Author Topic: bearing/GPS/Map differential  (Read 2296 times)

which way

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bearing/GPS/Map differential
« on: April 14, 2016, 05:37:12 PM »
Hi all,
Just been playing with my Satmap and compass.
took a bearing using my compass from SH 6723 TO SH 6038.
Made sure everything was lined up,  and my reading was 056 degrees, yet mySatmap tells me it's 051 degrees.
The map I am using is dated 2010 and if my calculations are correct I think adding 1 degree for declination is correct.
Do I need to be at the location for my Satmap to be correct or am I doing something wrong?

« Last Edit: April 14, 2016, 07:31:43 PM by which way »

ianj37

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Re: bearing/GPS/Map differential
« Reply #1 on: April 16, 2016, 07:17:40 PM »
Hi

Interesting! I've tried the equivalent of what you did (but used a different grid ref) with my SatMap 12 and an Silva Explorer compass and I get only a 2 degree difference (incidentally SatMap Xpedition mapping is 1 degree out from the GPS). BUT -

1) I would have said 5 degrees difference is acceptable when comparing SatMap with a compass as you're not really comparing 'like with like' and as far as I know variation between compasses can be up to 3 degrees. When doing what you did it's also important to make sure as far as you can that you are comparing like for like by, for example, not increasing the magnification on the SatMap above that of the actual map and using the same scale on SatMap as the paper map you're using.
2) I guess you've done this but check that the compass setting on the SatMap are what you expect them to be.
3) I think that maybe you've only given us half the grid ref. If not then you will get major inaccuracies as the grid ref you're giving is for a 1km square.

As far as I can see you don't need to be at the location for SatMap to be correct using the 'set marker' function and then using the joy stick to get to the position to take a bearing to(I'm assuming you did it this way) - mine was OK and I used an area on Kinder Scout and I'm 30 miles as the crow flies from there.

Hope this helps.

which way

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Re: bearing/GPS/Map differential
« Reply #2 on: April 16, 2016, 09:08:23 PM »
Thanks for your help, shows you can always learn. I honestly believed it would make no difference what scale map you use when taking a bearing. Thought,  no matter what scale a map was, the subject was in exactly the same direction.
You are totally correct when you said I took the reference from a 1:25000 map. My Satmap was in 1:50000 mode and set my marker via the joy stick.
If I am in the hills with my comfort blanket (Satmap 10) running 1:50000 mapping, and using a 1:25000 map to navigate but felt lazy. I used the joy stick from my fix to my destination and noted the bearing (do not like using electronic compasses) and set my base plate compass to that bearing, would it be wrong?
Once again thanks for your informative answer.

ianj37

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Re: bearing/GPS/Map differential
« Reply #3 on: April 17, 2016, 02:29:22 PM »
I think I may have confused you rather than helping.
What follows may or may not help!!
Things are always in the same place geographically but the 25k map will show more detail on natural features than the 50k so the potential is that the points on the maps  that you use to line up may be different. Did you sign up for SatMaps Xpedition software? If so try this which I think illustrates my point. With the map at 1:50k take a bearing (ie plan a route as a strainght line from the tip of the spire on the church symbol in Edale SK12314 85808 to the start of the stream at SK 10147 87338 - which I think is 305 deg. Now change the scale of the map to 1:25k and you will find that whilst the church is in the same position because it is what it is and a symbol can't be shown in more detail, at the other end of the route the stream is now shown in more detail with several tributaries and the end of the route which was on the end of the stream at 1:50 is now below it.

When using the bearing from the SatMap you need to be sure that before you transfer it to your compass you know what it is telling you degrees M is magnetic, degrees G is grid. That said I'm not sure how reliable the SatMap conversion is as where I am the declination is 1 degree 59 minutes but the difference in SatMap is only 1 degree, whereas I would have rounded up to 2 degrees. Also bear in mind that walking on a bearing is not 100% accurate and is worse the longer you make the stages- I think on the Mountain Leadership exam  requires 2% accuracy but I may be wrong. 

captain paranoia

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Re: bearing/GPS/Map differential
« Reply #4 on: April 17, 2016, 07:54:23 PM »
Going from 6723 to 6038 (in that direction), you are heading 7km W and 15km N. Using grid angle, this is 335 degrees. (I calculated it using 270 + arctan(15/7))

That's assuming you're using 4 figure (1km) grid reference.

However, like Ian, I suspect you're not giving us the full GR, as you'd need a big compass to measure that bearing, and it comes out nowhere near either of your bearings...
« Last Edit: April 21, 2016, 01:53:44 AM by captain paranoia »

which way

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Re: bearing/GPS/Map differential
« Reply #5 on: April 18, 2016, 09:37:21 AM »
Hi to Ian and captain,  First of all thank you both for your replies.
Ian, I think I get a grasp of what you are saying.  If I am going to cross map for the sake of taking a bearing, ensure it's to a features that really cannot be added to by using an higher scale map. Is that correct?
Captain,  I would not be surprised if I got the ref wrong, if I have, I will of still learnt a valuable lesson,  DOUBLE CHECK!! Thank you. When I get a chance I will have another look.
Never the less, I have still gained food for thought from Ians replies.
Once again thank you both.   

which way

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Re: bearing/GPS/Map differential
« Reply #6 on: April 18, 2016, 11:49:50 AM »
How embarrassing is that cock-up? Total brain f... , and I could not see the trees for the forest.
I did of course intend to send two x 8 figures ref, when instead I sent one. Which looked like two four figure refrences, no wonder I baffled the captain.
Seriously though, I do apologise for been such a numpty and would like to thank the captain and Ian for being so patient and assure them that I have learnt something.
 :-[

captain paranoia

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Re: bearing/GPS/Map differential
« Reply #7 on: April 18, 2016, 01:33:47 PM »
No worries. It's always useful to everyone to think through challenges like these; we all learn something.

The other thing to consider is the distance between the two points; if they're very close together, then any small errors in choosing end points on the map (such as the church symbols Ian mentions) will cause a larger angular error; that's simple geometry. The larger the distance between the points, the more accurate the bearing measurement is likely to be (given identical error bounds on the end points).

To illustrate this, you can draw error circles at the end of lines of different lengths, and then draw lines between opposite sides of the error circles at each end of the connecting line, so that they cross in the middle. You'll see that the greater the separation of the end points, the smaller the angle between the crossed lines (which is the potential error angle).

ianj37

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Re: bearing/GPS/Map differential
« Reply #8 on: April 18, 2016, 04:15:53 PM »
As CP says not a problem - we all make mistakes.
I think the key thing is to get out there and relate the map and the SatMap with what you can see is there. Important to appreciate that nothing is 100% accurate - the GPS grid ref, the compass, your ability to walk on a bearing or measure distances in the field.
Also get to appreciate that even when using a 1:25k map a lot of ground features won't show on the map because they're too small. What focused my thinking was that a house from ground to ridge is a little under 10 metres so if you have a map that uses a 10m contour interval (1:50k and some 1:25k maps) then unless intermediate contours have been used (I think only Harvey maps use them to support their 15m contour interval) then you could come across a hill the height of a house which doesn't show on the map!

captain paranoia

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Re: bearing/GPS/Map differential
« Reply #9 on: April 18, 2016, 06:16:50 PM »
Attached is an exaggerated example, showing the error angles for two different distances, with identical error circles.


Lost Soul

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Re: bearing/GPS/Map differential
« Reply #10 on: April 18, 2016, 08:27:30 PM »
Hi Which Way,

Better to make errors in the relative safety of your own training environment and discuss them in this forum so we can all learn.  Than have a Coroner trying to second guess what went wrong, and probably getting it wrong too. 

Lost Soul.

which way

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Re: bearing/GPS/Map differential
« Reply #11 on: April 19, 2016, 09:12:42 AM »
To be honest CP you lost me a little with the graph.... could not quite grasp that. However,  overall I do get the general concept thanks to you and Ian.
Quite correct lost soul, better to ask questions in relative safety than wait until it's too late and wish you had asked.
That's always been my mantra,  so all you boffins please keep up the good work and be ready to help people like myself.
Once again many thanks

captain paranoia

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Re: bearing/GPS/Map differential
« Reply #12 on: April 20, 2016, 12:30:02 PM »
The diagram is trying to show attempts to plot a bearing of a 'leg' from a map, where there's uncertainty on the true position of points at each end of the 'leg'. This uncertainty can be represented as an error circle, which contains the 'true' point, somewhere within the bounds of the circle.

The red case shows a very short leg, and the blue case shows a longer leg. The error circles are the same on both cases.

The straight lines represent the edges of the compass in the worst case error situations, connecting the opposite outside edges of the error circles. One line takes one extreme pair, and the other line takes the other pair. The 'true bearing' lies somewhere between these two lines, so the angle between the lines is the potential error in reading the bearing.

You can see that the angle between the two red lines is much bigger than the angle between the two blue lines, showing that, with a short distance between the end points, the potential bearing error is larger. They're plotted to overlap in the centre, so the angles can be compared...

Then there's the error in aligning the compass bezel, but that's the same in both cases...
« Last Edit: April 20, 2016, 12:43:51 PM by captain paranoia »

which way

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Re: bearing/GPS/Map differential
« Reply #13 on: April 20, 2016, 11:48:31 PM »
You're good CP. I now fully understand the diagram and what it represents.
Thanks for taking the time to explain.