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Guest

Posted: Fri May 13, 2005 5:29 pm Post subject: Calibrating OSGB Map 


I am currently evaluating a demo version of Pathaway for PalmOS. It looks like an excellent product, but I am having some difficulty converting OSGB maps using the Map Manager tool.
I have seen the instructions in the FAQ about using the Projection
calibration and I have tried to follow the instructions, but with limited
success.
One possible area of confusion relates to the input formats for the
calibration points. For instance, for an OS grid reference of SO 982 840, I
enter "SO" for grid, "840" for Northing and "982" for Easting. Is this
correct?
After applying these inputs, I note that the format changes to "0000840" and "0000982". What format should I use? Are there any other tips I should be aware of? 

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Guest

Posted: Fri May 13, 2005 5:33 pm Post subject: Calibrating OSGB Map 


The grids you mentioned:
"OS grid reference of SO 982 840, I enter "SO" for grid, "840" for Northing
and "982" for Easting"
It looks to me that the grids you mention are in 100 metre units, and
PathAway is expecting Metres.
So you should enter "SO" for grid, "84000" for Northing and "98200" for
Easting"
Your grid numbers depend on the actual resolution of the original map. See this OSGB Grid description I found at http://www.bangor.ac.uk/is/iss025/osgbfaq.htm below:
Quote:  A full reference which is unique across Britain will consist of two letters
followed by (usually) six digits giving resolution to 100 metres eg (SS 599
939). Eight digits are needed for 10 metre resolution and 10 digits for one
metre. The two letters indicate the 100Km by 100Km square in which the
point lies and are often omitted since this is usually unambiguous.
The first letter gives the 500Km by 500Km square. 25 different letters
(omitting I) could be given here though only four actually map to the
covered land areas: S for south west, T for south east, N for the north and
Scotland and H for the Shetland Islands. The second letter gives the 100Km by 100Km square within that and is laid out with A in the North West corner,
Z in the South East thus:
ABCDE
FGHJK
LMNOP
QRSTU
VWXYZ
The digits should be split into two groups  in the above example of SS 599 939 the point is 59.9 Km east and 93.9 Km north of the south west corner of square SS. 


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Guest

Posted: Fri May 13, 2005 5:34 pm Post subject: Calibrating OSGB Map 


Thanks for your help. This does solve my problem. 

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bobulos Guest

Posted: Tue May 31, 2005 8:14 pm Post subject: Calibrating OSGB Maps 


Hi,
I am having trouble calibrating OSGB maps, could someone just confirm what I doing is correct
Calibration Type  Projection
Map Datum  Ord Srvy GB
Coord Grid  OSGB
Projection Parameters
lon_0 W2.0000000
Lat_0 N49.0000000
x_o 400000
y_0 100000
I then choose two calibration points (top left and bottom right)
TL
SK 563 062 entered as 56300 06200
BR
SK 603 023 entered as 60300 02300
I would be very grateful if anyone could help me at all.
Cheers
Mark 

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bobulos Guest

Posted: Thu Jun 23, 2005 3:52 pm Post subject: Calibrating OSGB Maps 


Hi,
I still cannot get the maps to calibrate correctly. I have just come back from the Yorkshire Dales and used Pathaway a few times with some maps I calibrated.
I used the projection as below, as wel as using top left , bottom right calibration.
Quote:  Calibration Type  Projection
Map Datum  Ord Srvy GB
Coord Grid  OSGB
Projection Parameters
lon_0 W2.0000000
Lat_0 N49.0000000
x_o 400000
y_0 100000 
<br><br>
I calibrated on points that were exactly in the cross of an Ord Surv grid
and as far apart as possible on the map so they should have been correct. However when it came to using it live the position it showed was still a few hundred feet out, any ideas on what I am doing wrong ? 

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lacoleman
Joined: 19 Mar 2006 Posts: 2

Posted: Wed Mar 22, 2006 7:19 am Post subject: Re: Calibrating OSGB Maps 


bobulos wrote:  Hi,
I am having trouble calibrating OSGB maps, could someone just confirm what I doing is correct
Calibration Type  Projection
Map Datum  Ord Srvy GB
Coord Grid  OSGB
Projection Parameters
lon_0 W2.0000000
Lat_0 N49.0000000
x_o 400000
y_0 100000
I then choose two calibration points (top left and bottom right)
TL
SK 563 062 entered as 56300 06200
BR
SK 603 023 entered as 60300 02300
I would be very grateful if anyone could help me at all.
Cheers
Mark 
This is a great question for anyone calibrating the OSGB maps. An example like this is sorely needed in the manual where OSGB calibration is discussed. The discussion there is fine for anyone who already knows how to do it, but not  definitely not  for anyone trying to figure it out for the first time.
If the parameters indicated here are correct, someone needs to say so. If they are not, someone should correct them for the benefit of all of us interested in OSGB calibration. 

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TheSnake
Joined: 27 Mar 2006 Posts: 44 Location: S.England

Posted: Mon Mar 27, 2006 2:57 am Post subject: 


I have found that entering 4 calibration points on a 6km x 6km map works fine. I locate them on grid intersections using unique Eastings.
The Map Calibrator doesn't really help, for two reasons...
1. Northings are prompted for before Eastings (?!)  this confuses me no end!
2. The Projection/Datum/Lat_0 etc. calibration values do not seem to be saved after calibrating a map, therefore you have to start from scratch with a new OSGB map, possibly introducing errors.
Paul 

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TheSnake
Joined: 27 Mar 2006 Posts: 44 Location: S.England

Posted: Thu Mar 30, 2006 3:29 pm Post subject: 


There also seems to be a bug in Map Manager using OSGB where the Remove option doesn't work properly, its liable to just set one of the Grid Refs to a completely wild value, which isn't immediately apparant.
My tip with Map Manager is to check and recheck each and every point before saving the calibrated map, then reload it and check again. 

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