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Lat/long to OS co-ordinates problems


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The short answer is "don't bother" - all cachers use WGS84 lat/long, so don't worry too much about the accuracy of the converted OS coordinates on the cache page.

 

The longer answer would mean delving deeper - can you post a example? It's possible you've maybe got your GPS wrongly configured - ensure you use the OSGB36 datum when taking OS coordinates, and the WGS84 datum for lat/long coordinates. There are also a variety of methods of converting between the two coordinate systems, though I believe most product reasonably accurate results....

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Another tip when recording the coords of your placed cache is to take several readings (I usually take about 6) by walking to the cache from different directions (from about 20/30 feet away). Then average out the readings. It will improve the accuracy.

 

Chris

It is also recommended to do this over several days, so different GPS constellation patterns can give a better accuracy also. But, I may just take too much time over placing caches...

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Another tip when recording the coords of your placed cache is to take several readings (I usually take about 6) by walking to the cache from different directions (from about 20/30 feet away). Then average out the readings. It will improve the accuracy.

 

Chris

It is also recommended to do this over several days, so different GPS constellation patterns can give a better accuracy also. But, I may just take too much time over placing caches...

 

Well in that case, you'd also better wait the 20 mins or so it takes for the GPS to download the almanac from the EGNOS satellites (assuming your GPS can 'see' them (or it)). Then, with accuracy down to 6 feet start walking!

Chris

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Hey,

 

Whilst taking more than one reading certainly improves accuracy, it is also likely that the conversion algorithm (maths to convert from one datum to another) is slightly different on your unit than that of the website software, was the difference in values small? It only needs one number to be rounded and your conversion results can be skewed. I know that my Garmin produces slightly diff OSGB36(02) coords to that of Geocaching.com....not done any tests to see which is more accurate.

Check out www.gps.gov.uk for more info and high precision conversion tools.

 

T

Edited by 6tangos
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Hey,

 

Whilst taking more than one reading certainly improves accuracy, it is also likely that the conversion algorithm (maths to convert from one datum to another) is slightly different on your unit than that of the website software, was the difference in values small? It only needs one number to be rounded and your conversion results can be skewed. I know that my Garmin produces slightly diff OSGB36(02) coords to that of Geocaching.com....not done any tests to see which is more accurate.

Check out www.gps.gov.uk for more info and high precision conversion tools.

 

T

Different algorithms will produce different results, since OSGB is based upon a transverse mercator projection - there will always be distortion. The OS conversion site may be accurate, but it certainly isn't simple to use (raw eastings and northings is rather cumbersome.

 

Being a curious person, I thought I'd test the conversion discrepancies.

 

Using WGS84 coords N 51 24.206' W 0 7.647' (top of a Trig pillar in South London, coords taken with Google Maps)

 

My eTrex: TQ 30347 68825

Geocaching.com: TQ 30347 68825

OS (OSTN02) convertor: 530346.801,168819.340 which is TQ 30346.801 68819.340

TrigpointingUK convertor: TQ 30348 68819

Another: TQ 30347 68819

 

I'll let you make your own conclusions.

Edited by Edgemaster
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Wow! I like how this topic has started off with a fairly nice and easy help type question and its gone into a really big Tefal expert answer! [:unsure:] Don't get me wrong its all quite valid. And it helps reminds us what a great job they've done with GPS and routing etc the fact it is a consumer item but we forget about all the clever maths and technology that tick away behind the scenes.

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The short answer is "don't bother" - all cachers use WGS84 lat/long,

 

Perhaps we are the exception . :unsure:

Being "plot it on the map fans "when seeking caches we always use OS coordinates to lead us to the general vicinity of the cache ,or stage ,then convert to the WGS84 when we arrive to seek the container .

 

But never checked on the accuracy of the conversions ,never been a problem .

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The short answer is "don't bother" - all cachers use WGS84 lat/long,

 

Perhaps we are the exception . :)

Being "plot it on the map fans "when seeking caches we always use OS coordinates to lead us to the general vicinity of the cache ,or stage ,then convert to the WGS84 when we arrive to seek the container .

 

But never checked on the accuracy of the conversions ,never been a problem .

 

Actually your not :unsure: . I've had to deal with quit a few cases were the cache owner has placed the cache using OS GB, but when they check the cache page the OS GB coordinates are well out [and I'm not talking the known small difference the site produces] the majority of the time they have not correctly changed the settings on their GPSr, when obtaining the Lat and long coordinates for the cache.

 

Personally I'd hazard a guess that there is a large No of cachers who use OS GB and not WGS 84. But it's not obvious as they never post to the forums.

 

it's like the use of feet and metres, personally I prefer metres, but my Reviewer replies are usually in feet as that's the measurements I most see.

 

And a after thought, isn't the on screen display for both OS GB and WGS 84 a conversion, the GPSr actually uses something different for the actual location No crunching?

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It's like the use of feet and metres, personally I prefer metres, but my Reviewer replies are usually in feet as that's the measurements I most see.

That's what's nice about OS Grid References, they are in meters, so you can get a rough idea of distances between places. (and exactly, using the relatively simple pythagorean theorem) With Lat/Long, its muuch harder maths.

 

And a after thought, isn't the on screen display for both OS GB and WGS 84 a conversion, the GPSr actually uses something different for the actual location No crunching?

I'd assume so, continual re-conversion would lead to some error. It probably uses WGS84 decimal degrees internally, and just converts to other positioning formats for display.

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I like the way that this topic has opened up!!!

 

Hey,

 

Whilst taking more than one reading certainly improves accuracy, it is also likely that the conversion algorithm (maths to convert from one datum to another) is slightly different on your unit than that of the website software, was the difference in values small? It only needs one number to be rounded and your conversion results can be skewed. I know that my Garmin produces slightly diff OSGB36(02) coords to that of Geocaching.com....not done any tests to see which is more accurate.

Check out www.gps.gov.uk for more info and high precision conversion tools.

 

T

Different algorithms will produce different results, since OSGB is based upon a transverse mercator projection - there will always be distortion. The OS conversion site may be accurate, but it certainly isn't simple to use (raw eastings and northings is rather cumbersome.

 

Being a curious person, I thought I'd test the conversion discrepancies.

 

Using WGS84 coords N 51 24.206' W 0 7.647' (top of a Trig pillar in South London, coords taken with Google Maps)

 

My eTrex: TQ 30347 68825

Geocaching.com: TQ 30347 68825

OS (OSTN02) convertor: 530346.801,168819.340 which is TQ 30346.801 68819.340

TrigpointingUK convertor: TQ 30348 68819

Another: TQ 30347 68819

 

I'll let you make your own conclusions.

 

Cool test!

the OS conversion is certainly more precise - I would definitely recommend it for accurate work as I've seen the algorithms behind it at uni and they're good!

but I doubt a geocacher would ever need that level of detail!

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It's like the use of feet and metres, personally I prefer metres, but my Reviewer replies are usually in feet as that's the measurements I most see.

That's what's nice about OS Grid References, they are in meters, so you can get a rough idea of distances between places. (and exactly, using the relatively simple pythagorean theorem) With Lat/Long, its muuch harder maths.

 

And a after thought, isn't the on screen display for both OS GB and WGS 84 a conversion, the GPSr actually uses something different for the actual location No crunching?

I'd assume so, continual re-conversion would lead to some error. It probably uses WGS84 decimal degrees internally, and just converts to other positioning formats for display.

 

I agree - GPS recievers always use WGS84. anything else you see on screen is a transformation, unless you have a reciever that supports Glonass etc...

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I would also recommend the excellent coordinate transformation tools at nearby.org.uk, but unfortunately, the site seems to be down.

 

I didn't test out the OS spreadsheet, or C&M's waypoint workbench. I may just go and test those now...

 

WGS84 Lat/Long: N 51 24.206' W 0 7.647'

OSGB Conversions:

My eTrex: TQ 30347 68825

Geocaching.com: TQ 30347 68825

OS (OSTN02) convertor: 530346.801,168819.340 which is TQ 30346.801 68819.340

TrigpointingUK convertor: TQ 30348 68819

Another: TQ 30347 68819

OS Spreadsheet: 530348.621,168819.360 = TQ 30349 68819

Waypoint Workbench: TQ 30235 68874

 

This is the most disappointing result of the lot. I'm not sure how widely used Waypoint Workbench is any more, but it's accuracy seems to be comparatively poor.

Edited by Edgemaster
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The reason why retail level GPSrs and some apps such as GC.com's get OS co-ords so wrong is that most of them are using the oversimplified 3-parameter datum shift. By its very nature, that method cannot be accurate across the whole country. In central Scotland, for example, it's about 7 metres adrift. It's more than that elsewhere.

 

If you want accurate conversions, such as to plot single metre precision co-ords on large-scale mapping such as www.magic.gov.uk, then you really need to use the full polynomial datum shift thingy, such as the one on the OS website. If you want to have that ability on your own GPSr for use in the field in your own general area, then you need to make your own three parameters (dX;dY;dZ) and select the user-definable datum shift option on the menu of your GPSr.

 

Use a freebie package such as the NGA's GeoTrans to do the number crunching to find out what your local datum shift should be.

 

Take some WGS84 co-ords which are reasonably central to your caching area. Anywhere within 10 or 20 miles is quite adequate. Look up the 3D Cartesian co-ords for that location on WGS84, then on Airy/OSGB36, subtract one from t'other and voila, you have your own bespoke datum shift parameters for injection into the user definable option on your GPSr. That will give you metre-accuracy conversions on your own GPSr within a fair distance of the centre of your geocaching area.

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3-parameter datum shift.

the full polynomial datum shift

you need to make your own three parameters (dX;dY;dZ)

user-definable datum shift

your local datum shift

Look up the 3D Cartesian co-ords

Airy/OSGB36

metre-accuracy conversions

Alternatively, enter the numbers given on the cache page, follow the arrow, have a hunt around the general area and find a tupperware box <_<:laughing:

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I like OS GB, how are you supposed to plot Lat/Long grids on an OS map?

Not easy. :D

 

I also like OS for the same reason. OS grid is marked on the map, including numbers, once you have the map folded, you can still see the grid and numbers... Long/Lat is only printed on the very top and edge of the map -which requires unfolding the map, not easy in the wind and rain!

 

G

 

(GPSr is set to lat/long though!)

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I like OS GB, how are you supposed to plot Lat/Long grids on an OS map?

Not easy. :D

That's why you use conversion...

 

WGS84 lat/long is an international system, as is geocaching is an international sport.

 

The lat/long printed on OS maps is in theOSGB36 datum, not WGS84 - you'll still need conversion.

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The short answer is "don't bother" - all cachers use WGS84 lat/long, so don't worry too much about the accuracy of the converted OS coordinates on the cache page.

 

The longer answer would mean delving deeper - can you post a example? It's possible you've maybe got your GPS wrongly configured - ensure you use the OSGB36 datum when taking OS coordinates, and the WGS84 datum for lat/long coordinates. There are also a variety of methods of converting between the two coordinate systems, though I believe most product reasonably accurate results....

All cachers do not use lat /lon. I am an experienced fell walker and confirmed "plot it on the map" type person and I go geocaching to get in the fresh air and discover new places, not to get into trouble by slavishly following an arrow in the wilder parts of the country, even in my own fairly safe county of essex. We go caching with the GPS set to OSGB36, a map and a compass, and the knowledge that there are many more like us.

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All cachers do not use lat /lon. I am an experienced fell walker and confirmed "plot it on the map" type person and I go geocaching to get in the fresh air and discover new places, not to get into trouble by slavishly following an arrow in the wilder parts of the country, even in my own fairly safe county of essex. We go caching with the GPS set to OSGB36, a map and a compass, and the knowledge that there are many more like us.

It is merely as the GPS displays it. The GPS thinks in WGS84 - the OP notes that they take two sets of coords - one in WGS84 and the other as OSGB36 - in fact both will be stored as WGS84, and both are likely to be different due to normal GPS error.

Commonly, when placing it will not matter what system you take the coordinates in, as long as you average out multiple sets of coordinates and then use a reliable conversion to WGS84 lat/long.

The difference between the conversion algorithms is negligible for geocaching - since it is much less than the quoted accuracy of a GPS.

 

Personally, I also use my GPS in OSGB36 with a paper OS map - the arrow will still point as if it were WGS84.

 

It is always sensible to know where you are going - don't rely on the GPS alone - straight lines are not always the best...

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The difference between the conversion algorithms is negligible for geocaching - since it is much less than the quoted accuracy of a GPS.

Not so.

 

A modern GPSR with WAAS enabled, is good to ±3m or better. With just a couple of minutes averaging this can easily be brought down to well below the 2.1m resolution of the GC.com DDMM.mmm format.

 

The 3parameter datum shift used by systems such as Garmin and Magellan GPSrs induces an error of 7 metres or more.

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The difference between the conversion algorithms is negligible for geocaching - since it is much less than the quoted accuracy of a GPS.

Not so.

 

A modern GPSR with WAAS enabled, is good to ±3m or better. With just a couple of minutes averaging this can easily be brought down to well below the 2.1m resolution of the GC.com DDMM.mmm format.

 

The 3parameter datum shift used by systems such as Garmin and Magellan GPSrs induces an error of 7 metres or more.

 

So that's why I ended up on the wrong side of a stream yesterday!! B)

I too use OS maps and co-ordinates to plot my destination, just to avoid the odd lake, river, gorge, type of thing, even though some of my maps actually have Lat and Long in DD MM' down the side! :)

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The difference between the conversion algorithms is negligible for geocaching - since it is much less than the quoted accuracy of a GPS.

Not so.

 

A modern GPSR with WAAS enabled, is good to ±3m or better. With just a couple of minutes averaging this can easily be brought down to well below the 2.1m resolution of the GC.com DDMM.mmm format.

 

The 3parameter datum shift used by systems such as Garmin and Magellan GPSrs induces an error of 7 metres or more.

 

I use OS grid references too, because I use OS maps, and I mainly bag summits and trigs, which are usually displayed in that form. Whenever I download a cache gpx. file to my Garmin gps I try to make a point of checking the co-ords, as they are normally out a metre or two in both directions. But I'm not sure it makes any real difference, as the variations from one day/gps/cacher to the next probably outway this in practice.

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