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Fuck… OSGB 36, I’ve never heard of that one. I assume the 36 means it was made up in 1936?
With ArcGIS, the one GIS program I’m very familiar with you’d just do a Datum conversion from WGS84 to OSGB36 assuming OSGB36 was loaded into the software as a possible datum choice and everything will work out. I’m relatively sure you can do this in ArcGIS software, not sure if you can with ArcGIS online. I don’t have an extra site license to Arc although at least one of the several free programs should be able to do the conversion.
@Shakyamuni 985140 wrote:
Fuck… OSGB 36, I’ve never heard of that one. I assume the 36 means it was made up in 1936?
it was indeed started in 1936 as a military project although was only completed in 1962; and is uniquely British in many respects (I think other smaller countries such as NL and DK use or have used similar grids but they are all less well known outside the country).
Quite early on in the project the boffins realised that it made much more sense to start working in metric rather than use the Imperial unit standards which had become inaccurate and were not widely recognised internationally (post WW II the French were also helping out with the survey) – but attempts by OS, NPL and others to suggest full metrication in the 1960s and 1980s came up against popular resistance, so you have such anomalies as 1970s era OS Maps being published with grids that had worked with decimal fractions of an inch and modern street signs being shown in decimal fractions of a mile, with both distances being conversions of the metric equivalents anyway.
As an example, cycling home from work I see a set of signs with “warning, low bridge 500 yards ahead” with the height warning in metres then feet and inches below
Although even if a driver of a large vehicle (who might be from anywhere in Europe) knows how long 500 yards (and the height of their vehicle is usually marked on a sign inside the cab) is the position of the advance warning and levels of traffic means if they have ended up there it is is not a sufficient distance for them to do anything sensible other than stop the vehicle just before they strike the bridge and call the Emergency Services – who then have to stop the entire road from both ends for everyones safety whilst the vehicle is moved away.
I Had a look at ArcGIS and noticed that British Waterways (or whatever they are called nowadays) do use it but I got the impression that licenses are not cheap, a personal one wouldn’t be compliant for my purposes as many projects/experiments of mine are testing systems that ultimately get used in some form at my work and I would be able to use a work card to pay for the OS subscription (which is way cheaper than it once was; and even allows printing of whole maps.
@General Lighting 985138 wrote:
modern GPS systems use the WGS84 datum – unfortunately for those of us in Britain, the Pentagon/NATO decided to shift the meridian from the reference point that Ordnance Survey still use for the grid system. Apparently this also happens with other countries national mapping systems but I am only familiar with the OS grid having been taught to use this in high school long before kids could afford such things as GPS receivers…
Which explains the 100m position error but unless there is some way of this GIS software translating the lat/long from WGS84 to OSGB36 (I haven’t found it yet) means that I can’t use the grid references (which are far easier to work with if you are only dealing with waypoints a few tens of kilometres apart) directly from the GIS app (which is somewhat annoying; at least a subscription to the full OS mapping service is affordable these days)
BTW these are the links I found a while back (one is to convert the data)
Convert co-ordinates between WGS84 and OSGB36
And full 43 page PDF from OS about how co-ordinate systems work and why there are different ones in use in Britain – I still haven’t had the time to read and understand it to the point of being able to correctly configure the GIS app yet…
https://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf
Sorry, I haven’t read any of the links, not tbh all of your post yuet, but wtf made it seem like a good idea to move the meridian? Magnetic variation is real and has to be accounted for when using a compass and for any system of orientation really, but if the meridian has been in Greenwish for hundreds of years, and the internationa;l time zones are adjusted according to their proximity to the meridian, why the hell this was done…..
Out of interest could either of you look at a map, take a bearing, add the magnetic variation, which shifts by around half a degree per year, or was over 20 years ago?
When GPS came into existence was that when they moved the meridian and was it purely for GPS? I would be surprised if any other system in use ever used a fixed point the same as this.
WGS84 predates civillian use of GPS / GLONASS (the Soviet system; some receivers can use both) by about 15 years, as its name suggests it was devised around 1984 – but there have been several revisions of the datum and another one is planned! To compound the confusion there are some others and various attempts to internationalise the datum; all of which can skew GIS data by some metres unless accounted for or everything is definitely referenced to the same datum.
I would assume all this is being done for some sensible reason such as accounting for magnetic variation although it does raise the important point that blindly trusting any GPS receiver or associated software isn’t best practice – how do you know the data its outputting is actually correct?
larger OSGB paper maps as well as marine charts show the variations between Magnetic North and OS Grid North. I think these variations only become significant (and safety critical) though if you are going to sea or climbing mountains in poor visibility – for other mapping purposes on land or smaller scale areas you are limited in where you can go by the presence or otherwise of roads, legal property boundaries etc and these all provide plentiful landmarks.
For my purposes the radio transmitter antennas are at known fixed locations – what I am plotting is whether I can receive the signals at certain distances when I am out on my bike – I would have stopped somewhere safe and discreet to trigger off the pager transmitter (which is linked to a computer remotely controlled from my smartphone) as otherwise its just as illegal as doing so in a car – also if you do this in the wrong area folk these days are also likely to call 999 claiming I am some kind of terrorist / drug dealer / casing properties for burglaries. However there are also plenty of places outside the town centres where its not uncommon to see cyclists and people just think you are using a mapping or fitness tracker app…
So far I’ve found the smartphone apps to be quite accurate with regard to displaying OS grid references, no worse than the Garmin GPS receivers I used to use (which unfortunately seem to develop various defects after a few years of operational use)
Of course cross checking data is still a good idea – until I can get my head round this GIS stuff it is likely that I will just get the OS maps subscription; print off some maps on the big Japanese colour printers at my work and provided these print out at the correct scale (which I can check with a ruler), fix these to my wall and stick map pins at the waypoints – once again a combination of old and new tech may be the best….
yet more links here (the first one is easier to follow for those who are not cartographic/GIS experts)
https://www.ordnancesurvey.co.uk/resources/maps-and-geographic-resources/finding-north.html
https://confluence.qps.nl/pages/viewpage.action?pageId=29855173
I doubt strongly it is done to account for magnetic variation. Those satellites acocunt for relatavistic distortions in the clocks the carry on bard which is a far more complex, and therefore, hardware demanding than adding a relatively constant variable like magnetic variation.
@tryptameanie 985185 wrote:
Out of interest could either of you look at a map, take a bearing, add the magnetic variation, which shifts by around half a degree per year, or was over 20 years ago?
When GPS came into existence was that when they moved the meridian and was it purely for GPS? I would be surprised if any other system in use ever used a fixed point the same as this.
I think I could do that if pressed into service. Not sure they moved the Meridian, I think it was because Greenwich isn’t in the right spot.
@General Lighting 985186 wrote:
WGS84 predates civillian use of GPS / GLONASS (the Soviet system; some receivers can use both) by about 15 years, as its name suggests it was devised around 1984 – but there have been several revisions of the datum and another one is planned! To compound the confusion there are some others and various attempts to internationalise the datum; all of which can skew GIS data by some metres unless accounted for or everything is definitely referenced to the same datum.
I would assume all this is being done for some sensible reason such as accounting for magnetic variation although it does raise the important point that blindly trusting any GPS receiver or associated software isn’t best practice – how do you know the data its outputting is actually correct?
larger OSGB paper maps as well as marine charts show the variations between Magnetic North and OS Grid North. I think these variations only become significant (and safety critical) though if you are going to sea or climbing mountains in poor visibility – for other mapping purposes on land or smaller scale areas you are limited in where you can go by the presence or otherwise of roads, legal property boundaries etc and these all provide plentiful landmarks.
For my purposes the radio transmitter antennas are at known fixed locations – what I am plotting is whether I can receive the signals at certain distances when I am out on my bike – I would have stopped somewhere safe and discreet to trigger off the pager transmitter (which is linked to a computer remotely controlled from my smartphone) as otherwise its just as illegal as doing so in a car – also if you do this in the wrong area folk these days are also likely to call 999 claiming I am some kind of terrorist / drug dealer / casing properties for burglaries. However there are also plenty of places outside the town centres where its not uncommon to see cyclists and people just think you are using a mapping or fitness tracker app…
So far I’ve found the smartphone apps to be quite accurate with regard to displaying OS grid references, no worse than the Garmin GPS receivers I used to use (which unfortunately seem to develop various defects after a few years of operational use)
Of course cross checking data is still a good idea – until I can get my head round this GIS stuff it is likely that I will just get the OS maps subscription; print off some maps on the big Japanese colour printers at my work and provided these print out at the correct scale (which I can check with a ruler), fix these to my wall and stick map pins at the waypoints – once again a combination of old and new tech may be the best….
yet more links here (the first one is easier to follow for those who are not cartographic/GIS experts)
https://www.ordnancesurvey.co.uk/resources/maps-and-geographic-resources/finding-north.html
https://confluence.qps.nl/pages/viewpage.action?pageId=29855173
Couple of things, I’ve tried to use GPS to match the receiver to a sundial outside a planetarium that is at a point that is listed as precisely as possible and the GPS receiver is always off by a bit even if you’re standing exactly on the spot, the GPS stuff isn’t perfect but is pretty damn good.
As far as what you’re trying to do with GPS it seems pretty difficult without some knowledge of R the programming language or a good bit of practice with the software, seems easy to fuck up TBH even moreso with the freeware because it is less likely to have a guidebook or someone posting on a forum asking for help trying to do the same thing you’re trying to do. I’d definitely check it with the maps since you have access to them even if you were a GIS wizard.
@Shakyamuni 985188 wrote:
I think I could do that if pressed into service. Not sure they moved the Meridian, I think it was because Greenwich isn’t in the right spot.
WTF you mean Greenwich wasn’t in the right place???? Right place for what, and what was in the right place just over 100m away?
@tryptameanie 985190 wrote:
WTF you mean Greenwich wasn’t in the right place???? Right place for what, and what was in the right place just over 100m away?
Like when they calculated Greenwich as exactly opposite the International Date Line they were off by 100m.
@Shakyamuni 985191 wrote:
Like when they calculated Greenwich as exactly opposite the International Date Line they were off by 100m.
Exactly opposite is not 100m away.
Like I said they fucked up then they moved it, I would have just covered up my shoddy work and let it be.
To be fair, Uncle Sam does appear to have based the recalculation on international scientific consensus by asking every country’s national observatory to report the locations of stars and calculating the average positions. I did wonder why the UK didn’t simply shift the marker plates in 1984; but not only would that have caused utter confusion when comparing historical OS maps the entire observatory building is divided down the middle by the original meridian with the main entrance door positioned around it; and the Earths crust is shifting so in another century the actual meridian could change further.
I suppose a 102,5m error in an entire century (given that the 1884 location was worked out without the benefit of modern computers) isn’t that bad..
Topic: The Prime Meridian at Greenwich | Explore Royal Museums Greenwich
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Forums › Life › Computers, Gadgets & Technology › Social Media › INT/US : American prof uses cat pics to demonstrate online privacy issues
