> 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+2006&cd=1&hl=en&ct=clnk&ie=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 1:250000 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
