Heading ?

[Deleted account W]

In units setup on my Quest there is an option for Heading.

True, Magnetic, Grid or User.

I've got mine set on True.

Is this what I should be using (City Select Europe V7) ?

When would you need to use the others?

I'm also using the Datum WGS84. Whats all that about?

I was just wondering

 

[Mouse]

The WGS84 (World Geodetic System) datum is a mathematical model of the shape of the Earth - which isn't spherical. It's a "best fit" approximation to allow the coordinate system (lattitude and longitude) to be used with reasonable accuracy over the entire surface of the Earth.

One other datum you might want to use is OSGB1939 (I think that's the name, something like that anyway) which is a model that only covers the UK (or rather it only accurately models the Earth's surface in the vicinity of the UK). This is the datum used for the British National Grid (BNG) coordinate system, as used on OS maps etc.

I think if you were trying to find your position on an OS map, you'd tell the Quest to use the OSGB1939 datum and set its coordinate system to BNG - if it supports this. MapSource does.

 

[Clive]

MapSource does.

You use MapSource ?

I'd have thought you'd knocked up a DIY version, made up of a papier mache scale model of the globe, some pins, lengths of string, a ruler and some conversion charts

 

[Mouse]

I did but I found it a bit tricky to use on the bike at over 25mph.

 

[Mouse]

By the way the BNG datum is actually called OSGB36, I thought I was wrong about it earlier.

 

[Fanum]

True, Magnetic, Grid or User.

I've got mine set on True.

That bit's about the different 'Norths' that you could use.......

Grid North, Magnetic North & True North

There are three norths commonly in use in Great Britain:

Grid North: the direction of a grid line which is parallel to the central meridian on the National Grid.

True North: the direction of a meridian of longitude which converges on the North Pole.

Magnetic North: the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain.
Differences

The horizontal angular difference between True North and Magnetic North is called MAGNETIC VARIATION or DECLINATION. The horizontal angular difference between Grid North and Magnetic North is called GRID MAGNETIC ANGLE. It is this angle which needs to be applied when converting between magnetic and grid bearings.

Differences between the three norths are given on our 1:50000 scale OS Landranger® and OS Explorer 1:25000 maps.

On OS Landranger Maps, the difference between True North and Grid North is given for each corner of the map and is shown in the legend as is the difference between True North and Magnetic North. The information is shown diagrammatically in the middle of the North margin. In both cases the year for which the value was compiled is given and once its value has been corrected for the current year, its value can be accepted for the whole sheet.
Calculating Magnetic Variation

Example - Penzance

The Magnetic North to Grid North is given as 3 deg. 30' W in 1991 decreasing by about 30' in three years. Therefore the difference is 3 deg. 00' in 1994.

The difference between True North and Grid North at the sheet corners are given and, by interpolation, the difference at Penzance, (approximately central to the map) is 2 deg. 40' E.

Therefore the difference between Magnetic North and True North is 3 deg. 00' plus 2 deg. 40' equals 5 deg. 40' in 1994.
Example - Lowestoft

Magnetic North to Grid North is given as 6 deg. 30' W in 1994.

The difference between True North and Grid North at the sheet corners are given and, by interpolation, the difference at Lowestoft, which is close the SE corner, is 3 deg. W.

Therefore the difference between Magnetic North and True North is 3 deg. 30'

 

[Deleted account W]

That bit's about the different 'Norths' that you could use.......

Yep, I gathered that even with my mid-morning post red wine hangover haze.

But which one should I be using

 

[Fanum]

True north is as good as any.

It's irrelevant really unless you're only using paper maps and you're navigating across the Gobi with only a bearing and distance as your target.


If you get up beyond the arctic circle, you'll see your diplay rotate a teensy bit if you switch between them but down here it ain't worth losing any sleep over

 

[(RIP) Kaister]

Grid North: the direction of a grid line which is parallel to the central meridian on the National Grid.

Does that mean all the power lines and pylons are arranged in parallel lines ?

 

[Pukmeister]

If navigating by map and compass, you can use a 1.5 degree error correction rule to go between magnetic and grid north, using the follwing analogy:

Grid to mag: Add
Mag to Grid: Get Rid

It gets you close enough for Government work anyway......