Os To Lat/long Conversion

[+1100f]

I have a few OS196 grid refs I'd like to reasonably accuratly convert to WGS84 mm.mmm (our "normal" format)

 

But for some reason I'm getting the 100-200 feet or so close but not quite where I expected them to be syndrome.

 

This I achieved by changing the setting on my Garmin to OSGB, inputting them , then changing the setting back. I think I've done something wrong, as they are not "quite" where I expected them to be!

 

I tried a search on the web for convertors , and some calculations were close, some were miles out, so I'm not confident ther're right!

 

Eg I have SZ 405867, and on fugawi it shows up as being on a footpath, when It should be a pub on the road! (it comes out as N50 40.709 W001 25.690)

 

I have a map and can roughly there, but was hoping to create a route around this one and 6 other "waypoints" I have, all supplied to me in this format.

 

TIA

Edited March 9, 2005 by 1100f

[+leecee]

Do a search for Grid InQuest on Google. It does OSGB conversions with superb accuracy. It's also a free download once you've registered your name on the site.

[+68 GUNS]

You could try Co-ord Converter

[+-Phoenix-]

waypoint workbench

[+Simply B]

If you want to do it on a Pocket PC, my OS GPS Convertor utility converts either way and can then show the position on a map in TomTom Navigator or OziExplorer (from where it's easy to navigate to, create a waypoint, etc).

[+1100f]

I tried the co-ord convertor already , some worked, some were 60 miles out!

 

I've just tried the in quest, with similar failure!

 

For the one I think I know what it should be, I've been given SZ405867

 

My garmin converted it to N50 40.709 W1 25.690 (the closest so far!!!!)

I think It should be N50 40.7662 W1 25.8135

This is a diff of about 100m, and I have the in quest settings as follows:

Co-ord a : UK and ireland co-ord type, OSGB36 (OSTN02), Newlyn,

eastings 405000 northings 86700

 

This I convert on the other side:

Geodetic co-ord. etrs datum

 

and gives a value 50 40.796 N 01 55.835 W

Bloody miles out !!!

[+1100f]

Oh, and thanks for the super fast replies already!

[+Simply B]

[...]

For the one I think I know what it should be, I've been given SZ405867

 

My garmin converted it to N50 40.709 W1 25.690 (the closest so far!!!!)

I think It should be N50 40.7662 W1 25.8135

This is a diff of about 100m, and I have the in quest settings as follows:

Co-ord a : UK and ireland co-ord type, OSGB36 (OSTN02), Newlyn,

eastings 405000 northings 86700

[...]

Hi there

There's a mistake in your eastings figure - it should be 440500 for SZ 405 ...

Putting that in InQuest gives ETRS89 of:

N 50° 40.71282056'

W 001° 25.68880185'

and in my utility (which will be slightly less accurate) gives:

N 50° 40.7134'

W 001° 25.6894'

and the Streetmap website gives the same as mine - see its map here

All of these are very close to what your Garmin is telling you.

Whenever I get conversions that are about 100m out, my first suspicion is a datum issue, for example entering a lat and long in what I think is WGS84 datum but is actually OSGB36, or vice versa. On my second cache hunt I did that, and my Pocket PC was telling me "it's another 20m that way" as I stood on the edge of a sheer drop. I went home and did some studying that night!

[+1100f]

Thats almost exactly where my garmin put it, but i think it should be slightly west (nearest building when zoomed out a level on streetmap)

 

I'm beginning to wonder if this is where it is meant to be for those fiven co-ords , and os grid isn't giving me the accuracy I was expecting..............I was hoping for better than 100 m accuracy.

 

I think I'm just more confused that the calculators never agreed with my garmin "much".

 

I'm, not sure hoe the additional 4 works either

[+Simply B]

Hi again

 

"I was hoping for better than 100m accuracy"

A 6 digit OS grid ref like this one only defines a point to the nearest 100m - you need 8 digits for 10m, or 10 digits for 1m.

 

"I'm not sure how the additional 4 works either"

The two letters define a 100kmx100km square within the national grid. SZ is the square that starts 400km (=400000m) east and 0km north of the grid origin. The 405 and 867 add the next 3 digits of accuracy to those starting values, so 400000m becomes 440500m and 000000m becomes 086700m.

 

Taking your lat / long as input, my utility gives an OS grid ref, which should be good to within about 5m, of:

SZ 40353 86796

which is indeed by a building on the nearby road.

 

Hope this helps. The OS web site is a great source of information of this kind.

[+1100f]

Thanks for the replies, I had wondered how to get SZ in the co-ords, the garmin lets me put in SZ, in the calculatos I had to work out how many extra zeroes to put in, which made me think I was asking the impossible after all!

 

Thanks for the excellent info!

 

Phil

[+albertw]

I've been working on a script to do the GPS<-> Irish OS grid conversions. Currently you can test it at http://www.cademuir.net/geocaching.php

 

The source code is in php, so it would be suitable for simple web pages, and is available at http://sourceforge.net/projects/iegridconv/

 

It's still being worked on, so any feedback would be appreciated. It seems accurate to within 5 meters anywhere on the Island.

 

Hopefully this is of some use, I have not looked at the UK algorithm but I understand that it is very different to the Irish one.

 

Cheers,

~Al

[+Simply B]

Hi

My utility mentioned earlier does it for Irish Grid too, if you want to do it using a Pocket PC.

It uses the code from the OSGB website - I just had to change a few numbers, and add the different format grid square IDs, to get it running for both grids. It should be good to within 5m, and when I compared its results with a couple of samples on your web page, we agreed to within a couple of metres - so we're either both right or both wrong!

The Grid Inquest software freely downloadable via links from the OS web site also covers British and Irish grids, and does the conversions even more accurately.

Edited March 20, 2005 by BandMandAandA

[barryhunter]

My converter at http://www.deformedweb.co.uk/trigs/coord.cgi

can accept Irish Grid References, which it will convert to lat/long (amongst others)

 

Similarly entering a lat/long thats in Ireland will give you the Irish Grid Reference.

 

I've used http://search.cpan.org/~pkent/Geography-NationalGrid-1.6/

 

but comparing results with your converter, seems to be different, so I really must check thats mines working OK.

[+wildlifewriter]

Give me a few days, and I'll tell you which of those three gives the best and most consistent results for Ireland. (Statistically)

 

Having studied the code for two of 'em, I'd guess that it'll be pretty close.

 

(NB: Results will be e-mailed but NOT posted here without agreement from all those involved.)

 

-Wlw.

[+albertw]

Hi Barry,

 

I noticed the perl modules that you used, but I wanted to create something in php to give the option of using php rather than uploading perl modules to folks who wanted to use it on their websites.

 

It will be interesting to see what wildlifewriter finds with the tools, hopefully his conclusions will not force me to do a complete rewrite! :-)

 

Best Wishes,

~Al

[+wildlifewriter]

It will be interesting to see what wildlifewriter finds with the tools, hopefully his conclusions will not force me to do a complete rewrite! :-)

He's getting some interesting results, which show that some work is still needed on the two web-based tools.

 

I cannot test Barry D's utility after all because it only runs on PocketPC, which I don't use.

[+Simply B]

By all means send me a few of your test coordinates, and I'll send back the results I get using it. I'd be interested in the results, as I've not yet been in a position to test it on the ground in Ireland. (I'm BarryD on PocketGPSWorld and BandMandAandA here, as all the family is into geocaching).

Edited March 21, 2005 by BandMandAandA

[+wildlifewriter]

By all means send me a few of your test coordinates, and I'll send back the results I get using it.

I've made up the test data in the form of an Excel-compatible spreadsheet. 12 locations, you are testing conversion from ETRF89 to Irish Grid, but not the reverse.

 

I'll need a PM with your e-mail address.

Edited March 21, 2005 by wildlifewriter