OSGB 36 to WGS 84 on chart 883 IOS

cagey

cagey

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Evening,
Just bought Adm Chart 883 Isles of Scilly printed 1986 in Metres will bring it up to date with West Country Portfolio and NTM's, just noticed Datum is OSGB 36 , does anyone know what the offset is and if so how or where did you find this. Chartplotter is WGS 84 and dont want to mess with datums.
Only reason for chart is lovely huge scale, perfect for planning and getting feel of area.
Thanks
Keith
 
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NormanS said:
Usually it tells you on the chart somewhere, what offsets to apply.
Click to expand...
I don't expect it will on a chart that old. GPS only became fully functional in 1994.

I would to reset the Datum on the GPS when in the area, after putting a big sticker on it to remind me to reset it,
 
Ordnance Survey Great Britain 1936, based on the Airy 1830 ellipsoid. This is the land datum used for OS Maps.

Try a Google search for OSGB 1936. You may find something specific, but offsets to WGS84 (which was only published 2 years before your chart was printed!!) vary across the UK, and could be of the order of 70 metres and more. Also, bear in mind that plotter datum conversion programmes are not always accurate, (it depends on the algorithm used and location) so allow a greater margin of error if you choose to change datum on your machine.

Alternatively, just forget about GPS and use your nice chart the way it was intended: navigate by eye and natural transits along with depth. I cruised the Scilly Isles that way long before GPS was invented and it was much more interesting than playing with a plotter. Chart in the cockpit and a length of fine twine for picking out transits and showing the position line.
 
OSGB 1936 is a survey referenced datum, so there isn't a constant offset between it and an ellipsoidal datum like WGS84. The difference will vary across the chart; the offsets suggested in another post will probably work - but bear in mind they will be average values for the area of the chart, and may well be slightly off at different places on the chart. I'd guess that there will be a slightly different offset from island to island - it is likely each island has it's own survey network, loosely connect by tie-lines between islands.

Unfortunately, the offset between OSGB 1936 and WGS84 can also vary abruptly; there is a distinct "kink" in the contours of equal offset as you cross a line from approximately the Severn to the Tyne! This is because in 1936, the UK survey network was computed (using hand cranked calculators) in two chunks, and then "stitched" across this line.

Finally, there are a few places where they got the sums wrong and the old survey network does crazy things!

You should also remember that the Isles of Scilly are the end of the world as far as the UK National Grid is concerned, and offsets will be maximized here. The old chart also has no strong way of linking the Scilly Isles to the mainland (pre GPS, all I can think is Tellurometer and (perhaps) Transit - but Transit was probably not in use for long enough to affect a chart), so in addition to datum adjustments, the chart may have intrinsic offsets of 10s to 100s of metres.
 
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NormanS said:
Usually it tells you on the chart somewhere, what offsets to apply.
Click to expand...

Unfortunatelly the ofsett printed is to WGS 72. This is just a learning exercise a bit of fun, when I'm there it'll be Adm portfolio charts and plotter and old fashioned plotting, but looking at a large scale chart of the area is lovely after years of plotters and Adm leisure portfolios.
Thanks
Keith
 
AntarcticPilot said:
OSGB 1936 is a survey referenced datum, so there isn't a constant offset between it and an ellipsoidal datum like WGS84. The difference will vary across the chart; the offsets suggested in another post will probably work - but bear in mind they will be average values for the area of the chart, and may well be slightly off at different places on the chart. I'd guess that there will be a slightly different offset from island to island - it is likely each island has it's own survey network, loosely connect by tie-lines between islands.

Unfortunately, the offset between OSGB 1936 and WGS84 can also vary abruptly; there is a distinct "kink" in the contours of equal offset as you cross a line from approximately the Severn to the Tyne! This is because in 1936, the UK survey network was computed (using hand cranked calculators) in two chunks, and then "stitched" across this line.

Finally, there are a few places where they got the sums wrong and the old survey network does crazy things!

You should also remember that the Isles of Scilly are the end of the world as far as the UK National Grid is concerned, and offsets will be maximized here. The old chart also has no strong way of linking the Scilly Isles to the mainland (pre GPS, all I can think is Tellurometer and (perhaps) Transit - but Transit was probably not in use for long enough to affect a chart), so in addition to datum adjustments, the chart may have intrinsic offsets of 10s to 100s of metres.
Click to expand...

Thanks for replies,
Got diverted etc.
Did a Herr Google etc but answers here are as usual more interesting.
Its nice to do a time warp between techs, my thinking is when I get there I will stick a WP on a central posn with CPlotter on WGS 84 and then see how it disagrees with up to date charts and this old chart, as said earlier its just a bit of fun but a big sticker on chart , plotter and forehead is a brilliant idea.
Liked the bit of string idea, shows every day is a learning day, Bretton always slid now learnt a neat idea.
Thanks
Keith
 
taifun said:
The information is available on the latest edition of 833.
Remember to reverse the signs.
View attachment 19332
Click to expand...

Thanks for that, on my 1986 chart it shows: To agree with Satellite derived positions normally referred to WGS 72 Datum, such posns should be moved 0.03 min southwards and 0.06 mins eastwards to agree with this chart.
No big change, but didn't understand reversing thingy, not being smart arse just sunday pinot blurring.
Thanks
Keith
 
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cagey said:
Unfortunatelly the ofsett printed is to WGS 72. This is just a learning exercise a bit of fun, when I'm there it'll be Adm portfolio charts and plotter and old fashioned plotting, but looking at a large scale chart of the area is lovely after years of plotters and Adm leisure portfolios.
Thanks
Keith
Click to expand...

WGS72 is insignificantly different from WGS84. Offsets between the two are in metres or less.
 
cagey said:
Evening,
Just bought Adm Chart 883 Isles of Scilly printed 1986 in Metres will bring it up to date with West Country Portfolio and NTM's, just noticed Datum is OSGB 36 , does anyone know what the offset is and if so how or where did you find this. Chartplotter is WGS 84 and dont want to mess with datums.
Only reason for chart is lovely huge scale, perfect for planning and getting feel of area.
Thanks
Keith
Click to expand...

You could always download this freeware map coordinate calculator. It converts between numerous datums, including OSGB 1936 and of course WGS84
 
little_roundtop said:
You could always download this freeware map coordinate calculator. It converts between numerous datums, including OSGB 1936 and of course WGS84
Click to expand...

Unfortunately there are two incarnations of OSGB 1936. The older one is survey related, and is basically irregular and not related to a regular ellipsoidal datum; this is certainly the datum of the chart the OP is interested in judging by the dates. This datum cannot be accurately represented in coordinate calculators.

There is a later version of OSGB36 which is an ellipsoidal datum; sadly the OS re-used the same name! It is this one (which is NOT the same as the older one) that will be in the coordinate calculator. It is basically a "best fit" of an ellipsoidal datum to the old survey datum.
 
AntarcticPilot said:
Unfortunately there are two incarnations of OSGB 1936. The older one is survey related, and is basically irregular and not related to a regular ellipsoidal datum; this is certainly the datum of the chart the OP is interested in judging by the dates. This datum cannot be accurately represented in coordinate calculators.

There is a later version of OSGB36 which is an ellipsoidal datum; sadly the OS re-used the same name! It is this one (which is NOT the same as the older one) that will be in the coordinate calculator. It is basically a "best fit" of an ellipsoidal datum to the old survey datum.
Click to expand...

Ah. Thanks! :)
 
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