Hi all, I have updated the 7th series layer with improved accuracy. It seems as if the Helmert approximation of the OS grid reference to WGS84 was not accurate enough; I was getting transform errors of approximately 20m. Also, to my surprise the next biggest error seems to be surveying errors. For example, see around the Mull of Kintyre where the survey errors are around 40m (near NR730100).
http://www.openstreetmap.org/?lat=55.3339&lon=-5.5737&zoom=14&layers=B000FTF The best way of determining the error in rectification is to get the lat lon of a grid square corner. stick that into the OSTN02 transformation (http://gps.ordnancesurvey.co.uk/convert.asp). The true corner position can be found by rounding the resultant grid coordinates to the nearest thousand. (Assuming the error is less than 500m!) Rectification errors seem to be in the order of 0 to 20m depending on the area. If any areas have errors that exceed this, let me know and I will investigate it. The next steps are to rectify the other sheets we have scanned. http://wiki.openstreetmap.org/wiki/7th_Series Is there anyone who can help us with sheet calibration? Having a linux box with a decent amount of RAM is a requirement. If yes, get in touch with me and Steve Chilton. Tim _______________________________________________ Talk-GB mailing list Talk-GB@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-gb
