[MetaCRS] Motion Vote: CS-Map RFC 2 - Redesign Datum Transformation Engine

Wed Jul 7 11:24:20 EDT 2010

Hello Frank

Le 07/07/10 14:50, Frank Warmerdam a écrit :
> I would appreciate hearing a bit more about the downsides of using WGS84
> as a pivot datum since PROJ.4 is still based on this assumption. I understand
> that it tends to lead to kludges for some situations and makes chaining
> challenging, but these seem to be implementation issues more than fundamentals
> of using WGS84 as a pivot datum.

A quick search in the EPSG 7.5 database gave me 272 on a total of 1013 
Coordinate Operations (considering only methods 1031-1035, 1037-1039, 9603-9607 
and 9636) which don't use the WGS 84 datum (6326) neither as a source or target 
datum. This is 27% of the datum shifts listed in the EPSG database (using the 
above-cited criterion). For those who may want to know, I attached to this email 
the list of source and target datum that are not WGS 84 (the list is shorter 
than 273 entries because I omitted the operation methods variants having the 
same source and target datum).

If you apply the datum shifts using WGS 84 as a pivot (assuming that accurate 
Bursa-Wolf parameters exist for the "to WGS84" operation and the "from WGS84" 
operation - which is not garanteed), you may not get the same result for the 
following mathematical raison:

When using the Geocentric transformation method, performing the datum shift as 
two separated steps is equivalent to express the two Bursa-Wolf parameters as 
two affine transform matrix, multiply those matrix together, and use it for the 
Geocentric transformation. The matrix is derived from the Bursa-Wolf parameters 
as below:

http://www.geotoolkit.org/apidocs/org/geotoolkit/referencing/datum/BursaWolfParameters.html#getAffineTransform%28%29

Every Bursa-Wolf parameters can be expressed an an affine transform, but the 
converse is not true: an affine transform can be retrofitted in a Bursa-Wolf 
parameters only if the matrix is antisymetric. From my Geotoolkit experience, 
the result of the above-cited multiplication usually don't match this criterion.

In other words, the result of two consecutive Datum Shift is usually not 
expressible in a single set of Bursa-Wolf parameters, except if the datum shifts 
are only geocentric translations (no scale or rotation).

The above doesn't said that one method is more accurate than the other. It just 
said that the result are going to be different, except in the special cases 
where the result of the above-cited matrix multiplication is antisymetric. If a 
library wants to provide coordinate transformations matching the authority 
definition (without judgment about the accuracy of that definition), it is 
better to avoid going through a pivot.

The Geotoolkit.org and GeoTools libraries don't use any pivot: they apply the 
Bursa-Wolf parameters directly from the source to the target datum defined by 
EPSG, or apply the above-cited matrix multiplication when no direct path is found.

	Best regards,

		Martin
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"source_datum","datum_name","target_datum","datum_name"
1031,"MGI 1901",6258,"European Terrestrial Reference System 1989"
1031,"MGI 1901",6765,"Slovenia Geodetic Datum 1996"
1034,"Serbian Reference Network 1998",6258,"European Terrestrial Reference System 1989"
1037,"Cadastre 1997",1036,"Reseau Geodesique de Mayotte 2004"
6123,"Kartastokoordinaattijarjestelma (1966)",6258,"European Terrestrial Reference System 1989"
6124,"Rikets koordinatsystem 1990",6258,"European Terrestrial Reference System 1989"
6124,"Rikets koordinatsystem 1990",6619,"SWEREF99"
6127,"Tete",6130,"Moznet (ITRF94)"
6131,"Indian 1960",6324,"WGS 72 Transit Broadcast Ephemeris"
6133,"Estonia 1992",6258,"European Terrestrial Reference System 1989"
6134,"PDO Survey Datum 1993",6322,"World Geodetic System 1972"
6149,"CH1903",6258,"European Terrestrial Reference System 1989"
6150,"CH1903+",6151,"Swiss Terrestrial Reference Frame 1995"
6150,"CH1903+",6258,"European Terrestrial Reference System 1989"
6152,"NAD83 (High Accuracy Regional Network)",6759,"NAD83 (National Spatial Reference System 2007)"
6156,"Jednotne Trigonometricke Site Katastralni",6258,"European Terrestrial Reference System 1989"
6159,"European Libyan Datum 1979",6754,"Libyan Geodetic Datum 2006"
6160,"Chos Malal 1914",6221,"Campo Inchauspe"
6164,"South Yemen",6163,"Yemen National Geodetic Network 1996"
6168,"Accra",6324,"WGS 72 Transit Broadcast Ephemeris"
6171,"Reseau Geodesique Francais 1993",6258,"European Terrestrial Reference System 1989"
6173,"IRENET95",6258,"European Terrestrial Reference System 1989"
6178,"Pulkovo 1942(83)",6258,"European Terrestrial Reference System 1989"
6179,"Pulkovo 1942(58)",6258,"European Terrestrial Reference System 1989"
6180,"Estonia 1997",6258,"European Terrestrial Reference System 1989"
6181,"Luxembourg 1930",6258,"European Terrestrial Reference System 1989"
6182,"Azores Occidental Islands 1939",1041,"Autonomous Regions of Portugal 2008"
6189,"Red Geodesica Venezolana",6170,"Sistema de Referencia Geocentrico para America del Sur 1995"
6190,"Posiciones Geodesicas Argentinas 1998",6170,"Sistema de Referencia Geocentrico para America del Sur 1995"
6193,"Manoca 1962",6324,"WGS 72 Transit Broadcast Ephemeris"
6194,"Qornoq 1927",6747,"Greenland 1996"
6195,"Scoresbysund 1952",6747,"Greenland 1996"
6196,"Ammassalik 1958",6747,"Greenland 1996"
6197,"Garoua",6324,"WGS 72 Transit Broadcast Ephemeris"
6198,"Kousseri",6324,"WGS 72 Transit Broadcast Ephemeris"
6200,"Pulkovo 1995",6740,"Parametrop Zemp 1990"
6202,"Australian Geodetic Datum 1966",6283,"Geocentric Datum of Australia 1994"
6203,"Australian Geodetic Datum 1984",6283,"Geocentric Datum of Australia 1994"
6207,"Lisbon 1937",6258,"European Terrestrial Reference System 1989"
6208,"Aratu",6674,"Sistema de Referencia Geocentrico para America del Sur 2000"
6213,"Beduaram",6324,"WGS 72 Transit Broadcast Ephemeris"
6216,"Bermuda 1957",6762,"Bermuda 2000"
6218,"Bogota 1975",6686,"Marco Geocentrico Nacional de Referencia"
6220,"Camacupa",6324,"WGS 72 Transit Broadcast Ephemeris"
6221,"Campo Inchauspe",6694,"Posiciones Geodesicas Argentinas 1994"
6222,"Cape",6148,"Hartebeesthoek94"
6224,"Chua",6674,"Sistema de Referencia Geocentrico para America del Sur 2000"
6227,"Deir ez Zor",6324,"WGS 72 Transit Broadcast Ephemeris"
6229,"Egypt 1907",6322,"World Geodetic System 1972"
6230,"European Datum 1950",6231,"European Datum 1987"
6230,"European Datum 1950",6258,"European Terrestrial Reference System 1989"
6231,"European Datum 1987",6258,"European Terrestrial Reference System 1989"
6237,"Hungarian Datum 1972",6258,"European Terrestrial Reference System 1989"
6238,"Indonesian Datum 1974",6755,"Datum Geodesi Nasional 1995"
6242,"Jamaica 1969",6322,"World Geodetic System 1972"
6242,"Jamaica 1969",6758,"Jamaica 2001"
6247,"La Canoa",6189,"Red Geodesica Venezolana"
6248,"Provisional South American Datum 1956",6170,"Sistema de Referencia Geocentrico para America del Sur 1995"
6248,"Provisional South American Datum 1956",6189,"Red Geodesica Venezolana"
6259,"Malongo 1987",6264,"Mhast"
6265,"Monte Mario",6258,"European Terrestrial Reference System 1989"
6269,"North American Datum 1983",6140,"NAD83 Canadian Spatial Reference System"
6272,"New Zealand Geodetic Datum 1949",6167,"New Zealand Geodetic Datum 2000"
6273,"NGO 1948",6258,"European Terrestrial Reference System 1989"
6274,"Datum 73",6258,"European Terrestrial Reference System 1989"
6275,"Nouvelle Triangulation Francaise",6230,"European Datum 1950"
6275,"Nouvelle Triangulation Francaise",6258,"European Terrestrial Reference System 1989"
6275,"Nouvelle Triangulation Francaise",6322,"World Geodetic System 1972"
6277,"OSGB 1936",6230,"European Datum 1950"
6281,"Palestine 1923",6141,"Israel"
6284,"Pulkovo 1942",6126,"Lithuania 1994 (ETRS89)"
6284,"Pulkovo 1942",6133,"Estonia 1992"
6284,"Pulkovo 1942",6258,"European Terrestrial Reference System 1989"
6284,"Pulkovo 1942",6740,"Parametrop Zemp 1990"
6289,"Amersfoort",6258,"European Terrestrial Reference System 1989"
6299,"TM65",6258,"European Terrestrial Reference System 1989"
6300,"Geodetic Datum of 1965",6258,"European Terrestrial Reference System 1989"
6301,"Tokyo",6612,"Japanese Geodetic Datum 2000"
6307,"Nord Sahara 1959",6324,"WGS 72 Transit Broadcast Ephemeris"
6309,"Yacare",6170,"Sistema de Referencia Geocentrico para America del Sur 1995"
6310,"Yoff",6322,"World Geodetic System 1972"
6312,"Militar-Geographische Institut",6258,"European Terrestrial Reference System 1989"
6312,"Militar-Geographische Institut",6765,"Slovenia Geodetic Datum 1996"
6313,"Reseau National Belge 1972",6258,"European Terrestrial Reference System 1989"
6314,"Deutsches Hauptdreiecksnetz",6258,"European Terrestrial Reference System 1989"
6324,"WGS 72 Transit Broadcast Ephemeris",6322,"World Geodetic System 1972"
6618,"South American Datum 1969",6674,"Sistema de Referencia Geocentrico para America del Sur 2000"
6619,"SWEREF99",6258,"European Terrestrial Reference System 1989"
6623,"Centre Spatial Guyanais 1967",6624,"Reseau Geodesique Francais Guyane 1995"
6626,"Reunion 1947",6627,"Reseau Geodesique de la Reunion 1992"
6627,"Reseau Geodesique de la Reunion 1992",6626,"Reunion 1947"
6629,"Tahaa 54",6687,"Reseau Geodesique de la Polynesie Francaise"
6630,"IGN72 Nuku Hiva",6687,"Reseau Geodesique de la Polynesie Francaise"
6633,"IGN56 Lifou",6749,"Reseau Geodesique de Nouvelle Caledonie 91-93"
6634,"IGN72 Grande Terre",6749,"Reseau Geodesique de Nouvelle Caledonie 91-93"
6641,"IGN53 Mare",6749,"Reseau Geodesique de Nouvelle Caledonie 91-93"
6642,"ST84 Ile des Pins",6749,"Reseau Geodesique de Nouvelle Caledonie 91-93"
6644,"NEA74 Noumea",6749,"Reseau Geodesique de Nouvelle Caledonie 91-93"
6647,"International Terrestrial Reference Frame 1988",6656,"International Terrestrial Reference Frame 2000"
6648,"International Terrestrial Reference Frame 1989",6656,"International Terrestrial Reference Frame 2000"
6649,"International Terrestrial Reference Frame 1990",6656,"International Terrestrial Reference Frame 2000"
6650,"International Terrestrial Reference Frame 1991",6656,"International Terrestrial Reference Frame 2000"
6651,"International Terrestrial Reference Frame 1992",6656,"International Terrestrial Reference Frame 2000"
6652,"International Terrestrial Reference Frame 1993",6656,"International Terrestrial Reference Frame 2000"
6653,"International Terrestrial Reference Frame 1994",6656,"International Terrestrial Reference Frame 2000"
6654,"International Terrestrial Reference Frame 1996",6656,"International Terrestrial Reference Frame 2000"
6655,"International Terrestrial Reference Frame 1997",6656,"International Terrestrial Reference Frame 2000"
6656,"International Terrestrial Reference Frame 2000",6896,"International Terrestrial Reference Frame 2005"
6663,"Porto Santo 1995",1041,"Autonomous Regions of Portugal 2008"
6664,"Azores Oriental Islands 1995",1041,"Autonomous Regions of Portugal 2008"
6665,"Azores Central Islands 1995",1041,"Autonomous Regions of Portugal 2008"
6666,"Lisbon 1890",6258,"European Terrestrial Reference System 1989"
6670,"Istituto Geografico Militaire 1995",6258,"European Terrestrial Reference System 1989"
6673,"Chatham Islands Datum 1979",6167,"New Zealand Geodetic Datum 2000"
6676,"Vientiane 1982",6678,"Lao National Datum 1997"
6677,"Lao 1993",6678,"Lao National Datum 1997"
6688,"Fatu Iva 72",6687,"Reseau Geodesique de la Polynesie Francaise"
6689,"IGN63 Hiva Oa",6687,"Reseau Geodesique de la Polynesie Francaise"
6690,"Tahiti 79",6687,"Reseau Geodesique de la Polynesie Francaise"
6691,"Moorea 87",6687,"Reseau Geodesique de la Polynesie Francaise"
6692,"Maupiti 83",6687,"Reseau Geodesique de la Polynesie Francaise"
6695,"Katanga 1955",1033,"Reseau Geodesique de la RDC 2005"
6705,"Mhast (offshore)",6324,"WGS 72 Transit Broadcast Ephemeris"
6706,"Egypt Gulf of Suez S-650 TL",6324,"WGS 72 Transit Broadcast Ephemeris"
6741,"Faroe Datum 1954",6230,"European Datum 1950"
6743,"Karbala 1979",1029,"Iraqi Geospatial Reference System"
6745,"Rauenberg Datum/83",6258,"European Terrestrial Reference System 1989"
6746,"Potsdam Datum/83",6258,"European Terrestrial Reference System 1989"
6747,"Greenland 1996",6747,"Greenland 1996"
6750,"ST87 Ouvea",6749,"Reseau Geodesique de Nouvelle Caledonie 91-93"
6751,"Kertau (RSO)",6245,"Kertau 1968"
6761,"Croatian Terrestrial Reference System",6258,"European Terrestrial Reference System 1989"
6765,"Slovenia Geodetic Datum 1996",6258,"European Terrestrial Reference System 1989"