Datum Probs

[Lady SheWolf]

I'm just a tiny (alot) lost here.

 

And I'm probably doing it all wrong.

 

I went out today with 3 clues and co ords on a sheet of paper parked and started walking to the first clue. which was a House. Anyways gets to where what we thought was the right clue and the co-ords were way out.

 

On arrival home I checked the given co-ords with Streetmap.co.uk and it came up with the place we were standing out side.

 

Then I read about this datum thing........ but can't find in the gps or manual any reference to what mine is. I've searched on a girls best friend Google but still no luck.

 

I've a Medion 95000 from Aldis with the Medion 4.2 software.

 

Some advice please.

 

I did post this on the forums above but it was sugested I might try here. An update is I'was tolld to go to settings and find gPS and change datum. But there is nowhere in the settings to do that, just the option to either enable a gps icon or start automaticly when the pda is switched on.

 

If it's new software I need to go hunting can anyone suggest a map that will sit along side what I have?

TIA.

[+ribel]

Hi there.

 

I think the unit you have is the Mio 168. Is this correct?

 

My old man has this unit, I'll speak to him to see if I can help you out. I know he has successfully found his first caches with this unit, so he must be doing something right!

 

ribel

[Lady SheWolf]

Thats the one

 

Thanks . fingers crossed.

 

(I think I've finally found a hobby to get me out of this seat )

[chockhead]

Hi there, I'm ribel's old man. I got his message and as he informed you I too have the Medion pocket pc. I must say here though that I use memory map os software for geocaching and I suspect this is where your problem lies. I do not think you can alter the datum in medion's street map it is a basic (albeit good) street finder.

 

That said, I will play around with the medion software and see if it is possible to alter the datum and come back to you.

[Lady SheWolf]

Thanks. Thought I'm begining to suspect you are right. I can't find a setting anywhere.

 

Then again saying that I'm new to the world of gps and pdas too,.

Thanks again.

[+ribel]

Is it not possible to enter coordinates and have the gps direct you towards them, without the use of 3rd party mapping software?

[Lady SheWolf]

What was happening was when I was phiscilly outside the house (which was the clue) the software told me I was in the right place it was the co ords that were off. I knew it was the right place as I checked the given co ords on streetmap. I don't think you can put cordanates straight intot he gps on this. You have to use some sort of software. I think.

 

Anyways an update I've got got myself gps tuner and am going in a while to try that. Fingers crossed for me.

 

[+ribel]

That program looks good. Does it tell you distance to target (can't see it on any screen shots, but it's gotta be there)?

 

Fingers are crossed. Go get that tuppa!

[+tonycollins]

I started trying GPS Tuner yesterday.

 

It is excelleng for geocaching. On the compass screen, it gives you distance and heading to target.

 

One problem I have found is after manually entering a waypoint, it gave the distance and heading around 3000 miles south instead of one mile north west. All other waypoints were similarly off. When I stopped and restarted the software it seemed to sort itself out.

 

The other aspect I am not keen on is the compass reverts to north up when you stop moving. Other than these points, though it seems very good. There is a new version planned in the next few months, and they seem to have good support forums - I've posted a couple of suggestions in the "feature requests" forum.

 

You can also import cache locations as loc or gpx files (although it bombs out on large gpx files that hold all the cache details/logs etc.

 

This means I can use GPX sonar to hold all the cache details on the MIO, and export from this into GPS Tuner

 

I also use the fugawi software for OS Map provision - but this is far from necessary for geocaching.

[Lady SheWolf]

I found it I should say we it was a "family" outing .

 

But I must have a read on the how to use the software .

 

I was still out with the co ordnates by about a mile. Unless of course it's me just not doing something right. I've got the pda on charge now so will have a look and a play laters.

 

I found the compass nice and big to read saves taking my glasses off to see it.

Things are looking up

 

[questsmith]

Can I piggy-back this topic as I have more datum confusion ?

 

I have read elsewhere that the GC coordinates use the US WGS84 datum - this was also the default for my Garmin unit - so far so good.

 

As I also use my GPS for walking on the hills, I use it in conjunction with a compass and an OS Map. Having looked at the OS Map technical data it uses the OSGB (1936) datum. If I set my GPS to this datum will I be out of position trying to find the cache and conversely if I use the WGS84 datum, am I going to wander over a cliff in the fog ?! I don't really want to be switching datums backwards and forwards, so any suggestions on which is best ?

 

Also, does the age of the datum impact on its accuracy ?

 

Thanks !

Questsmith

[+wildlifewriter]

Can I piggy-back this topic as I have more datum confusion ?

 

I have read elsewhere that the GC coordinates use the US WGS84 datum - this was also the default for my Garmin unit - so far so good.

Strictly speaking, it's not "US WGS84", it's just "WGS84" which is a global system.

 

Having looked at the OS Map technical data it uses the OSGB (1936) datum. If I set my GPS to this datum will I be out of position trying to find the cache

Yes.

 

and conversely if I use the WGS84 datum, am I going to wander over a cliff in the fog

If you are foolhardy enough to go hillwalking in fog or low cloud, this is certainly a possibility. You cannot rely on your GPS to prevent such a thing happening, no matter what it is set to.

 

Hth,

[questsmith]

Thanks for the response ...

 

OK, so WGS84 is the one to use for caching.

 

if I use the WGS84 datum, am I going to wander over a cliff in the fog ?!

 

I used this expression for effect - what I was getting at is whether the WGS84 datum will give coordinates that don't cross-reference accurately to an OS Map ?

[+The Forester]

does the age of the datum impact on its accuracy ?

No.

 

OSGB36 is the datum, but the co-ords of the triangulation pillars have been recalculated at least twice since then.

 

Trigpoint co-ordinates are accurate to about 5.5 centimetres and can be expressed in either WGS84 or OSGB36.

 

The spheroid used by Ordnance Survey remains Airy's spheroid of 1849. It is optimised to be a best fit for the landmass of the UK, but is not a very good fit elsewhere in the world. WGS84 is optimised to be a best fit world-wide and was an improvement over its predecessor WGS72 which had been devised for the US Navy's Transit satellite navigation system.

 

The difference is position between OSGB36 and WGS84 is something of the order of a hundred metres, so it is important to ensure that you have selected the correct spheroid and datum for the task at hand. For geocaching, that is WGS84. For use with large scale mapping provided by the Ordnance Survey, that's Airy/OSGB36.

 

Cheers, The Forester

[questsmith]

Thanks that has helped clarify things.

 

So it looks like I will have to swap between the datums depending upon whether I am caching or navigating up Tryfan nothing is ever simple !

[+wildlifewriter]

So it looks like I will have to swap between the datums depending upon whether I am caching or navigating up Tryfan  nothing is ever simple    !

Well, you won't have to do much swapping.

 

If you are working from GPS-to-map, then use the appropriate setting to match the map. Then swap over when approaching the cache area. If using a Garmin unit, then changing the position format to the geocaching standard (Deg,mm.mmm) will alter the datum setting, automagically.

[questsmith]

OK, my head is beginning to hurt but I think I have nearly got it !

 

Thanks for helping me to understand it !!

[+wildlifewriter]

Before leaving home, set the GPSer to WGS84 and enter the waypoint for the cache. Switch the datum on the GPS to OSGB36 and the waypoint will be converted to the new datum by the GPSer. Now use the GPS and the maps together to find the cache.

Ignore this advice. The conversion method used in [almost all] handheld GPS units is not precise enough for the shortcut described above to work reliably in the UK. Depending on where you are in the country, the resulting error could be nine metres, or even a bit more.

 

dd-mm.mmm and WGS84 - that is all the Law and the Prophets..