Re: [talk-au] Why not to change coastlines automatically to ABS data.
On 05/10/2009, at 3:59 PM, Ross Scanlon wrote: So PLEASE look at the sat photos and already entered data before you go removing the coastline and using the ABS data automatically as the coastline. As a +1 comment, I'd also like to note that in many places the ABS follow the sand-grass/tree/dirt line, rather than the high-tide line that natural=coastline is supposed to represent. If there is a place where the ABS data is a consistently a bit inland of the PGS data for a stretch, check if it's a beach. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Why not to change coastlines automatically to ABS data.
I would be cautious about preferencing survey and satellite/aerial photography data over ABS. I have found errors in both of these. Survey data from GPS seems at times to have been either traced pooly from gpx tracks or based on innacurate position data, especially where there are tall objects like buildings and hills nearby. Similarly, imagery can be misleading when there is vegetation, like mangroves on the shore, not to mention to low resolution of the yahoo imagery itself. 2009/10/5 Ross Scanlon i...@4x4falcon.com Just noticed this: http://www.openstreetmap.org/?lat=-20.34647lon=148.95263zoom=16layers=B000FTF If you then go to edit and zoom in you find: Two restaurants that are now in the ocean. The airport road now in the ocean, this is a surveyed road and runs along the foreshore. The dam next to the airport overlapping the ocean. The marina disappeared totally. It's the area with the three ferry tracks going into it. So PLEASE look at the sat photos and already entered data before you go removing the coastline and using the ABS data automatically as the coastline. -- Cheers Ross ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Why not to change coastlines automatically to ABS data.
I would be cautious about preferencing survey and satellite/aerial photography data over ABS. I have found errors in both of these. Survey data from GPS seems at times to have been either traced pooly from gpx tracks or based on innacurate position data, especially where there are tall objects like buildings and hills nearby. Similarly, imagery can be misleading when there is vegetation, like mangroves on the shore, not to mention to low resolution of the yahoo imagery itself. You've missed the point here. What I'm saying is don't just go and change it from PGS coastline to the ABS boundary data without looking what's there. In the example given (Hamilton Island), at the points given, the ABS boundary data was grossly in error, more than 200m near the restaurants and approximately 500m near the airport. The ABS data more than likely came from aerial photos anyway as there's never been anyone actually survey (professional surveyor style) the coastlines in this area. I think everyone should have a read of this: http://74.125.155.132/u/AustralianBureauOfStatistics?q=cache:ijmG6hPI8egJ:www.abs.gov.au/Websitedbs/D3110122.NSF/4a255eef008309e44a255eef00061e57/8e860540d4a7505cca256bf300055f0d/%24FILE/technical%2520paper.pdf+%22digital+boundary%22+accuracy+2006cd=1hl=enct=clnkie=UTF-8 It is the html version of a pdf file from the abs website, as the pdf is corrupt and won't load (at least on windows). It's from 2001 but I could not find an equivalent document for 2006. The main area of interest is Appendix B and the section on topographic features, as below in part: A typical use of digital basemap in GIS is to select features which lie within, intersect, or are adjacent to other features. In most GIS these spatial relationships are determined by the latitude and longitude of the objects being analysed. If an object is close to a boundary then the absolute accuracy of the latitude and longitude becomes important. The PSMA dataset is digitised from maps at scales of from 1:4,000 to 1:250,000 and the accuracy of a latitude or longitude can therefore vary from 4 metres to 250 metres. Cartographic licence and data integration issues can all further erode the positional accuracy of basemap features. So there can be very significant discrepancies in the ABS data in regards to topographic features. Given that the only topo maps for this area are 1:25 then the errors can be in excess of 200m in the ABS data. Cheers Ross ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Why not to change coastlines automatically to ABS data.
Yup, Just because data is available, it doesn't mean it has to be used. This data can be used in conjunction with other data sources. The end result is to make the best map possable, regardless of the source. We arnt employees of ABS, nor TIGER, nor NRCan. So at least IMO, we take it as a bonus. This is why im promoting the 'oneway import' concept. (of the 3 step process) Hope that makes sence, cheers, Sam ps, sorry if im a couple steps ahead, your comments are de-ja-vue for me :) On 10/6/09, Ross Scanlon i...@4x4falcon.com wrote: I would be cautious about preferencing survey and satellite/aerial photography data over ABS. I have found errors in both of these. Survey data from GPS seems at times to have been either traced pooly from gpx tracks or based on innacurate position data, especially where there are tall objects like buildings and hills nearby. Similarly, imagery can be misleading when there is vegetation, like mangroves on the shore, not to mention to low resolution of the yahoo imagery itself. You've missed the point here. What I'm saying is don't just go and change it from PGS coastline to the ABS boundary data without looking what's there. In the example given (Hamilton Island), at the points given, the ABS boundary data was grossly in error, more than 200m near the restaurants and approximately 500m near the airport. The ABS data more than likely came from aerial photos anyway as there's never been anyone actually survey (professional surveyor style) the coastlines in this area. I think everyone should have a read of this: http://74.125.155.132/u/AustralianBureauOfStatistics?q=cache:ijmG6hPI8egJ:www.abs.gov.au/Websitedbs/D3110122.NSF/4a255eef008309e44a255eef00061e57/8e860540d4a7505cca256bf300055f0d/%24FILE/technical%2520paper.pdf+%22digital+boundary%22+accuracy+2006cd=1hl=enct=clnkie=UTF-8 It is the html version of a pdf file from the abs website, as the pdf is corrupt and won't load (at least on windows). It's from 2001 but I could not find an equivalent document for 2006. The main area of interest is Appendix B and the section on topographic features, as below in part: A typical use of digital basemap in GIS is to select features which lie within, intersect, or are adjacent to other features. In most GIS these spatial relationships are determined by the latitude and longitude of the objects being analysed. If an object is close to a boundary then the absolute accuracy of the latitude and longitude becomes important. The PSMA dataset is digitised from maps at scales of from 1:4,000 to 1:250,000 and the accuracy of a latitude or longitude can therefore vary from 4 metres to 250 metres. Cartographic licence and data integration issues can all further erode the positional accuracy of basemap features. So there can be very significant discrepancies in the ABS data in regards to topographic features. Given that the only topo maps for this area are 1:25 then the errors can be in excess of 200m in the ABS data. Cheers Ross ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au -- Twitter: @Acrosscanada Blog: http://Acrosscanadatrails.blogspot.com Facebook: http://www.facebook.com/sam.vekemans ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Why not to change coastlines automatically to ABS data.
On Mon, 5 Oct 2009 17:05:54 +1100 Liz ed...@billiau.net wrote: On Mon, 5 Oct 2009, Ross Scanlon wrote: So PLEASE look at the sat photos and already entered data before you go removing the coastline and using the ABS data automatically as the coastline. another paragraph in the wiki?? Done -- Cheers Ross ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
Re: [talk-au] Why not to change coastlines automatically to ABS data.
2009/10/5 Ross Scanlon i...@4x4falcon.com: Just noticed this: http://www.openstreetmap.org/?lat=-20.34647lon=148.95263zoom=16layers=B000FTF If you then go to edit and zoom in you find: Two restaurants that are now in the ocean. The airport road now in the ocean, this is a surveyed road and runs along the foreshore. The dam next to the airport overlapping the ocean. The marina disappeared totally. It's the area with the three ferry tracks going into it. So PLEASE look at the sat photos and already entered data before you go removing the coastline and using the ABS data automatically as the coastline. That would be my mistake, I thought I had moved the ABS boundary to be in line with the coast line I didn't quite succeed it would seem, also in Qld the property boundary data is useful here too. ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
[talk-au] Why not to change coastlines automatically to ABS data.
Just noticed this: http://www.openstreetmap.org/?lat=-20.34647lon=148.95263zoom=16layers=B000FTF If you then go to edit and zoom in you find: Two restaurants that are now in the ocean. The airport road now in the ocean, this is a surveyed road and runs along the foreshore. The dam next to the airport overlapping the ocean. The marina disappeared totally. It's the area with the three ferry tracks going into it. So PLEASE look at the sat photos and already entered data before you go removing the coastline and using the ABS data automatically as the coastline. -- Cheers Ross ___ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
