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

Andy Robinson (blackadder-lists) ajrlists at googlemail.com
Sun Oct 18 21:15:36 BST 2009


Tim,

What's the scan file size? Will happily help look at calibration as I'm
doing it all the time for the 1:25k so set up to do it easily. If I know the
file size I can try a similar size on my box here.

How many calibration points are you needing per sheet to do a reasonable
job?

Cheers

Andy 

>-----Original Message-----
>From: talk-gb-bounces at openstreetmap.org [mailto:talk-gb-
>bounces at openstreetmap.org] On Behalf Of TimSC
>Sent: 18 October 2009 4:11 PM
>To: talk-gb at openstreetmap.org
>Subject: [Talk-GB] OS 7th series, accuracy update,request for calibration
>help
>
>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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