[Talk-GB] OS 7th Series offset

mapping at sheerman-chase.org.uk mapping at sheerman-chase.org.uk
Wed Oct 7 09:45:32 BST 2009

Hi all,

I am doing most of the image processing for the 7th series scans. I  
want to stress again that it is a work in progress :)

The rectification is done in two steps:

1) Polynomial-based image warp from the source image into a square  
grid. This should get rid of any rotation of the map due to the  
scanning process and any deformation of the paper. This requires  
control points to make marked on the image to calculate the  

2) Transform from GBOS grid to slippy map tile projection  
(EPSG:900913). This is a port of  
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/ProjectionandTransformationCalculations.xls into C++. This is very much a non-linear transformation. Any ideas on a more open  

I have not found the source of The far right sheet (75) has about 20  
calibration points and the accuracy is good. The others only have 9  
calibration points and the accuracy seems worse. I suspect that the  
inaccuracy is in step one.

I don't normalise the colour yet :)

Further investigation needed...


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