lat/long vs osgb

[+Wild-Skye]

Another stupid question from a newbie (pls be patient a) am a newbie am ancient anyway but on the other hand can use map and compass and have found a couple of caches just using these and description so at least I know if my GPS batteries give up I won't be the one lost on the hill:-)

Right - so the stupid question - the OS grid refs are 10 digits long - that gives me (in theory) 1 m sq?? Tried as experiment comparing my satnav for car (8 yr old garmin nuvi) set on pedestrian with latitude and longitude as co-ordinates and compared with etrex 20 with OSGB. the car sat nav was miles out - well more than 100m anyway at more than one cache. Is this just my satnav? What accuracy should the lat/long give?

Edited October 9, 2013 by Wild-Skye

[+kunarion]

Tried as experiment comparing my satnav for car (8 yr old garmin nuvi) set on pedestrian with latitude and longitude as co-ordinates and compared with etrex 20 with OSGB. the car sat nav was miles out - well more than 100m anyway at more than one cache. Is this just my satnav? What accuracy should the lat/long give?

I saw the same thing with my Nuvi, which I used for a couple of days to find caches. The problem with "Pedestrian Mode" is that it still routes to the center of streets. The difference is "Pedestrian Mode" ignores things like 1-way signs, maybe? So I was standing in a park, and the Nuvi placed the cache icon over the cache, yet routed me entirely around the park, then made a straight line to the icon. It was not directing me in a crow-flies direction to that icon, it was pointing to the start of the route. I had to just select the raw lat/lon display and then walk and try to get it to match the cache location (what a chore that was!). But the accuracy was good enough to get to a cache spot, then use the hint and some Geosense.

 

Anyway, it's not way off. But it may be taking the long way to a cache, since it's designed to direct you to streets. I've since discovered there's another setting, "Off Road", and maybe that causes it to ignore roads completely. But it still rounds to tenths of a mile, so it would still require the lat/lon countdown at some point in the hunt.

 

Yours may be set up a little differently than my Nuvi. And some even have a compass rose just like a hiking GPSr. I don't think that the map datum changed the device's accuracy. But if you make a mistake entering the numbers, that could introduce inaccuracy.

Edited October 9, 2013 by kunarion

[+Kacher82]

I've used my Nuvi to find a few geocaches, though I prefer my handheld (Etrex 20). You want to make sure it's set to pedestrian and off-road. It worked fairly well for me once I figured out the settings.

[+Amberel]

Another stupid question from a newbie (pls be patient a) am a newbie am ancient anyway but on the other hand can use map and compass and have found a couple of caches just using these and description so at least I know if my GPS batteries give up I won't be the one lost on the hill:-)

Right - so the stupid question - the OS grid refs are 10 digits long - that gives me (in theory) 1 m sq?? Tried as experiment comparing my satnav for car (8 yr old garmin nuvi) set on pedestrian with latitude and longitude as co-ordinates and compared with etrex 20 with OSGB. the car sat nav was miles out - well more than 100m anyway at more than one cache. Is this just my satnav? What accuracy should the lat/long give?

It's not a stupid question, but the answer can be as long and complicated as you want it to be :-) I'll try to keep it brief, but that means most of it will be simplified or left unsaid.

 

When we give lat/long co-ordinates we are giving a position relative to a datum. There are a hundred or so commonly used datums, and any given position will have a different lat/long depending on which one is used. While you may think this sounds barmy, there are thoroughly sound historical and technical reasons for it. The Ordnance Survey Grid is based on the OSGB datum. The GPS system uses the WGS84 datum. The difference over most of the UK is 100-200 metres. How you deal with this depends on the make and model of GPS receiver you have - most if not all will allow you to select the datums, some may automatically select OSGB when you choose to display by OS grid ref, some might not.

 

Please accept that for geocaching purposes, it is best if you set your GPSr to WGS84 datum and to display co-ordinates as lat/long in the degrees/minutes/decimal minutes format. It is probably not what you are used to, but it is pretty much completely universal in caching and if you don't do that you will be fighting the system the whole time.

 

Incidentally, the OS grid refs used on the cache pages are calculated from the lat/long so the accuracy is less than the written resolution. Furthermore, they use a very poor conversion algorithm that imposes a fixed error of about 18 feet. Stick to WGS84 lat/long.

 

Rgds, Andy

Edited October 9, 2013 by Amberel

[+Amberel]

... The only real time you need worry is if you need to compare the readout on your GPS with some sort of map or someone else, in which case both need to be on the same format/datum.

Geocaching uses WGS84 degrees/minutes/decimal minutes as its primary co-ordinate system. When a cache is published the co-ordinates are entered in this format, and all other systems are derived from it with some loss of accuracy in the conversion. Additional waypoints are published in this format. With very few exceptions, if you do a multi or a puzzle you will need to calculate and enter co-ordinates in this format.

 

Rgds, Andy

[+Amberel]

I agree that there may be cases, eg puzzle caches, where you might be forced to use lat/long/WGS84 ... but apart from this sort of exception (which I mentioned) there is really no need to use lat/long.

 

My Etrex 20 is set to OS Grid and I have never had a problem finding caches with it, never noticed any 'loss of accuracy'.

If you load your ETrex using a PQ (or pretty any method other than manually) then the co-ordinates it uses will be the WGS84 lat/long (the files don't even contain the OS grid refs) and you will find no loss of accuracy - in this case the OS grid co-ordinates are being used merely for display purposes and any conversions are being performed in the GPSr.

 

If you enter the co-ordinates manually and use OS grid refs printed on the cache page, the co-ordinates will be less accurate than using the WGS84 lat/long. Most conversion algorithms lose accuracy, typically up to about 1.5 metres, but the Groundspeak one is very odd in that there is an additional constant error of approx 6 metres. Whether you would notice this error or not depends on a great many factors.

 

Maybe you don't do many multis, but few caching days go by without me doing at least one, and it would be a right royal pain if I has my GPSr set to OS grid refs. Unless the OP is only ever going to do trad caches, it really is a lot less hassle to use WGS84 lat/long. The only reason I can think of at present for wanting to use OS grid refs at all (other for an almost vanishingly small fraction of OS specific puzzles or multis) is if you are transcribing your position onto a paper map - if you are using a map on the GPSr you can just look at the screen to see where you are. I do normally carry paper maps but I don't need to use a grid ref display to see where I am.

 

Rgds, Andy

[+agentmancuso]

I switch back and forward all the time - it's easy enough to do. Probablly OS grid about 90% of the time to be honest.

[+Amberel]

I switch back and forward all the time - it's easy enough to do. Probablly OS grid about 90% of the time to be honest.

I use WGS84 lat long format for:

 

Entering co-ords when I set a cache

Entering solutions to multi-caches

Entering parking waypoints into my SatNav

Puzzle solutions

Occasionally for entering cache location if I have a printout but the waypoint isn't on the device

 

Can you say for what purposes you use the OS grid format?

 

Rgds, Andy

[+Wild-Skye]

WEll --- I expected a simple answer but I will have to read the replies again with care. General impression is that I wld be better using lat/long. So far i have been entering OS grid ref by hand ( simply because I don't know where my data cable is so cannot connect Etrex to computer)

 

thanks to all for your patience with a numpty

[+Amberel]

Only the first occasionally

Don't do multi-caches

Use OS grid or direct from the screen on my satnav

Don't do puzzles

Caches always on my GPS

 

Mostly use the OS grid format when out hiking ... tends to go better with OS maps

 

Should I need lat/long then it's simple enough to switch ... as agentmanusco pointed out.

Sorry to labour this, but I'm really trying to understand the advantages of using OS grid format. What you are saying is that specifically for your restricted pattern of caching it is not usually a disadvantage, but that is not the same as being an advantage.

 

I already acknowledged the possibility of it tending to go better with OS maps, but that only applies if you actually read off the co-ordinates from the GPSr and plot them on your map with any frequency. I use OS maps, but I use them in conjunction with the maps on my GPSr - I can see exactly where I am on the paper map without having to use the co-ordinates. I enjoy long hikes in remote places, and always carry and use a paper map, but in 6 years or more caching I can't remember ever having to use the grid ref display to locate myself on it.

 

It's not a major task to change the display format, but it is a bit fiddly and I would find it quite annoying to have to keep doing it several times a day for no advantage.

 

Rgds, Andy

[+Amberel]

I don't think we're disagreeing

 

For the way you use yours you find it more convenient in lat/long, for my use I find it more convenient in OS grid.

 

Where this started is the assertion that you should use lat/long ... and for some (like me and presumably agentmanusco) that simply isn't true.

Actually I was advising the OP to use WGS84 lat/long, at least until they are experienced enough to be confident in making their own choices, and given the information we have I remain convinced that was the correct advice.

 

Rgds, Andy

[+The Patrician]

I see the OP is using an Etrex 20 which I think has various profiles which can be used.

 

I've got my GPS62 set with profiles for walking (OSGB so that it matches my maps) and lat & long for geocaching purposes. It's only a couple of button presses to switch between the two. Apologies if this has been mentioned further up the thread, I haven't waded through it all!

[+MartyBartfast]

I see the OP is using an Etrex 20 which I think has various profiles which can be used.

 

I've got my GPS62 set with profiles for walking (OSGB so that it matches my maps) and lat & long for geocaching purposes. It's only a couple of button presses to switch between the two. Apologies if this has been mentioned further up the thread, I haven't waded through it all!

 

Yes the Etrex20 does allow this, in fact I have set up an OSGB profile on my Etrex30 but so far I've found no scenario where I've wanted to use it.

 

To the OP, I would say that unless you have a burning desire to stick with OSGB for some reason then you will be far better off going with WGS84 and sticking with it. I don't think OSGB settings can give you anything other than being a format you may be familiar with, and personally I didn't find it at all difficult to switch to WGS84 and now I find it much more logical and easier to visualise.

[+agentmancuso]

...

 

Can you say for what purposes you use the OS grid format?

 

 

Corrected coords, puzzle answers etc I do on GSAK. Only occasionally do I enter legs for multicaches in the field - that's when I switch over to WSG84.

 

I've been hillwalking with OS coords for 30 years. The numbers actually mean something to me, in a way that lat/lon never will. Serious summit bagging and trigpointing/benchmarking are all done via the OS grid too.

 

It's just a personal preference, either way.

[+Wild-Skye]

Have just been trying to hide my first caches. Through habit - cos Grid refs are what I understand and am used to - I checked the locations on different days with grid refs. When I came to fill out the online form - I realised it wouldn't accept Grid refs so changed my gps to read lat/long (at home not at caches). I then entered the co-ordinates given into the online form for new caches and it converts them into a grid ref which doesn't correspond to the original grid refs I had from my gps. What am I doing wrong?

 

Clearly in future - I'll just have to set GPS to lat/long for geocaching

[+Amberel]

Have just been trying to hide my first caches. Through habit - cos Grid refs are what I understand and am used to - I checked the locations on different days with grid refs. When I came to fill out the online form - I realised it wouldn't accept Grid refs so changed my gps to read lat/long (at home not at caches). I then entered the co-ordinates given into the online form for new caches and it converts them into a grid ref which doesn't correspond to the original grid refs I had from my gps. What am I doing wrong?

 

Clearly in future - I'll just have to set GPS to lat/long for geocaching

I'm not wholly clear on what you've done, but I'm assuming you're not just reading your home co-ordinates off the GPSr. Have you have set one or more waypoints, noted the grid ref readings at the time, and are now reading those same waypoints back in lat/long?

 

The first thing is you have to make sure that you have set it to WGS84 lat/long, as you have had it set to display grid refs up to now maybe you have set it to OSGB36 lat/long? The difference is likely to be about one or two hundred metres.

 

If you have the right datum set, then be aware that the algorithm used by Groundspeak to calculate grid refs is very poor, and gives a fairly constant approx 6 metre error.

 

Is the error you are seeing close to either of these numbers?

 

Rgds, Andy

[+The Patrician]

There's some good info on coordinate systems here:

 

http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf

 

http://tinyurl.com/ow5ewur

 

See Page 4 for some of the differences in position Andy mentions above.

[+Wild-Skye]

Thanks for these replies

[+flipflopnick]

DO NOT USE OSGB given on geocaching.com as it is inaccurate as are coordinates taken with a phone. Use your phone to find caches only. Streetmap convert coords better than Groundspeak, more accurately. Grid refs are converted from another system. Your GPS converts from lat long given by satellites. OS have their own software to download for free which converts many ways.

 

Groundspeak use WGS84 datum, which thinks the globe is round, where as OSGB think the UK is flat, IMHO. OSGB use a datum in Newlyn, Cornwall.

Generally Groundspeak grid refs are approximate , they get you near the GZ whereas lat long gets you exactly to GZ.

 

The further away from Newlyn, the more inaccurate the conversion by Groundspeak, IMHO. So GCs near Thurso ...