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

TimSC mapping at sheerman-chase.org.uk
Mon Oct 19 18:31:28 BST 2009

Thanks for the offers of help. The task is a very "on again, off again" 
affair so expect quiet times and busy times! (Depending on availability 
of new scans.) The steps are as follows:

1) Load map into gimp or similar. The images are 17000 by 21000 pixels 
which can be a strain on computer memory.

2) Resize to 50% of orginal. This seems sufficient and makes later 
stages faster to process.

3) Save as jpg

4) Note 9 calibration points that are appoximately evenly placed in a 
3x3 grid. The map corners are used as calibration points. Each point has 
a map reference and pixel position.

5) Save that information in a text file as specified here:


6) To check the calibration, the calibration file can be used with the 
warp-gbos tool to create a KML file. The warp-gbos tool needs to be 
compiled from source. Details at the above link.

7) This can be compared to google-earth as a sanity check.

8) Send me the calibration file :)

I will send people who have expressed an interest a link to download a 
map sheet to play with.



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