Lat and Long

[sherwoodranger]

Help, New to Geocaching and just found my first cach today with the Wife and Children but found it dificult to do as Lat and Long coordinates were up to 100m out on all the clues leading up to the cach. I have an Etrex whitch I picked up in a px in Iraq in 2004 and it has served me well since then. Can any one explane why caches have Lat and Long and not easier to use grid ref, and why when on the altenative grid page there seames to be extra numbers added to the front of the british grids? and the Eastings are before the Northings. As a British Army Map Reading Instructor I find this strange, so what anyone who is new to Geocaching and Map Reading must think when British Ordinance Survey Maps are in somthing different must seem very confusing

[+The Blorenges]

Welcome to these forums

 

As you're in the UK you might find this site useful: "Follow the arrow" Geocaching resources for UK and Ireland. If you scroll down the first page a bit you'll see a useful section on co-ordinates and map datum which explains which is used for consistency throughout the geocaching world.

 

MrsB

[+coggins]

Looks like a different way of displaying the BNG:

http://en.wikipedia.org/wiki/British_natio...grid_references

[sherwoodranger]

Welcome to these forums

 

As you're in the UK you might find this site useful: "Follow the arrow" Geocaching resources for UK and Ireland. If you scroll down the first page a bit you'll see a useful section on co-ordinates and map datum which explains which is used for consistency throughout the geocaching world.

 

MrsB

 

Thanks for that, But as i said i do know that my Gps is set to the correct map datum and co-ordiates, what i cant understand is the use of Lat and Long. If people have to use a map as well as thier GPS then most people could not use lat and long co-ordinates on a osmap, please try to do this and just see how succesful you are. Also as i said on the british grids that are given on the other pages if you did not know that the eastings and northings where the wrong way round and they had an extra nummber at the front then those grids would not work as well. Just wish some one from Geocaching would explane this. But as i said thanks for your reply and please try Lat and Long on a map just to see how you get on and let me know.

[+Ant89]

point SV 0 0 on the OS grid is the start point of the british grid using the 0 northing line and the 0 easting line from that point as line of origin ( X and Y axis) mesure to a point within the grid in metres the further from the X and Y axis the greater the distance and therefore the greater the numbers.

 

hope that makes sense.

Edited January 1, 2010 by Ant89

[+Ant89]

Welcome to these forums

 

As you're in the UK you might find this site useful: "Follow the arrow" Geocaching resources for UK and Ireland. If you scroll down the first page a bit you'll see a useful section on co-ordinates and map datum which explains which is used for consistency throughout the geocaching world.

 

MrsB

 

Thanks for that, But as i said i do know that my Gps is set to the correct map datum and co-ordiates, what i cant understand is the use of Lat and Long. If people have to use a map as well as thier GPS then most people could not use lat and long co-ordinates on a osmap, please try to do this and just see how succesful you are. Also as i said on the british grids that are given on the other pages if you did not know that the eastings and northings where the wrong way round and they had an extra nummber at the front then those grids would not work as well. Just wish some one from Geocaching would explane this. But as i said thanks for your reply and please try Lat and Long on a map just to see how you get on and let me know.

 

You could always change the format once you have put the coords into the GPS.

[+The Blorenges]

Help, New to Geocaching and just found my first cach today with the Wife and Children but found it dificult to do as Lat and Long coordinates were up to 100m out on all the clues leading up to the cach. I have an Etrex whitch I picked up in a px in Iraq in 2004 and it has served me well since then. Can any one explane why caches have Lat and Long and not easier to use grid ref, and why when on the altenative grid page there seames to be extra numbers added to the front of the british grids? and the Eastings are before the Northings. As a British Army Map Reading Instructor I find this strange, so what anyone who is new to Geocaching and Map Reading must think when British Ordinance Survey Maps are in somthing different must seem very confusing

 

Hi

The 100m error is a classic symptom of having the GPS set to the wrong map datum in the UK. You need to use Lat/Long on the WGS84 map datum. If you've set the units 'position format' to Hdd°mm.mmm (lat long degrees decimal minutes that geocaching uses) you need the map datum set to WGS84. If you've left the datum on Ordnance Survey GB then that is actually WGS36 and that gives the 100m error.

You can run the GPSr on OSGB if you want but you must set both the position format and datum to OSGB.

The reason we use Lat/Long on cache pages is that it is 'universal'. The OS grid reference you refer to is UK specific and only found on the paper based (or digital PC version) OS maps. Someone in Germany for example wouldn't have a clue what the OS Grid Reference is but they would know Lat/Long.

The cache page displays the OSGB grid for our benefit in the UK. Any additional waypoints that are displayed on the cache page (parking, stages etc) are not converted. If you download the waypoints into your GPS from the cache page or via a Pocket Query you'll get them in Lat/Long degrees decimal minutes and that is the best setting to leave your GPSr on.

 

Re the eastings and northings. The grid reference shown on a cache page (example N 51° 35.547 W 002° 44.586 British Grid ST 48621 88443) is correctly shown. The first group of 5 digits is the easting and the second group the northing. This is more usually written as two groups of 3 (ST 486 884). The 5 digits take you down to 1 meter. Example: S (500Km square) T (100Km square) 4 (10Km) 6 (1km) 6 (100m) 2 (10m) 1 (1m).

 

You can check this on an excellent website that uses the Streetmap OS maps http://benchmarks.org.uk/map.php?&scale=50&bm&c= by entering the lat/long and the OSGB. They show the same location.*

 

Hope this helps.

 

Chris (MrB)

 

UK Geocaching Information & Resources website www.follow-the-arrow.co.uk

* Thanks to Gary and Jane for this excellent link to Streetmap

Edited January 2, 2010 by The Blorenges

[+purplywurply]

Help, New to Geocaching and just found my first cach today with the Wife and Children but found it dificult to do as Lat and Long coordinates were up to 100m out on all the clues leading up to the cach. I have an Etrex whitch I picked up in a px in Iraq in 2004 and it has served me well since then. Can any one explane why caches have Lat and Long and not easier to use grid ref, and why when on the altenative grid page there seames to be extra numbers added to the front of the british grids? and the Eastings are before the Northings. As a British Army Map Reading Instructor I find this strange, so what anyone who is new to Geocaching and Map Reading must think when British Ordinance Survey Maps are in somthing different must seem very confusing

 

Hi

The 100m error is a classic symptom of having the GPS set to the wrong map datum in the UK. You need to use Lat/Long on the WGS84 map datum. If you've set the units 'position format' to Hdd°mm.mmm (lat long degrees decimal minutes that geocaching uses) you need the map datum set to WGS84. If you've left the datum on Ordnance Survey GB then that is actually WGS36 and that gives the 100m error.

You can run the GPSr on OSGB if you want but you must set both the position format and datum to OSGB.

The reason we use Lat/Long on cache pages is that it is 'universal'. The OS grid reference you refer to is UK specific and only found on the paper based (or digital PC version) OS maps. Someone in Germany for example wouldn't have a clue what the OS Grid Reference is but they would know Lat/Long.

The cache page displays the OSGB grid for our benefit in the UK. Any additional waypoints that are displayed on the cache page (parking, stages etc) are not converted. If you download the waypoints into your GPS from the cache page or via a Pocket Query you'll get them in Lat/Long degrees decimal minutes and that is the best setting to leave your GPSr on.

 

Re the eastings and northings. The grid reference shown on a cache page (example N 51° 35.547 W 002° 44.586 British Grid ST 48621 88443) is correctly shown. The first group of 5 digits is the easting and the second group the northing. This is more usually written as two groups of 3 (ST 486 884). The 5 digits take you down to 1 meter. Example: S (500Km square) T (100Km square) 4 (10Km) 6 (1km) 6 (100m) 2 (10m) 1 (1m).

 

You can check this on an excellent website that uses the Streetmap OS maps <a href="http://benchmarks.org.uk/map.php?&scale=50&bm&c=" target="_blank">http://benchmarks.org.uk/map.php?&scale=50&bm&c=</a> by entering the lat/long and the OSGB. They show the same location.*

 

Hope this helps.

 

Chris (MrB)

 

UK Geocaching Information & Resources website www.follow-the-arrow.co.uk

* Thanks to Gary and Jane for this excellent link to Streetmap

Thanks for the explanation. I have an Etrex set to WGS84 and Hdd°mm.mmm. Strangely when I change the options to use the OS Grid (which I would like to do, as I am teaching my daughter to use maps and read grid references) then the GPS changes the very last digit in one of the coordinates.

 

So, a few questions....

 

1) Why does my gps change the coordinates when I change from one system of coordinates to another?

2) Is lat/long more accurate than OSGrid?

3) What degree of accuracy is lost if the very last digit in the coordinates changes?

4) Is it fair to assume that the hider used lat/long to log the cache?

[Tahosa and Sons]

Help, New to Geocaching - Can any one explane why caches have Lat and Long and not easier to use grid ref,

Not even familar with the grid system you are talking about. But it is true that L & L don't work well on map work. I use the UTM system with TOPO and do most of my map plotting with that system. I have my GPS set to UTM's and WGS 84.

[+Heresomewhere]

Help, New to Geocaching and just found my first cach today with the Wife and Children but found it dificult to do as Lat and Long coordinates were up to 100m out on all the clues leading up to the cach. I have an Etrex whitch I picked up in a px in Iraq in 2004 and it has served me well since then. Can any one explane why caches have Lat and Long and not easier to use grid ref, and why when on the altenative grid page there seames to be extra numbers added to the front of the british grids? and the Eastings are before the Northings. As a British Army Map Reading Instructor I find this strange, so what anyone who is new to Geocaching and Map Reading must think when British Ordinance Survey Maps are in somthing different must seem very confusing

 

Hi

The 100m error is a classic symptom of having the GPS set to the wrong map datum in the UK. You need to use Lat/Long on the WGS84 map datum. If you've set the units 'position format' to Hdd°mm.mmm (lat long degrees decimal minutes that geocaching uses) you need the map datum set to WGS84. If you've left the datum on Ordnance Survey GB then that is actually WGS36 and that gives the 100m error.

You can run the GPSr on OSGB if you want but you must set both the position format and datum to OSGB.

The reason we use Lat/Long on cache pages is that it is 'universal'. The OS grid reference you refer to is UK specific and only found on the paper based (or digital PC version) OS maps. Someone in Germany for example wouldn't have a clue what the OS Grid Reference is but they would know Lat/Long.

The cache page displays the OSGB grid for our benefit in the UK. Any additional waypoints that are displayed on the cache page (parking, stages etc) are not converted. If you download the waypoints into your GPS from the cache page or via a Pocket Query you'll get them in Lat/Long degrees decimal minutes and that is the best setting to leave your GPSr on.

 

Re the eastings and northings. The grid reference shown on a cache page (example N 51° 35.547 W 002° 44.586 British Grid ST 48621 88443) is correctly shown. The first group of 5 digits is the easting and the second group the northing. This is more usually written as two groups of 3 (ST 486 884). The 5 digits take you down to 1 meter. Example: S (500Km square) T (100Km square) 4 (10Km) 6 (1km) 6 (100m) 2 (10m) 1 (1m).

 

You can check this on an excellent website that uses the Streetmap OS maps <a href="http://benchmarks.org.uk/map.php?&scale=50&bm&c=" target="_blank">http://benchmarks.org.uk/map.php?&scale=50&bm&c=</a> by entering the lat/long and the OSGB. They show the same location.*

 

Hope this helps.

 

Chris (MrB)

 

UK Geocaching Information & Resources website www.follow-the-arrow.co.uk

* Thanks to Gary and Jane for this excellent link to Streetmap

Thanks for the explanation. I have an Etrex set to WGS84 and Hdd°mm.mmm. Strangely when I change the options to use the OS Grid (which I would like to do, as I am teaching my daughter to use maps and read grid references) then the GPS changes the very last digit in one of the coordinates.

 

So, a few questions....

 

1) Why does my gps change the coordinates when I change from one system of coordinates to another?

2) Is lat/long more accurate than OSGrid?

3) What degree of accuracy is lost if the very last digit in the coordinates changes?

4) Is it fair to assume that the hider used lat/long to log the cache?

 

1) If you shift from one map system to another the GPSr will move to the nearest coordinate in the new system, as the two systems use different data they never quite match up.

2) On paper, no, but on a GPSr yes. The GPSr is designed to work with lat/long on the WGS84 datum, the OS grid ref position shown is an estimate based on the lat/long.

3) About a metre.

4) The cache should be logged with the WSG84 datum.

 

PS Take a look at the altitude difference between the OS map and the GPSr(WSG84) reading. The GPSr reads about 100 feet higher than the map.

[+The Blorenges]

So, a few questions....

 

1) Why does my gps change the coordinates when I change from one system of coordinates to another?

2) Is lat/long more accurate than OSGrid?

3) What degree of accuracy is lost if the very last digit in the coordinates changes?

4) Is it fair to assume that the hider used lat/long to log the cache?

 

1) I assume you mean you leave the 'position format' as Lat and long (degrees and decimal minutes) but change the Map Datum to OS Grid? That would give an error as it is the wrong datum for the position format. But be careful because even though you may be standing still and have a good satellite signal you'll find the last digit may 'drift' a bit due to the vagueries of the satellites and atmosphere.

2) both are accurate as they are indicating a point on the earth using a grid reference method - there is a slight descrepancy between them but it isn't much.

3) in the UK at our lattitude and longitude the last digit of the Latitude is about 6 feet (2 meters) and the last digit of the longitude is about 3 feet (1 meter)

4) No, never assume anything! All cache pages have to be submitted with the coordinates in Lat/Long so you'd think it was OK. I'm also one of the UK cache Reviewers (Graculus) and have published caches that no one can subsequently find. The cache owner then asks me to update the coordinates explaining they had their GPS on OSGB and then converted it or didn't even use a GPS but took the coordinates from a paper OS Map!

 

Hope this helps

 

Chris (MrB)

Edited January 2, 2010 by The Blorenges

[+Ant89]

Regarding formats and datrums

 

Using my garmin GPS I entered the coords of one of my (unpublished) caches in decimal degrees using WGS84 datrum then converting on the GPS to the british grid but leaving the datrum on WGS84 resulted the point being approx 500m away. the OS datrum put the point within a few metres of the actual location.

 

Moral of the post, DO NOT use WGS84 if you change the format to the British Grid.

[+Heresomewhere]

Regarding formats and datrums

 

Using my garmin GPS I entered the coords of one of my (unpublished) caches in decimal degrees using WGS84 datrum then converting on the GPS to the british grid but leaving the datrum on WGS84 resulted the point being approx 500m away. the OS datrum put the point within a few metres of the actual location.

 

Moral of the post, DO NOT use WGS84 if you change the format to the British Grid.

 

That's because the datum point for the Ordinance Survey (OSGB36) is 49°N,2°W and the datum for WGS84 is the Earth's centre of mass.

 

The Ordinance Survey and GPS were designed with very different objects in mind. GPS was designed by the United States Air Force to provide a world wide positioning capability, while the OS was, originally, designed by the British Army to help suppress Jacobite rebellions.

Edited January 3, 2010 by Heresomewhere