Hi, Does anyone know if there is a simple method to calculate a reasonable approximate distance from oneself to a distant landmark using just a silva type 4 compass only (with no map or other tools), is there a simple formula (or not) that can be used in the field that doesnt require excessive maths?, thankyou. --- I have heard it can be done somehow using your thumb but wondered if it was possible to do so using a compass, the thumb method seems to require the known size of objects to begin with unless Ive misunderstood it.
calculating distance with compass only
- Thread starter Joonsy
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given half an hour, a short walk, pencil, paper, and calculator (as well as a compass, preferably a prismatic one) i could get a relatively accurate idea of distance, but there'd be a fair bit of head scratching involved and i really wouldn't be too trusting of my results. as for working out distances using just a compass (and maybe a thumb 
) i'd love to be told of a method, but i'll be surprised if there is one
sorry for being no help at all
stuart
) i'd love to be told of a method, but i'll be surprised if there is onesorry for being no help at all
stuart
maybe that's the case, all replies are helpful anyhow if only to dismiss the idea, thanks.
good point but i would only require reasonable estimation, but the more contour lines the less accurate obviously and hence less useful
Joonsy,
The "thumb shift" method works for known size objects that are in the medium distance range - say a few feet to a hundred yards or so. You hold out your thumb and by closing first one eye then the other, estimate the apparent side-shift distance - multiply this by a factor of 8 (though each person will be slightly different) to get the range to the target - it relies upon the correlation of distance between your eyes and from your dominant eye to your thumb which is somewhere near 1:8 ratio.
Using a compass would necessitate some ultra accurate bearing reading and a good dollop of trigonometry. You'd have to take two bearings to the "distant object" moving along a line perpendicular to one of the bearing lines - building a right angled triangle. The two angles would need to be subtracted, then calculated with the distance you moved sideways to get an approximation as even tiny fractions of a degree error would mean huge errors in the predicted distance. I suppose if you were to get somewhere towards the 60 to 90 degree intersection you may get something reasonably accurate - but you'd end up knackered from making all the bearing taking rather than actually closing on your target!
You can make a "range card", calibrated to yourself to indicate the size of common items at known distances (person, vehicle, building etc) but it is something else to loose.
ATB
Ogri the trog
The "thumb shift" method works for known size objects that are in the medium distance range - say a few feet to a hundred yards or so. You hold out your thumb and by closing first one eye then the other, estimate the apparent side-shift distance - multiply this by a factor of 8 (though each person will be slightly different) to get the range to the target - it relies upon the correlation of distance between your eyes and from your dominant eye to your thumb which is somewhere near 1:8 ratio.
Using a compass would necessitate some ultra accurate bearing reading and a good dollop of trigonometry. You'd have to take two bearings to the "distant object" moving along a line perpendicular to one of the bearing lines - building a right angled triangle. The two angles would need to be subtracted, then calculated with the distance you moved sideways to get an approximation as even tiny fractions of a degree error would mean huge errors in the predicted distance. I suppose if you were to get somewhere towards the 60 to 90 degree intersection you may get something reasonably accurate - but you'd end up knackered from making all the bearing taking rather than actually closing on your target!
You can make a "range card", calibrated to yourself to indicate the size of common items at known distances (person, vehicle, building etc) but it is something else to loose.
ATB
Ogri the trog
see, told you i'd be scratching my head a lot, and that i wouldn't be trusting my results, ogri's done a great job of explaining why
thanks ogri the trog, the fact you have to know known sizes using the thumb method is why i was trying to find an alternative method, for instance if i could see a distant rockface and wanted to know how far away it was then using the thumb method i would either need to know the approximate height or width of rockface to begin with to find it's distance (if i understand correctly) but to know those sizes would be impossible to begin with, thanks for reply Ogri.
in respects to what Ogri said i've been looking at the method on this link i've attached however to use the method in link you presumably would have to have a table of 'Tangents' with you in the field which would be unlikely http://www.zoom-one.com/distance.htm
You could always look at the map!
yes probably the best advice so far
thanks i'm just inquisitive.
Food for thought....
http://www.vendian.org/mncharity/dir3/bodyruler_angle/
http://www.vendian.org/mncharity/dir3/bodyruler/
.. some distance estimation can be a bit of fun but someone always wants to be "more accurate"!
Ogri the trog
http://www.vendian.org/mncharity/dir3/bodyruler_angle/
http://www.vendian.org/mncharity/dir3/bodyruler/
.. some distance estimation can be a bit of fun but someone always wants to be "more accurate"!
Ogri the trog
Here is a method I sometimes teach the students on my courses:
The object of this process is to determine the range to a known point.
In order to accomplish this we need the target to be in visual range and furthermore we need knowledge of the physical size of a prominent feature near the target.
1: Tie a piece of string (50cm long) to your compass.
2: Use the compass to measure the apparent "length" (in mm) of some feature of known size at the point to which the distance is to be measured. Hold the compass exactly 50 cm from your eye.
3: Mulitply the number of mm (from above sightning) by 2
4: Divide the lenght (or width) (in meters) of the known feature by the number achieved in (3)
5: The result is the range (in kilometers) to the target
Explanation:
The above process is actually a variation of a artillery range-finding procedure from "old times". It uses the concept of "angular mils"
An angular mil is the angle which precisely can cover a length of one meter at a distance of one kilometer.
By sightning with the compass we can measure the angular mills - knowing that one mm equals two angular mils (provided that the compass is kept at a distance of 50 cm from the eye)
An example:
You observe a tent across a valley and would like to know how far there is to the tent:
Firstly, measure the apparent angle (in mm) of the tent = 2mm
Multiply by two the get the measurement in angular mils = 4 angular mils.
The length of the tent is determined to be 4 meters.
Then divide this length with the number of angular mills: 4 meters / 4 angular mils = 1 kilometer.
//Kim Horsevad
The object of this process is to determine the range to a known point.
In order to accomplish this we need the target to be in visual range and furthermore we need knowledge of the physical size of a prominent feature near the target.
1: Tie a piece of string (50cm long) to your compass.
2: Use the compass to measure the apparent "length" (in mm) of some feature of known size at the point to which the distance is to be measured. Hold the compass exactly 50 cm from your eye.
3: Mulitply the number of mm (from above sightning) by 2
4: Divide the lenght (or width) (in meters) of the known feature by the number achieved in (3)
5: The result is the range (in kilometers) to the target
Explanation:
The above process is actually a variation of a artillery range-finding procedure from "old times". It uses the concept of "angular mils"
An angular mil is the angle which precisely can cover a length of one meter at a distance of one kilometer.
By sightning with the compass we can measure the angular mills - knowing that one mm equals two angular mils (provided that the compass is kept at a distance of 50 cm from the eye)
An example:
You observe a tent across a valley and would like to know how far there is to the tent:
Firstly, measure the apparent angle (in mm) of the tent = 2mm
Multiply by two the get the measurement in angular mils = 4 angular mils.
The length of the tent is determined to be 4 meters.
Then divide this length with the number of angular mills: 4 meters / 4 angular mils = 1 kilometer.
//Kim Horsevad
Hi Kim Horsevad, thankyou very much for your information, I have read it carefully. In your example I presume the tent stated is already known in advance to be 4 meters in length (and is not the 4 angular mils when you multiplied apparent angle x 2), if for example that same tent was known to be 7 meters wide instead would the distance to that tent using the same formula of length divided by mils which would then be 7 / 4 = 1.75 kilometres distance to tent, or if the tent was 13 metres in length then the distance would be 13 / 4 = 3.25 kilometres distance to tent --- in other words you are not calculating the length of tent using angle/mils but actually already know it’s length, and hence you would have to know the size of a particular object in view in advance in order to calculate it’s distance from you.
Yes, by knowing the size of that object you are "shortcutting" the otherwise needed trigonometry (which in itself is rather useful and interesting, but sometimes is a little to daunting for the students)
The system was originally used by artillery officers, so they would know the sizes of the enemy vehicles (for instance) - thereby enabling a very fast and quite accurate range measurement.
//Kim Horsevad
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