Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
--- On Fri, 12/6/09, Liz wrote: > I think that often the amount was greater than 10 > metres. Now that you helped me to figure out relations I'm finally able to fix up the NSW/QLD border into 1 bondary instead of 2 wrong ones, the ABS boundary is in places 80-120m off the 29 degree south mark. The only cause of this error has to be import problems, because it doesn't match up with any other source of error I can think of. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
On Fri, 12 Jun 2009, Delta Foxtrot wrote: > --- On Fri, 12/6/09, Liz wrote: > > I'll ask the offspring. He is 7/8 of a surveyor > > with one semester to go! > > Can you also ask the bearing the drift is going, all I've been able to find > out is approx north or north north east, but haven't been able to find > anything on the net that clarifies it. adrian advises Making me drag out the Geodesy notes Ok, here we go. This may be incredibly boring, but it was boring in class. When GDA94 was defined one of the driving factors was GPS and the need because of it for an Earth centred datum that matched WGS84 closely. GDA94 takes the same origin as WGS84, the centre of mass of the Earth and the same semi-major ellipsiodal axis, but a tiny difference in the semi-minor to match the geoid over Australia better. This results in the original position difference between the 2. This positional difference is fairly irrelevant to any but Geodetic surveyors, as most people do not have equipment to measure that accurately and the general Cadastral surveyors use predetermined marks to reference their surveys to. For those who are not following at the moment, an ellipsiod is a lot like a circle, but has a varying radius. It is defined by an origin, the largest radius (semi-major axis) and the smallest radius (semi-minor). All Earth models assume it is an ellpsiod of revolution, around the semi-minor axis to create a flattened sphere. It is a purely mathematical surface. The geoid is a surface defined by being perpendicular to the line of gravity at every point. It also is a surface where gravity has the same force at any point on the surface. If you level something with a spirit level, it is level with respect to the geoid. Back to the datums, I don't have any current misalignment data between GDA94 and WGS84. GDA94 is fixed to the position of reference stations on the Australian continent as of 1st January 1994. At that point the difference between the 2 was regarded as irrelevant at the DGPS level, so it should have been within 0.3m. I have a document from late 2002 saying that the difference was negligible, the continental drift was approx 7cm per year across the continent, and at that point the 2 datums had diverged by a further half-metre. Assuming the same continental drift, it has diverged by another half-metre. So, currently, GDA94 and WGS84 are within 1.3m of each other. Who would know this? In Australia, the Intergovernmental Council on Surveying and Mapping (ICSM) is supposed to deal with datums. Someone in there should be keeping track on the shifts. The shift at a given point can be calculated quite accurately with a survey grade receiver and a few hours worth of recordings. On to continental drift. While I mentioned above that it is approx 7cm per year, I can do better then that. The International Earth Rotation Service is responsible for keeping track of various receivers that measure the differences between their locations by several different methods, each to at least centimetre position. The data from these stations is freely available, but a pain to find. By finding the released position data used as the basis for the establishment of consecutive International Terrestrial Reference Framework (ITRF) datums the continental drift can be calculated for any participating station. From the horrible to navigate SINEX files provided by the IERS, in 1997 the continent was doing about 6cms/y at Canberra and Hobart, 7cms/y at Pearth, and 13cms/y at Darwin. In 2000, the values are almost exactly the same. As for what direction it is going, the IERS releases files in 3D X,Y,Z Earth centred co-ordinates. These are a pain to work with, and I can't tell you from them what the direction is in a nice easy co-ordinate system. I always thought that it was in a North to North Westerly direction, an azimuth of around 330 degrees. References: IERS: http://www.iers.org/MainDisp.csl?pid=163-253 ICSM: http://www.icsm.gov.au/ Going Geocentric, Version 1, October 2002. Queensland Government Department of Natural Resources and Mines for the ICSM. (I should see if I can put this cd and it's information somewhere that you can all read it. It is quite a good overveiw of GDA94, and also Datums in general.) Geodesy Notes, Geoff Sandford, 2008 and 2009. -- BOFH excuse #288: Hard drive sleeping. Let it wake up on it's own... ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
On Sat, 13 Jun 2009, Franc Carter wrote: > Yes, back in my early osm days I yahoo'd Saint Clair in Western Sydney, > only to fin much > later when I went out with a gps that the alignment was really bad ;-( Can't locate the place right now - it was somewhere between Penrith and Campbelltown. -- Your step will soil many countries. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
Yes, back in my early osm days I yahoo'd Saint Clair in Western Sydney, only to fin much later when I went out with a gps that the alignment was really bad ;-( On Fri, Jun 12, 2009 at 9:43 PM, Liz wrote: > On Fri, 12 Jun 2009, Delta Foxtrot wrote: > > > I've come to the realisation that every road (and > > > everything that was placed in reference to the roads) in the > > > suburb where I live is out by about 8-10m. Much of > > > Brisbane's road system seems to have been traced off the > > > Yahoo imagery, apparently without aligning it to fixed > > > points before hand. > > > > 8-10m isn't that much of an error to be honest. There really isn't any > such > > thing as perfect data if you are using consumer grade equipment of any > > sort, things will improve over the next 10 to 20 years with other > > constellations of GPS like sats starting to broadcast, but until then 10m > > is as good as it gets for the most part for most things unless you spend > a > > lot of time getting very very accurate positioning. > > > There are definitely suburbs in SW Sydney where we have seen the same - the > photos have not been aligned with any known points. > There are tricks in JOSM for moving these areas en masse - can't confirm > for any other editor. > > > I think that often the amount was greater than 10 metres. > But we aren't making a perfect plan of the area - we are making a map, and > to me it is an aid to navigation, so the exact placement is not as important > as the relative positions of the objects. > > > But we can improve the accuracy over time as DF notes. > > > > > ___ > Talk-au mailing list > Talk-au@openstreetmap.org > http://lists.openstreetmap.org/listinfo/talk-au > > -- Franc ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
On Fri, 12 Jun 2009, Delta Foxtrot wrote: > --- On Fri, 12/6/09, Liz wrote: > > I'll ask the offspring. He is 7/8 of a surveyor > > with one semester to go! > > Can you also ask the bearing the drift is going, all I've been able to find > out is approx north or north north east, but haven't been able to find > anything on the net that clarifies it. I've forwarded the emails He won't resist a chance to answer but I'm not sure when we will hear from him. -- He that is giddy thinks the world turns round. -- William Shakespeare, "The Taming of the Shrew" ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
--- On Fri, 12/6/09, Liz wrote: > But we can improve the accuracy over time as DF notes. Well according to wikipedia, there are 31 US NAVSTAR sats broadcasting, the chinese are planning to put up 35, the europeans are planning 27 and the russians plan to have their system back up to 30 by 2011. All going well to their respective planning there will be in excess of 120 sats broadcasting on several diff diff frequencies which will eliminate virtually all positioning errors relative to WGS84. This means we should see accuracy into the sub-metre range or sub-centremetre range for consumer grade hand held devices. Surveryors who currently spend large chunks of cash getting that sort of accuracy now, will be into the sub-mm range. While the maps might look strange until they are fixed I still think taking advantage of more accurate information where possible is beneficial even if it doesn't seem like it at the time with maps potentially looking weird, otherwise it just shifts the work to someone else later on that is annoyed by the incorrect placement of POIs or roads to fix up. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
On Fri, 12 Jun 2009, Delta Foxtrot wrote: > > I've come to the realisation that every road (and > > everything that was placed in reference to the roads) in the > > suburb where I live is out by about 8-10m. Much of > > Brisbane's road system seems to have been traced off the > > Yahoo imagery, apparently without aligning it to fixed > > points before hand. > > 8-10m isn't that much of an error to be honest. There really isn't any such > thing as perfect data if you are using consumer grade equipment of any > sort, things will improve over the next 10 to 20 years with other > constellations of GPS like sats starting to broadcast, but until then 10m > is as good as it gets for the most part for most things unless you spend a > lot of time getting very very accurate positioning. There are definitely suburbs in SW Sydney where we have seen the same - the photos have not been aligned with any known points. There are tricks in JOSM for moving these areas en masse - can't confirm for any other editor. I think that often the amount was greater than 10 metres. But we aren't making a perfect plan of the area - we are making a map, and to me it is an aid to navigation, so the exact placement is not as important as the relative positions of the objects. But we can improve the accuracy over time as DF notes. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Mass realignment of roads? (was Mapping things by importance)
--- On Fri, 12/6/09, James Livingston wrote: > Unless you have a known point of reference, clearly visible > in the imagery and placed via something accurate (i.e. gps), > they can be out a fair bit. You can also get into datum issues, OSM uses WGS84, which differs from DGA94 by about 1m at present (my best guess, still trying to find a surveyor to confirm), however as time goes on this increases by about 7cm per year due to the Australian continental drift. However that isn't the biggest source of error, most consumer grade GPSr units are only qualified accurate to about 10m, although this can be minimised by doing an average of location data over a 24hr period, although this is fairly time consuming obviously. Until we have dual frequency receivers to eliminate sources of error with atmospheric conditions 10m accuracy should be acceptable for most purposes, unless you are a surveyor of course. > I've come to the realisation that every road (and > everything that was placed in reference to the roads) in the > suburb where I live is out by about 8-10m. Much of > Brisbane's road system seems to have been traced off the > Yahoo imagery, apparently without aligning it to fixed > points before hand. 8-10m isn't that much of an error to be honest. There really isn't any such thing as perfect data if you are using consumer grade equipment of any sort, things will improve over the next 10 to 20 years with other constellations of GPS like sats starting to broadcast, but until then 10m is as good as it gets for the most part for most things unless you spend a lot of time getting very very accurate positioning. > When I've been going around recently, I started noticing > that POIs on the north-east corner of an intersection looked > like they were on the SW corner. Obviously the best solution > is to go out and capture all the roads with my GPS, but I'm > wondering what to do in the mean time. That can happen at diff zoom levels because of the x,y offset position that they are plotted from also. > If I leave the roads where they are, the data I'm putting > in will look wrong as it's not consistent with where the > roads have been placed. Moving just the intersection will > make the road appear to bend in places it doesn't, and would > be a fairly arbitrary thing. I could go around and move > every road in the suburb the same distance, but that'd be a > fair bit of work, and I'd probably screw something up. IMHO if you have a better source of information it should be used, and over time errors should be corrected by others till things do look right, since seeing mistakes motivates people to fix them and in some cases to go on and do other things too. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
