map reference conversion

General discussion about trigpoints, the TrigpointingUK website etc

Re: map reference conversion

Postby physicalthing69 » Wed Oct 23, 2019 12:17 pm

You're right in assuming that the OS coordinate converter is the only accurate one. All th consumer grade receivers and most of the online converters use a simplified algorithm, that typically has an accuracy of a few metres at best. The OS produce a (free) piece of stand-alone software called GridInquest, that does the same job as the online converter, they also produce a software extension to allow developers to build their own software using the OS conversion (also free). The OS Give various figures for the accuracy of their conversion, see the OS Net bit of their website for all the FAQs. There are distortions in the OS grid, as much as 20m, between one end of the country and the other, due the way it was measured, in the pre satellite days. There's a book called 'The history of the re-triangulation of great Britain' which was available as a (large) PDF download from the OS website. You won't get anything approaching 2cm accuracy from any GPS receiver, without collecting and processing the raw data, as opposed to just averaging the stand-alone positions that the receiver generates, I'm not that familiar with UBlox receivers, to know whether they all generate raw data, or what format they use to store it. You basically use either some post-processing software or an online processing service, to combine your data with that from another receiver at a precisely surveyed point. The OS provide the reference data (free) to download on their website. Unfortunately, there isn't a UK based online processing site, although the overseas ones will process UK data. You are also right that the best way to express precise locations is via Cartesian or Geodetic coordinates, the OS say that their grid is specifically for use on their maps and doesn't possess such a high degree of accuracy. You need to check whether your receiver stand-alone heights are above local sea level or the WGS ellipsoid, consumer grade receivers use local sea level, it's more use for navigation, professional receivers use the ellipsoid, as it's better for processing or using with things like the OS coordinate converter. Where I live, the ellipsoid height is 25m higher than the sea level height, so if you fed the wrong height into the converter, you could imagine it would make a huge difference to the resultant height. Hope this helps. If you need any more information,
physicalthing69
 
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Re: map reference conversion

Postby physicalthing69 » Wed Oct 23, 2019 12:28 pm

Please leave a supplementary question on the forum. Your main problem is likely to be post-processing software, most of it is commercial stuff and very expensive. although, there are a couple of free/open source projects out there. whatever method of processing you use, you'd need to be able to store the raw data from your receiver, and convert it to the appropriate format to process it. You typically need to leave your receiver over the point you're measuring for an hour or more, much longer if you want accurate height information, although it depends on your distance from your closest OS reference station. Reference station map is available on OS Net bit of OS website. The latest versions of android will collect raw satellite data from the phone/tablet GPS receiver, and their are a couple of free apps out there, to collect it and convert it to a format suitable for processing. Not all phones or tablets are able to collect quality data, because they switch their GPS receiver on and off, to save battery power, and there's no way for the user to override this function. There's quite a bit of information online about which phones and tablets are suitable to use. something to think about if/when you upgrade your phone. Hope all this helps.
physicalthing69
 
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Joined: Sun Oct 20, 2019 10:33 pm

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