• Welcome to The Bushcraft Forum

    You are currently viewing the site as a guest and some content may not be available to you.

    Registration is quick and easy and will give you full access to the site and allow you to ask questions or make comments and join in on the conversation. If you would like to join then please Register

Compasses - Advice needed

A question regarding storage: I've seen some info saying not to store a compass next to ferrous metals. Will it really damage it if it's stored in my pouch with folding knife, ferro rod and DC4?
 
Could interfere with the accuracy Paul, especially the folding knife unless it’s nonmagnetic.... best away from all metal. Top outside pocket, am sure I read somewhere that the static between clothes can affect it also.... my memory is keek so it’s not gospel lol
 
OK so I had another go at this but keep finding I'm an hour out.

I suspect that's something to do with the fact that were only a couple of weeks away from BST? Can anyone advise on this?

Here's my method:

Calculating the time using a compass on a sunny day
  • Find a vertical shadow
  • Align the compass along the edge of the shadow
  • Adjust the dial so that the needle points to North
  • Adjust for declination. E.g.
    • for 4deg W then adjust so that the north needle points to 356deg
    • for 4deg E then adjust so that the north needle points to 4deg
  • Read the azimuth e.g. 141deg
  • Subtract azimuth from 180deg. E.g. 180-141=39
  • Divide this by the hourly degrees of the sun’s travel (360deg/24hr=15deg/hr)
    • E.g. 39/15=2.6 hours
      • If azimuth is less than 180 then this means 2.6 hours before 12:00
      • If azimuth is greater than 180 then this means 2.6 hours after 12:00
  • 2.6 hrs before 12:00 = 09:24
  • Actual time was 10:25

Another attempt at actual time 10:51
With the dial set to adjust for 4deg W declination
Azimuth is 148
180-148=32
32/15=2.13hours before 12:00
= 09:54 (actually 10:51)

20210313_105126.jpg
 
Well I got responses from both Tristan Gooley and Paul Kirtley on this question (Dave Canterbury has not replied yet) and got a helpful response from one of the guys on the Astronomy Scotland Facebook group.

One of the main points to note is that solar noon, as the sun is crossing the meridian at 180deg, is not 12:00 but today was 12:24 here.

Therefore taking an azimuth bearing on the sun and adding the hours difference to 12:24 should be closer to the actual time.

at 17:34 today I read an azimuth of 260deg to the sun.
That's 80deg past 180deg (due south).
80/15=5.33hours = 5h20m
Solar noon today was 12:24 + 5h20m = 17:44

Only 10 mins out but I'm not sure if I should be adjusting for a declination difference which would have to be 2.5deg (Az 257.5deg) to achieve a calculation of 17:34
 
I was actually (originally) adjusting for 4deg longitude but it seems the declination currently here is about 1.79deg
So, using that adjustment from the 260deg
That's 78.2deg past 180deg (due south).
78.2/15=5.21hours = 5h13m
Solar noon today was 12:24 + 5h13m = 17:37 Only 3 mins out!
 
Impressive stuff Paul......glad you solved the things that were going wrong with your first calculations...... and cracked it :thumbsup:
I guess the more times you do it now.......it will become second nature.
 
Yes, I will need to do it at lots of different times of day to ensure that it's always accurate(ish). If it's reliable I'll do a wee video.

It does rely on knowing the magnetic declination of the area you are in and also the solar noon for that day, both of which would be unlikely if you were in a survival situation in an unknown area. But in that situation you probably don't need to know the precise time anyway.
 
Seems a lot of work Paul, just to find out when it’s rum’o’clock 🤷🏼‍♂️

it’s interesting Paul, I had a little nose on the web and I will admit it’s over my head, not something I’ve bothered to study.... might give it a go tho as am impressed with your progress mate 👍🏻
 
I've created a couple of lookup tables to save trying to do the degrees/15 and the decimal conversion to minutes maths in the field.

IF the sun comes out today I'll have another go
 

Attachments

  • Compass Time Lookup Tables.pdf
    89.3 KB · Views: 134
Suns oot paul......🌅....now all you have to do is find a place that's quiet.....passed bumper to bumper cars parked on single track road yesterday🧭🧭🧭🧭
 
One other correction I've had to make is that the sun's apparent motion is a little over 15deg per hour since the length of day is 23hrs and 56mins making the angular motion around 15.04 degrees per hour.
Using this adjustment on my tests this morning makes the error between calculated time and actual seems to be consistently about 11 to 13 minutes which is probably down to the inaccuracy of reading the azimuth from the compass.
 
Of course there are a few fairly major flaws with the set up of this which are prerequisites for this working accurately:
  • First, you must be doing this on a level surface (does anyone carry a spirit level? :whistle: )
  • Second, the shadow you're using must be from a completely vertical object. Plumb line is easiest outdoors, unless it's windy.
  • Third, you need to know the time of solar noon at your location
  • Fourth, you need to know the magnetic declination at your location
  • Fifth, this will give the time in UTC and you'll need to adjust by an hour if you're in summer time
Of course, if you're only looking for an approximate time of day it should work reasonably well.
 
  • Like
Reactions: Bam
I just noticed that Asda (aka Walmart) have Silva style compasses in for £5. I have bought cheap Silva clones before and the only thing to check is the friction when you turn the compass housing - too lose and they move when you don't want them to. Other than that they work fine.
 
I just noticed that Asda (aka Walmart) have Silva style compasses in for £5. I have bought cheap Silva clones before and the only thing to check is the friction when you turn the compass housing - too lose and they move when you don't want them to. Other than that they work fine.
Personally, I love magnetic compasses. Funny how the Silva compass design hasn’t changed in decades…

These days with everybody following GPS directions, navigating with a compass and charts is a dying art.

Funnily enough it’s still the basis for aviation navigation training. Younger people, used to smart phone technology, really struggle with going back to paper aeronautical charts and slide rule. Of course, once they have their ticket, everybody just ‘follows the magenta line’. 😂

I can’t speak for Silva compass design or quality. Haven’t seen one in years. But they were the staple of every school geography class, Boy Scout navigation exercise and Duke of Edinburgh award expedition.

Awesome. Just awesome.

Alan
 
Personally, I love magnetic compasses. Funny how the Silva compass design hasn’t changed in decades…

These days with everybody following GPS directions, navigating with a compass and charts is a dying art.

Funnily enough it’s still the basis for aviation navigation training. Younger people, used to smart phone technology, really struggle with going back to paper aeronautical charts and slide rule. Of course, once they have their ticket, everybody just ‘follows the magenta line’. 😂

I can’t speak for Silva compass design or quality. Haven’t seen one in years. But they were the staple of every school geography class, Boy Scout navigation exercise and Duke of Edinburgh award expedition.

Awesome. Just awesome.

Alan
From my time in the military I've always used prismatic compasses including ones using mils as opposed to degrees. We were doing radio shots which had to accurate to the enth degree and something like a silva couldn't give us the required accuracy so for me it's prismatic every time
 
Back
Top