Re: [talk-au] Australia "changing coordinates"
On 8/2/2016 3:37 PM, Andrew Davidson wrote: WGS84 is a datum fixed; as in dated 1984 and the data does not change. I'm not sure that you've understood exactly what a semi-dynamic datum is. Have a read of this: http://ceur-ws.org/Vol-1142/paper6.pdf particularly sections 3.4 and 3.5. I'm reading http://earth-info.nga.mil/GandG/publications/tr8350.2/wgs84fin.pdf The document is dated in 1996 so more recent things are not in it. But I think the principles would hold. It lists; WGS 84 1 Jan 87 - 1 Jan 94 WGS 84 (G730) 2 Jan 94 - 28 Sept 96 WGS 84 (G873) From 29 Sept 96 You will observe the refinements of WGS 84 (to reduce error/uncertainty). I would expect that suffix would be used in professional circles. The (G) simply gives a refinement to WGS84, does not change the basis, and the refinements are getting smaller. This document also introduces the concept of 'uncertainties', a good move. Page 16 of 175 states in part "As of 2 Jan 94, a set of geodetic coordinates shall include a designation of the reference frame and epoch of the observations" Note that this is for any datum. I expect this date requirement will become more common and a requirement with earth centric datums. "In summary, the refinements which have been made to WGS 84 have reduced the uncertainty in the coordinates of the reference frame, the uncertainty of the gravitational model and the uncertainty of the geoid undulations.They have not changed WGS 84. " http://ceur-ws.org/Vol-1142/paper6.pdf In section 3.5 the error is introduced in making the first transform from GDA94 to WGS84. Not clear if this transform was made at all, or if made was not to the correct date. The error could have been avoided by the correct transform in the first datum conversion? Or you have access to survey points/marks and their data in the local area. LOL. Reminds me of this quote: "You'll never find a survey mark on the ground with coordinates referenced to WGS-84. If you do, it's wrong." A global datum will have more compromises than a local datum. For that reason alone I would expect survey marks to be in the local datum.. e.g. NSW survey marks are GDA94 using MGA http://spatialservices.finance.nsw.gov.au/surveying/scims_online I also expect these will be translated to the new earth centric datum as a mathematical operation some time in 2017. I would expect that any local datum can be transformed into WGS84 with a sufficient level of accuracy for the task, if required (i.e. if the end equipment requires WGS84, or its refinements) ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Australia "changing coordinates"
I found this explanation helpful http://www.geoproject.com.au/gda.faq.html#q16 On Tue, 2 Aug 2016, at 01:57 PM, Andrew Davidson wrote: > What to do about it? Unfortunately the changeover to GDA2020 is not > going to happen overnight, the plan is for a three year transition > starting in 2017. If it did happen overnight then we could just shift > all of the nodes in Australia to their new positions can keep on > mapping. What's actually going to happen is that the various different > organizations are going to switch at different times and the situation > for aerial imagery mosaics will be interesting if they aren't > retrospectively re-projected (you'll have some places on GDA2020 and > others still on GD94). So I'm guessing the approach of wait and see > might be the one to take. Obviously if organizations start releasing > data in GDA2020 and people start importing it into OSM we're going to > have to make the call: move everything to GDA2020 or convert imports > back to GDA94? It's only going to be a real issue once we get access to > aerial imagery that's in GDA2020 or people get general access to GPS > with decimetre accuracy. If you have suggestions on better ways to handle this within OSM, please add them as a suggestion for the 0.7 API at https://wiki.openstreetmap.org/wiki/API_v0.7 Whether this means including a date together with the coordinates (for correcting WGS84 to account for drift), or supporting GDA2020 coordinates directly. ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Australia "changing coordinates"
WGS84 is a datum fixed; as in dated 1984 and the data does not change. I'm not sure that you've understood exactly what a semi-dynamic datum is. Have a read of this: http://ceur-ws.org/Vol-1142/paper6.pdf particularly sections 3.4 and 3.5. Or you have access to survey points/marks and their data in the local area. LOL. Reminds me of this quote: "You'll never find a survey mark on the ground with coordinates referenced to WGS-84. If you do, it's wrong." ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Australia "changing coordinates"
On 8/2/2016 1:57 PM, Andrew Davidson wrote: It's interesting because it highlights one of the foundation myths of OSM; which is that it uses the "WGS84" co-ordinate system. This is a convenient myth and if you're talking about only mapping to the nearest 5m then it is in effect true. However, once you start talking about sub-metre accuracy it stops being true for a number of reasons: WGS84 is a datum fixed; as in dated 1984 and the data does not change. 1. Unless you are cleared by the US DoD you don't have access to this level of accuracy. Or you have access to survey points/marks and their data in the local area. 2. If the accuracy of your GPS is supplemented by external data then this is inevitably based on one of the realisations of the ITRS; which is not "WGS84". No. The survey points can be in one datum and coordinate system and those can be translated to any other datum and/or coordinate system. Datums are definitions .. they do not rely on nor based on something else. 3. WGS84 is a semi-dynamic datum. Every 1st of January the USAF uploads new co-ordinates for each of their ground control/monitoring stations (where they will be at 1st July). Which means that every location in WGS84 co-ordinates needs an epoch to indicate when the measurement was taken but the database schema for OpenStreetMap doesn't have an entry for this. No .. WGS84 if fixed, it does not change. https://en.wikipedia.org/wiki/World_Geodetic_System And that in part is incorrect ... read http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html Documents EGM 96 ... as in 1996. _"__Third Edition, 4 July 1997 has been amended to correct errata found in the original printing of this edition. There are no changes to the definition of WGS 84 and this does not constitute a "new" WGS 84." _If WGS84 were to be 'updated' then I would expect a different number e.g. WGS94 for a 1994 model. Otherwise it is not possible to distinguish between the two datums! So what happens in OSM is that each country gets mapped to their local approximation of WGS84. In North America this is NAD83, Europe has the ETRS89 and the various national variations. The UK must be the most confusing as the OS seems to be maintaining two system OSGB36 and OS Net but a least they are far enough apart to be obvious. WGS84 is not approximated to any local datum. Rather a local datum is modelled for the best fit of the local surface. A global model must make more compromises/approximations so will always be worse than a local model (assuming the same data, effort and level of complexity). Translation of data from one datum to another has approximations, but the datums themselves have no approximation, they may be derived (contrived) from approximations but they define the thing. If 'the' inch is defined as the length of the Quenn Of Britians thumb .. then that is what it is .. no approximation. (I would buy before the thumb nail is trimmed, and sell after it is trimmed.) In Australia it means that we've been happily mapping to GDA94 and ignoring the fact that we're racing north at something like 70-90 mm a year. People mapping with consumer grade GPS are only within 5m anyway so don't care. People with access to survey grade GPS understand the difference and convert back to GDA94. Depends on what you set your GPS datum too ... mine is set to WGS84. The GPS knows what datum the map is, and adjusts for the required datum. With more or less accuracy depending on the model and the complexity implemented inside the GPS. As my GPS is obsolete by at least 2 generations I don't see it having any capacity for the new earth centric datums. What difference will it make to OSM? It depends. If you're mapping some where that has good quality aerial imagery and people have carefully traced things then it's going to be obvious (https://goo.gl/sWWfYm). On the other hand most of OSM in Australia can be described as roughly sketched and if all you have is Bing imagery and some crudely traced streets then I doubt you'll be able to tell (https://goo.gl/B7WVfz). As the imagery is set for WGS84 it should not be a problem'. Problems may arise if (when) open sourced data is in some 'other' datum and imported without adjustment to whatever datum OSM uses. What to do about it? Unfortunately the changeover to GDA2020 is not going to happen overnight, the plan is for a three year transition starting in 2017. If it did happen overnight then we could just shift all of the nodes in Australia to their new positions can keep on mapping. What's actually going to happen is that the various different organizations are going to switch at different timr84 o WGes and the situation for aerial imagery mosaics will be interesting if they aren't retrospectively re-projected (you'll have some places on GDA2020 and others still on GD94). So I'm guessing the approach of wait and see might be the one to take. Obviously
Re: [talk-au] Australia "changing coordinates"
It's interesting because it highlights one of the foundation myths of OSM; which is that it uses the "WGS84" co-ordinate system. This is a convenient myth and if you're talking about only mapping to the nearest 5m then it is in effect true. However, once you start talking about sub-metre accuracy it stops being true for a number of reasons: 1. Unless you are cleared by the US DoD you don't have access to this level of accuracy. 2. If the accuracy of your GPS is supplemented by external data then this is inevitably based on one of the realisations of the ITRS; which is not "WGS84". 3. WGS84 is a semi-dynamic datum. Every 1st of January the USAF uploads new co-ordinates for each of their ground control/monitoring stations (where they will be at 1st July). Which means that every location in WGS84 co-ordinates needs an epoch to indicate when the measurement was taken but the database schema for OpenStreetMap doesn't have an entry for this. So what happens in OSM is that each country gets mapped to their local approximation of WGS84. In North America this is NAD83, Europe has the ETRS89 and the various national variations. The UK must be the most confusing as the OS seems to be maintaining two system OSGB36 and OS Net but a least they are far enough apart to be obvious. In Australia it means that we've been happily mapping to GDA94 and ignoring the fact that we're racing north at something like 70-90 mm a year. People mapping with consumer grade GPS are only within 5m anyway so don't care. People with access to survey grade GPS understand the difference and convert back to GDA94. What difference will it make to OSM? It depends. If you're mapping some where that has good quality aerial imagery and people have carefully traced things then it's going to be obvious (https://goo.gl/sWWfYm). On the other hand most of OSM in Australia can be described as roughly sketched and if all you have is Bing imagery and some crudely traced streets then I doubt you'll be able to tell (https://goo.gl/B7WVfz). What to do about it? Unfortunately the changeover to GDA2020 is not going to happen overnight, the plan is for a three year transition starting in 2017. If it did happen overnight then we could just shift all of the nodes in Australia to their new positions can keep on mapping. What's actually going to happen is that the various different organizations are going to switch at different times and the situation for aerial imagery mosaics will be interesting if they aren't retrospectively re-projected (you'll have some places on GDA2020 and others still on GD94). So I'm guessing the approach of wait and see might be the one to take. Obviously if organizations start releasing data in GDA2020 and people start importing it into OSM we're going to have to make the call: move everything to GDA2020 or convert imports back to GDA94? It's only going to be a real issue once we get access to aerial imagery that's in GDA2020 or people get general access to GPS with decimetre accuracy. Whatever we do it'll be a good test to see what should happen in OSM when NAD83 gets replaced by 2022 and everything in the USA moves 1~1.5m (http://www.geodesy.noaa.gov/datums/newdatums/index.shtml). On 29/7/16 22:40, Andy Mabbett wrote: See you when you reach England ;-) http://www.bbc.co.uk/news/technology-36912700 But seriously: what impact might this have, on OSM? ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Australia "changing coordinates"
On 7/30/2016 1:57 PM, Andrew Harvey wrote: On Sat, 30 Jul 2016, at 08:40 AM, Warin wrote: The change, distance wise, is upto 1.5 metres, well within commercial GPS uncertainties. The change is to the datum. How this will work out with future global datums we will have to wait and see. In another 30 years there will probably be another change of around 1.5 metres again. As receivers become more accurate and folks start use other ground based positioning technology that gives millimetre accuracy, then that data in OSM will over time become more out of sync as the tectonic plate moves while the coordinate system remains fixed. When that happens it would be good to see a future OSM API that lets us upload data in a coordinate system like GDA so that we don't need to be constantly updating coordinates to reflect the move in the techtonic plate. Sigh. Simply changing from AGD66 to GDA94 can produce a change of around 100 metres in location of a coordinate. So a change in datum can correct the drift without changing the coordinates. From the above you can see that OZ moves at upto about 50 mm per year. At present there is no datum system that 'drifts' over time, that would give a system that would need little alteration and fewer of them. People who require the accuracy take into account the drift (usually professional surveyors with very expensive receivers, antennas etc.). ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Australia "changing coordinates"
On Sat, 30 Jul 2016, at 08:40 AM, Warin wrote: > The change, distance wise, is upto 1.5 metres, well within commercial > GPS uncertainties. > > The change is to the datum. How this will work out with future global > datums we will have to wait and see. > > In another 30 years there will probably be another change of around 1.5 > metres again. As receivers become more accurate and folks start use other ground based positioning technology that gives millimetre accuracy, then that data in OSM will over time become more out of sync as the tectonic plate moves while the coordinate system remains fixed. When that happens it would be good to see a future OSM API that lets us upload data in a coordinate system like GDA so that we don't need to be constantly updating coordinates to reflect the move in the techtonic plate. ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Australia "changing coordinates"
On 7/29/2016 10:40 PM, Andy Mabbett wrote: See you when you reach England ;-) http://www.bbc.co.uk/news/technology-36912700 But seriously: what impact might this have, on OSM? Very little! The change, distance wise, is upto 1.5 metres, well within commercial GPS uncertainties. The change is to the datum. How this will work out with future global datums we will have to wait and see. In another 30 years there will probably be another change of around 1.5 metres again. ___ Talk-au mailing list Talk-au@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-au
