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bearing/GPS/Map differential

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which way:
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?


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:
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.

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:
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...


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