[Talk-GB] OS 7th series, accuracy update, request for calibration help

TimSC mapping at sheerman-chase.org.uk
Sun Oct 18 16:11:08 BST 2009


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





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