Gps Questions

[+kbootb]

I had 2 thoughts nagging at me yesterday and can't find the definitive answers.

 

1) I live in London. If I enter a waypoint on the far side of the globe, day in the US, does the distance to the cache measure a straight line i.e. through the globe, or is it the more sensible distance on the surface, something like a great circle.

 

2) My gps also has an electronic compass that kicks in when I stand still. Is a GPS (under satellite and when I'm moving) set for true or magnetic north? What about when I stand still and the arrow is now driven by the compass? Is that now using magnetic north or is it compensated to match the standard GPS mode?

[+McDeHack]

Pass! Next question?

[+-Phoenix-]

Electronic compases dont use satalites to work, they use the magnetic field, GPS only returns a point position, heading speed and distance are all calculated by the handset by integrating the change in position

[+Bill D (wwh)]

I'm sure someone will come along with a definitive answer, but in the meantime:

 

1. You should be able to find out by trial and error. Try it and see - if the answer is around 7,900 miles it's a straight line through the earth, whereas if it's around 12,500 miles it's a great circle or something on those lines.

 

2. My MLR has the means to input magnetic variation, so I imagine it comes down to what you've entered, and you'll get true north unless you've told it otherwise.

[+kbootb]

Electronic compases dont use satalites to work, they use the magnetic field, GPS only returns a point position, heading speed and distance are all calculated by the handset by integrating the change in position

Yeah I know.

 

My Etrex knows when I stand still. It knows my position as it still has sat lock.

 

However, a static gps has no idea which way you are facing. i.e. if you turn round on the spot the arrow is no longer pointing at the waypoint.

 

However, on my model the electronic compass will kick in and drive the arrow so it continues to point to the cache. When I start moving again the traditional GPS as you have described takes over again.

[+-Phoenix-]

Well, unless you have an offset in for true north then it will be magnetic north, all compasses point to magnetic north, and some electronic ones have a progamable offset for an offset north, but otherwhise it will be magnetic north.

 

Because the compas takes over driving the heading arrow, it doesnt matter what north the compass uses, since it will maintaine the heading relevant to wat it was whilst you were moving.

[+kbootb]

Well, unless you have an offset in for true north then it will be magnetic north, all compasses point to magnetic north, and some electronic ones have a progamable offset for an offset north, but otherwhise it will be magnetic north.

 

Because the compas takes over driving the heading arrow, it doesnt matter what north the compass uses, since it will maintaine the heading relevant to wat it was whilst you were moving.

Yep, good thinking. There is no offset to program in, and as you say, it doesn't matter what north it is using when pointing at a cache.

 

I was asking as it has a mode where you can sight at a distant object and capture the bearing. So in this case the magnetic/true/grid north might be significant.

 

But then again, there are so many inaccuracies e.g. the unit must be exactly level when you both calibrate and use the compass that I bet there is more than 5 deg error anyway.

[+Team Hydro]

I haven't got it with me, but I think my Etrex has an option in it somewhere to select which North to reference to. Anyone able to look at their options before I get to my Etrex?

 

And not all compasses necessarily point to Magnetic North........

[+Roberts-tribe]

1)  I live in London.  If I enter a waypoint on the far side of the globe, day in the US, does the distance to the cache measure a straight line i.e. through the globe, or is it the more sensible distance on the surface, something like a great circle.

Inverting my home co-ordinates to get a position near New Zealand, I get a distance to of 20,037 Km ( 12,450 miles ). So, it's the great circle distance. ( Garmin legend C - but I suspect they all do the same ).

Edited April 22, 2005 by Roberts-tribe

[+Chris n Maria]

The answer to No. 1 is Great circle distance for sure.

 

Chris

[+Brenin Tegeingl]

The basic yellow etrex has the user option of True/ Magnetic/Grid/User and you can set the variance a well, so I presume the beter spec models will have the same options. On the etrex it's in the set up page.

 

Dave

[+The Forester]

Q1:

It is almost certainly not a geodesic (straight line between two 3-D points) as that is a worthless mesurement.

It is almost certainly either a Great Circle or a Rhumb Line.

 

The way to determine which is which is to look at the bearing to/from the two selected points. If they are reciprocal bearings (ie + or - 180° of eachother), then it's a Rhumb line. Otherwise it's a Great Circle.

 

Worked example:

Forester Towers: 55° 55.636N 3° 44.264W

faroff cache: 23° 26.305N 58° 34.148E

Great Circle distance = 6,152.049km

Rhumb line distance = 6,292.207km

Great Circle forward bearing = 099°.115

Great Circle reverse bearing = 322°.921

Rhumbline bearing = 125°.01/305°.01

 

Magellan calc: 6152km @ 099°(T) and a reverse bearing of 323°(T)

 

Clearly the GPSr is using the Great Circle calculation, not the Rhumbline.

 

Q2:

You can select which you want of: True, Magnetic, or Grid bearings.

 

Your GPSr has an internal mathematical model of the fluxlines of the Earth's geomagnetic field. It automatically compensates for the Variation by means of what amounts to a lookup table which is a model of this illustration of magnetic Variation at various places on the planet.

 

You can select the output format of its derived bearing(s). You can order it to display bearings in either True or Magnetic (or grid)

 

Cheers, The Forester

[Deego]

Whoosh

[+kbootb]

Thanks to The Forester as ever for the completeness of his reply, really useful and the map is great, I actually needed that for something else.

 

Thanks to all the other replies. I have now found all the relevent settings on the GPS. More to the point I have now found the GPS. The other half of the team had tidied it into a drawer she has never tidied it to before.