Offset Coordinates

[+The HERB5]

Did Katherine's Cross (GCB938) at the weekend and at the end he/she said

"The cache is hidden 15 metres East, North East of this spot."

 

This seems totally logical to me and I can't understand why more cache setters don't do it.

 

Is this classed as cheating or am I missing something?

 

On the same theme, are there any techniques which can help when using a GPS?

[Team 'James W']

Not sure... That kind of approach has caused problems as I think it can get a bit vague... Especially when you take elevation etc into account.

 

To be fair though, you're talking abut 15m, my disappointment had an offset of 200m..

 

[+Kitty Hawk]

I'm OK with that, set your existing point as a waypoint, press "go to" and walk away with the compass showing for direction.

 

I think on the Garmin Vista there is a way of projecting a waypoint, so your 200m NNW would be no worries as long as the setter was as accurate as you.

 

I wouldn't regard it as cheating, in fact I wish I'd thought of it last week, when 90ft from a cache I set were the coordinates W 001.22.222, but it was in an open space.

 

Adrian

[+wildlifewriter]

This seems totally logical to me and I can't understand why more cache setters don't do it.

 

It's not cheating - there are just different ways of achieving the same thing. (If everyone laid out their caches in exactly the same way, the hobby would be very boring. )

 

Here's an alternative way of doing it:

 

Let's take the car park location of that same cache as a starting point: N52º 01.968 W000º 30.497. Suppose the hiding place was the stated 15m ENE from there - but it's under tree cover, where a GPSr won't work very well...

 

We could take the initial co-ordinates and work out the REAL position for the cache - even though we can't use a GPSr to establish that position directly. It's N52º 01.972 W000º 30.486 (give or take a couple of metres.) This is called "projecting" a waypoint and it's quite easy to do (on flattish terrain) with some basic arithmetic.

 

The finder has to reverse the process: get as close as possible to a place which is 15m WSW of those co-ords, and then pace off the intervening distance.

 

There are a number of caches which are laid out exactly like this. The only issue is with the direction: strictly speaking we should always specify whether a compass bearing is (M)agnetic or (T)rue when stating it.

[+Pharisee]

Did Katherine's Cross (GCB938) at the weekend and at the end he/she said

"The cache is hidden 15 metres East, North East of this spot."

 

This seems totally logical to me and I can't understand why more cache setters don't do it.

 

Is this classed as cheating or am I missing something?

 

On the same theme, are there any techniques which can help when using a GPS?

As it's my cache that Nebias is referring to, I'll explain. The cache is a small box hidden in a hole in the base of a tree at the bottom of a steep bank which surround the cache on two sides. This cuts off signals from any satellites that are not pretty much directly overhead. There is also a fair amount of tree cover as well so GPS reception isn't good at the best of times. To give the seeker a chance I took the co-ordinates in the middle of what open ground there was and then quoted a direction and distance to the tree. As it was only 15 metres, I didn't consider 'magnetic' or 'true' to be particularly relevant (but as I used my Silva compass, it is in fact magnetic) when the only obvious hiding place was a fully grown maple tree, but I do acknowledge it's importance for greater distances.

 

And anyway... I like to do things a little differently sometimes

[+The Forester]

Pharisee is right.

 

Every good navigator knows how to rapidly do the mental trigonometry using the 1 in 60 rule. Magnetic Variation in that part of the world is approximately 4°, so at 15m that must be 1 metre (a quarter of 60). If that distance is less than the resolution of geocaching's ddmm.mmm format, then it's negligible.

 

Every schoolkid who has been educated in Geography to GCSE is aware that a minute of Longitude at the Equator is a nautical mile (1,852m) and that the distance decreases with increasing Latitude, so the geocaching resolution of a thousandth of a minute of Longitude at 52° must be approximately 1.85m x Cos52, which is about 1.14m. Therefore the resolution of the GC.com format of Lat Long at this location is approximately 2.18m which is substantially more than the approximately 1 metre difference between measuring the offset in True or Magnetic.

 

To put actual numbers to that, from the quoted cachepage: the Magnetic Variation at that location is actually 3° 47'W.

Therefore the difference between 15m ENE(M) and 15m ENE(T) is 0.87m.

The Latitude resolution of the GC.com format at this location is 1.85m and the Longitude resolution is 1.14m.

 

Therefore the original 1-in-60 rule based assessment that the difference between a Magnetic and a True Bearing over this distance is negligible in the context of the ddmm.mmm format of geocaching is correct and Pharisee was quite right to ignore the difference between True and Mag on this occasion.

 

Another aspect to this is that it is good practice to ignore small variations if their magnitude is less than and close to the resolution of the measurement or co-ordinate system, because Sod's Law decrees that you will accidentally apply the correction the wrong way, thus taking the error over the limit into the non-negligible range.

 

 

Cheers, The Forester

[+Lost in Space]

.... at the bottom of a steep bank which surround the cache on two sides....

 

And anyway... I like to do things a little differently sometimes 

 

Bl***y typical of you!!

[+Pharisee]

.... at the bottom of a steep bank which surround the cache on two sides....

 

And anyway... I like to do things a little differently sometimes 

 

Bl***y typical of you!!

 

Get out of bed the wrong side this morning, did you Neil.... G.O.G.

 

There still seems to be a couple of logs for 'Your Mission....' that are conspicuous by their absence. S'pect it's too tough for you old folk

[+Chris n Maria]

Every schoolkid who has been educated in Geography to GCSE is aware that...

I doubt every kid is aware of that - most of the kids in my school had enough trouble finding the geography class. Edited April 20, 2005 by Chris n Maria