[Talk-GB] OS 7th series, accuracy update, request for calibration help
mapping at sheerman-chase.org.uk
Sun Oct 18 16:11:08 BST 2009
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).
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.
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.
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