R.E., Emery, K.O. and Enfield, D.B. 1989. Geodetic fixing of tide gauge bench marks. Woods Hole Oceanographic Institution Technical Report, WHOI-89-31, 44pp.
and
Carter, W.E. (ed.) 1994. Report of the Surrey Workshop of the IAPSO Tide Gauge Bench Mark Fixing Committee. Report of a meeting held 13-15 December 1993 at the Institute of Oceanographic Sciences Deacon Laboratory. NOAA Technical Report NOSOES0006. 81pp.
The latter workshop's findings were influenced considerably by the recent developments of the International GPS Service for Geodynamics (IGS). The IGS can provide a mechanism whereby GPS data obtained from receivers at the tide gauges can be analysed within consistent reference frames and with common analysis techniques. The concept of this IGS technique is described in the report:
Zumberge, J., R. Liu (ed.) 1995, "Proceedings of the IGS Workshop on the Densification of the ITRF through Regional GPS Networks", Nov. 30 -Dec. 2, 1994, IGS Central Bureau, JPL Publication 95-11, Pasadena, CA, 131pp. and a status report and background on the IGS are in press:
Zumberge, J., R. Liu, R. Neilan (ed.) 1995, "International GPS Service for Geodynamics 1994 Annual Report", IGS Central Bureau, JPL Publication 95-XX, Pasadena, CA
Both are available from the IGS Central Bureau.
Future Data Banking Arrangements
It has been agreed between the IGS and PSMSL, and endorsed by several meetings of GPS and tide gauge experts, that GPS bench mark (GPS BM) coordinates and velocities will be provided to the PSMSL through IGS channels, some of which have yet to be defined. This arrangement means that GPS data will not be analysed by the PSMSL itself but by IGS-associated GPS experts who are most familiar with the data type and who are interested to join this joint activity. The PSMSL data bank will not hold either raw GPS data, or even sets of coordinates for GPS BM's for different epochs. That is to say, it will not itself construct time series and trends of vertical land movements. (This arrangement will replace that implied in a PSMSL/WOCE Sea Level Centre letter which followed the earlier "Carter et al." workshop).
Survey of GPS Work at Tide Gauge Sites
Following meetings of the PSMSL and IGS in early 1995, it was agreed that a survey of the status of GPS monitoring of tide gauge benchmarks was urgently required. We know that several countries now have permanent GPS receivers at some gauges; other countries have performed repeated short campaigns at gauge sites (perhaps of 2-5 days duration repeated each year); other countries are just beginning this sort of work. In many cases, lessons have been learned which should be shared with others.
In Annex 1 of this letter we list a set of questions which we would be grateful if you could answer for your organisation (or please pass to the appropriate person to answer). Annex 1 should not be regarded as a formal questionnaire. Please feel free to provide answers as you think best, or supply additional relevant questions and answers. Our main objectives are to determine the overall level of observations, what problems exist in attempting to tie the tide gauge benchmarks to the GPS bench marks, and how these problems can be shared and solved.
Of course, there are many tide gauges around the world, and our main interest is towards the GPS monitoring of BM's at tide gauges which make up the 'global network' of tide gauges (perhaps defined as an extended version of 'GLOSS'). Such a network would certainly include:
(i) sites which have long tide gauge records. Measured rates of vertical land movements might be retrospectively applied to the tide gauge relative sea level trends to give rates of real sea level change.
and (ii) sites at ocean islands. These are of potential interest for long term altimeter calibration purposes, possibly for ocean circulation studies, and even for GPS orbit improvements.
As an example of such a 'global network', we enclose as Annex 2 to this letter a table from the Carter (1994) report which gives a 'schematic list' of stations at which GPS recording might be useful. However, once again, please treat this list as simply a guide to present ideas, and not a definition of them (e.g. European sites are under-represented in GLOSS). So, if you know of GPS recording at other sites, we would be pleased to hear about them.
Survey Responses and Findings
We would be very grateful if you could answer this letter in electronic form, either as email (to plw@pol.ac.uk), or as an ascii file on floppy disk addressed to Dr.P.L.Woodworth at the PSMSL. That will help us considerably in editing and synthesising what we hope will be many detailed responses. Of course, if it is difficult to reply that way, please just write to us on paper or fax us as is most convenient.
Our present intention is that the findings of the survey should form the basis of a PSMSL/IGS meeting in 1997, possibly alongside a GLOSS or IAG meeting that year. This workshop will focus on problems, solutions, and preliminary scientific results.
Once again, many thanks for your help with this project.
Yours sincerely,
Philip Woodworth Ruth Neilan Permanent Service for Mean Sea Level IGS Central Bureau
Please fill out the questionaire below and return to:
plw@pol.ac.uk
or as an ascii file on floppy disk addressed to:
Dr.P.L.Woodworth
Proudman Oceanographic Laboratory,
Bidston Observatory,
Birkenhead, Merseyside L43 7RA, U.K.
or FAX to : +44 - 151 - 653 6269
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Annex 1
-------
Questions On GPS Recording at Tide Gauge Sites
----------------------------------------------
Your Name:
Organisation:
Address:
Email address:
Telephone:
Fax numbers:
Date of your reply:
For each station at which GPS recording has been made (or planned), please
provide information as follows (if information is the same for several
sites, just insert "same as for site (insert Tide Gauge Station name)"):
Tide Gauge Station name:
Latitude:
Longitude:
Tide gauge type:
Gauge owner/contact:
GPS Station name:
GPS four Character ID:
Is this an IGS station? (Y/N):
If not, Station owner/contact:
Dates of GPS Observations ddd/mmm/year to ddd/mmm/year:
or continuous GPS Observations since ddd/mmm/year:
Organization holding/archiving GPS data:
Organization processing GPS data:
Are results available:
Has a local survey tie been done between tide gauge BM and GPS BM(Y/N):
Contact who performed the survey tie:
Organization:
Contact for survey tie results (if different):
Briefly describe how the survey tie was performed:
Please describe any problems that you experienced (e.g. multipath in ports,
troposphere, site security, etc.) and how did you solve them?
Do you object to the information you provide above being edited into any
widely-circulated document on the conclusions of the survey (Y/N):
Are you be in favour of a PSMSL/IGS workshop in 1997 to discuss the findings
of the survey and explore further cooperation and research (Y/N):
Further comments:
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Annex 2 (from Carter 1994)
--------------------------
STATIONS USEFUL FOR SEA LEVEL STUDIES OF VARIOUS KINDS
======================================================
The following stations were identified, in a `schematic' way, by the
working group chaired by Gary Mitchum as being of interest for
GPS measurements for several oceanographic reasons. An `*' in the
column headed 'LONG TERM STUDIES' flags GLOSS stations with at least
40 years of data in the PSMSL dataset, while `+' flags several stations
in data sparse areas which could be of potential importance to long
term studies. Unflagged stations were selected primarily from the
WOCE sea level network, and are mostly at ocean islands. These are
of interest from the points of view of interannual variability of
deep ocean sea level, and as sites which could provide a densification
of the IGS network in oceanic regions. Station names are mostly those
used in PSMSL reports.
LONG
TERM
STATION GLOSS NO. LATITUDE LONGITUDE STUDIES
--------------------------------------------------------------------------------
Djibouti 2 11-36.0N 43-90.0E
Aden 3 12-47.0N 44-59.0E *
Salalah 4 16-56.0N 54-00.0E
Mombasa 8 4-03.0S 39-40.0E
Exuma 12 23-46.0N 76-06.0W
Durban 13 29-53.0S 31-02.0E
Reunion 17 20-55.0S 55-18.0E
Port Louis Harbour 18 20-09.0S 57-30.0E
Rodrigues 19 19-40.0S 63-25.0E
Marion Island 20 46-52.0S 37-52.0E
Crozet Island 21 46-25.0S 51-52.0E
Mawson 22 67-36.0S 62-52.0E
Kerguelen Island 23 49-21.0S 70-12.0E
Mirny 25 66-33.0S 93-01.0E
Diego Garcia 26 7-00.0S 72-30.0E
Gan 27 0-42.0S 73-10.0E
Male 28 4-11.0N 73-31.0E
Karachi, Manora Island 30 24-48.0N 66-58.0E *
Cochin 32 9-58.0N 76-16.0E *
Madras 34 13-06.0N 80-18.0E *
Vishakhapatnam 35 17-41.0N 83-17.0E *
Ko Lak 39 11-48.0N 99-49.0E *
Ko Taphao Noi 42 7-50.0N 98-26.0E *
Cocos Island 46 12-07.0S 96-53.0E
Christmas Island 47 10-25.0S 105-40.0E
Fremantle II 53 32-03.0S 115-44.0E *
Esperance 54 33-52.0S 121-54.0E
Hobart 56 42-53.0S 147-20.0E
Sydney, Fort Denison 57 33-51.0S 151-14.0E *
Bundaberg 59 24-50.0S 152-21.0E
Darwin 62 12-28.0S 130-51.0E
Rabaul 65 4-12.0S 152-11.0E
Honiara 66 9-26.0S 159-57.0E
Bitung 69 1-32.0N 124-50.0E
Jolo, Sulu 70 6-04.0N 121-00.0E *
Davao 71 7-05.0N 125-38.0E *
Legaspi, Albay 72 13-09.0N 123-45.0E *
Manila, S. Harbor 73 14-35.0N 120-58.0E *
Port Elizabeth 76 33-58.0S 25-38.0E
Quarry Bay 77 22-18.0N 114-13.0E +
Pusan 84 35-06.0N 129.02.0E +
Kushimoto 85 33-28.0N 135-47.0E *
Mera 86 34-55.0N 139-50.0E *
Yuzhno Kurilsk 90 44-01.0N 145-52.0E *
Petropavlovsk-Kamchatsky 93 52-59.0N 158-39.0E +
Syowa 95 69-00.0S 39-35.0E +
Kaliningrad 97 54-57.0N 20-13.0E *
Tuapse 98 44-06.0N 39-04.0E *
Wellington II 101 41-17.0S 174-47.0E *
Chichijima 103 27-05.0N 142-11.0E
Wake Island 105 19-17.0N 166-37.0E
Midway Island 106 28-13.0N 177-22.0W *
Honolulu 108 21-18.0N 157-52.0W *
Johnston Island 109 16-44.0N 169-32.0W *
Kwajalein 111 8-44.0N 167-44.0E *
Majuro 112 7-06.0N 171-22.0E
Tarawa 113 1-22.0N 172-56.0E
Nauru 114 0-32.0S 166-54.0E
Ponape 115 6-59.0N 158-14.0E
Truk Atoll 116 7-27.0N 151-51.0E
Kapingamarangi 117 1-06.0N 154-47.0E *
Saipan 118 15-14.0N 145-45.0E
Malakal 120 7-20.0N 134-28.0E
Funafuti 121 8-32.0S 179-13.0E
Suva 122 18-08.0S 178-26.0E
Noumea 123 22-18.0S 166-26.0E
Auckland II 127 36-51.0S 174-46.0E *
Chatham Island 128 43-50.0S 176-30.0W
Bluff Harbor 129 46-36.0S 168-21.0E
Macquarie Island 130 54-30.0S 158-56.0E
Dumont d'Urville 131 66-40.0S 140-01.0E
Isla da Pascua 137 27-09.0S 109-27.0W
Rikitea 138 23-08.0S 134-57.0W
Rarotonga 139 21-12.0S 159-46.0W
Papeete 140 17-31.0S 149-30.0W
Nuku Hiva 142 8-56.0S 140-05.0W
Penrhyn 143 9-01.0S 158-05.0W
Pago Pago 144 14-17.0S 170-41.0W *
Kanton 145 2-48.0S 171-43.0W
Christmas 146 1-59.0N 157-28.0W
Guam 149 13-26.0N 144-39.0E *
Seward 150 60-07.0N 149-26.0W *
Sitka 154 57-03.0N 135-20.0W *
Prince Rupert 155 54-19.0N 130-20.0W *
Tofino 156 49-09.0N 125-55.0W *
San Francisco 158 37-48.0N 122-28.0W *
La Jolla 159 32-52.0N 117-15.0W *
Cabo San Lucas 161 22-53.0N 109-55.0W
Socorro Island 162 18-44.0N 111-01.0W
Quepos 167 9-24.0N 84-10.0W
Balboa 168 8-58.0N 79-34.0W *
Galapagos Islands 169 0-26.0S 90-17.0W
Buenaventura 170 3-54.0N 77-06.0W *
Tumaco 171 1-50.0N 78-44.0W *
La Libertad II 172 2-12.0S 80-55.0W *
Callao 173 12-10.0S 77-12.0W
Antofagasta 174 23-39.0S 70-25.0W *
Valapariso 175 33-02.0S 71-38.0W *
Juan Fernandez 176 33-37.0S 78-50.0W
San Felix 177 26-17.0S 80-80.0W
Puerto Williams 180 45-56.0S 67-37.0W
South Georgia 187 54-15.0S 36-45.0W
Faraday 188 65-15.0S 64-16.0W
Puerto Deseado 190 47-45.0S 65-55.0W *
Isla Fiscal 195 22-52.0S 43-08.0W +
Fernando de Noronha 198 3-52.0S 32-25.0W
Porto de Natal 197 5-46.0N 35-12.0W +
Penedro Sao Pedro e Paulo 199 1-00.0N 29-23.0W
Marseille 205 43-18.0N 5-21.0E *
San Juan 206 18-28.0N 66-07.0W
Cartagena 207 10-24.0N 75-33.0W *
Settlement Point 211 26-41.0N 79-00.0W
Progresso 213 21-18.0N 89-39.0W +
Siboney 215 23-05.0N 82-28.0W
Key West 216 24-33.0N 81-48.0W *
Galveston II 217 29-19.0N 94-48.0W *
Miami 218 25-54.0N 80-07.0W
Duck, N.C. 219 35-13.0N 75-38.0W
Atlantic City 220 39-21.0N 74-25.0W *
Bermuda 221 32-22.0N 64-42.0W *
Halifax 222 44-40.0N 63-35.0W *
Alert 225 82-20.0N 62-20.0W +
Angmagssalik 228 65-30.0N 37-00.0W
Reykjavik 229 64-09.0N 21-56.0W
Barentsburg 231 78-04.0N 14-15.0E *
Maloy 235 61-56.0N 5-07.0E *
Newlyn 241 50-06.0N 5-33.0W *
Brest 242 48-23.0N 4-30.0W *
La Coruna I 243 43-22.0N 8-24.0W *
Ponta Delgada 245 37-44.0N 25-40.0W *
Cascais 246 38-41.0N 9-25.0W *
Gibraltar 248 36-07.0N 5-21.0W
Ceuta 249 35-54.0N 5-19.0W
Dakar 253 14-38.0N 17-27.0W
Porto Grande 254 16-52.0N 24-59.0W
Ascension 263 7-55.0S 14-25.0W
St. Helena 264 15-58.0S 5-42.0W
Ilha da Trindade 265 20-30.0S 29-18.0W
Tristan da Cunha 266 37-03.0S 12-18.0W
Simonstown 268 34-11.0S 18-26.0E
Bouveteya 269 54-22.0S 3-22.0E
Port Victoria 273 4-40.0S 55-28.0E
Murmansk 274 68-58.0N 33-03.0E *
Casey 278 66-17.0S 110-32.0E
Cuxhaven 2 284 53-52.0N 8-43.0E *
Nawiliwili 285 21-57.0N 159-22.0W
Kahului 286 20-45.0N 156-28.0W
Hilo 287 19-44.0N 155-04.0W *
Pensacola 288 30-24.0N 87-13.0W *
Fort Pulaski 289 32-02.0N 80-54.0W *
Newport 290 41-30.0N 71-20.0W *
South Caicos 296 22-00.0N 72-00.0W
Zanzibar 297 6-09.0S 39-11.0E
Montevideo 300 34-54.0S 56-15.0W *
Adak 302 51-52.0N 176-38.0W *
Massacre Bay 303 52-50.0N 173-12.0E
Port Stanley 305 51-45.0S 57-56.0W
Signy 306 60-42.0S 45-36.0W
Dikson 312 73-32.0N 80-39.0E +
Tiksi 313 71-40.0N 128.45.0E +
Arica 18-28.0S 70-20.0W
Caldera 27-04.0S 70-50.0W
Diego Ramirez 56-33.0S 68-40.0W
Heard Island 53-00.0S 73-25.0E
Malpelo Island 4-00.0N 81-21.0W
Amsterdam Island 38-43.0S 77-35.0E
Scott Base 77-51.0S 166-40.0E
Prudhoe Bay 70-12.0N 148-15.0W
Provideniya 64-18.0N 173-07.0W
St. Croix 17-42.0N 64-46.0W
S Sound, Gd Cayman 19-16.0N 81-23.0W
Lake Worth 26-37.0N 80-02.0W
Hualien 23-58.0N 121-37.0E
Ishigaki 24-20.0N 124-09.0E
Naze 28-23.0N 129.30.0E
Nishinoomote 30-44.0N 131-00.0E
Spring Bay 42-32.9S 147-55.8E
Cresent City 41-45.0N 124-11.0W
Dutch Harbor 53-53.0N 166-32.0W
Ketchikan 55-20.0N 131-38.0W
Kodiak Island 57-48.0N 152-24.0W
Lobos 6-56.0S 80-43.0W
Neah Bay 48-22.0N 124-37.0W
San Diego 32-43.0N 117-10.0W
Talara 4-35.0S 81-17.0W
Hanimaadu 6-46.0N 73-10.0E
Pointe La Rue 4-40.0S 55-32.0E
Charleston 32-47.0N 79-56.0W
Lome 6-08.0N 1-17.0E