Distance estimation and Dead Reckoning

[JohnHuth]

Hi - I'm doing a freshman seminar called "Primitive Navigation" - some people expressed interest in this.

I posted a first set of slides on distance estimation and dead reckoning (mainly for the hiker, we'll get to dead reckoning on the water in future meetings). I'd love some feedback, if you have the time and inclination.

If you go to:

http://huhepl.harvard.edu/~huth/

and click on primitive navigation, you'll get the powerpoint slides.

[rcohn]

I posted a first set of slides on distance estimation and dead reckoning (mainly for the hiker, we'll get to dead reckoning on the water in future meetings). I'd love some feedback, if you have the time and inclination.

Very interesting. I assume your visual references for judging distance are based in 20/20 vision? Some of us may overestimate distances using those values.

Ralph

[JohnHuth]

Yes, those values tend to be for the best conditions - most things will degrade the visual acuity - so those are ideal.

You didn't hear some of the words I gave with the photos, but in some cases, my daughter was closer than an indicated distance (800 meters) and because she was back-lit, she seemed further away.

[paulariegel]

Hi John,

Thank you for sharing this thought provoking presentation. Many wonderful bits of information and fodder for discussion.

After reading the presention, I was curious why, on the 1st slide, dead reckoning would be considered the simplest and most common means of navigation. Wouldn't piloting also be considered a simple and comon means of navigation since it involves using landmarks? The person navigating, of course, would have to know what the landmarks are either from map, chart or other source (i.e. local knowledge). From my general experience on land and sea, most people will use a combination of the two. Straight dead reckoning ususally comes in when there's a significant lack of visibility (i.e. fog or nighttime) or when out of site of any landmarks (e.g. major ocean crossings) . Even when hiking out in woods without a map and no trail and wanting to get back to starting point, I will make mental notes of landmarks passed. (E.g. I passed by that funky looking rock formation going out, so if I pass by it coming back, I know I'm heading in the right direction).

Thanks again for sharing this.

Peace & Cheers,

Paula

[JohnHuth]

Hey, Paula -

You raise a good point and it's certainly something one can debate. The name of the course is "primitive navigation" - but then an issue comes up as to "what environment"?

Out in the ocean, Polynesians had a very funky (to our thinking) way of navigating. They would mentally keep track of where they were between their departure location and destination island by imagining some third island and how it moved across the "compass points". In this case the compass points were defined by the azimuth of rising and setting stars. This is called a "sidereal compass" The only way they could really do this tracking was to have some sense of time, their speed and the direction of travel (not to mention currents and leeway). That, to my mind is dead reckoning for sure.

The same thing could probably be said of the Vikings, who practiced latitude sailing - finding a location on the Norwegian coast that corresponded to a latitude of, say, Iceland, or the Faroe Islands and then sailing due west for some length of time. I'd also call this dead reckoning, although the Vikings did correct their course from the noon-time height of the sun, and zenith stars. So, mostly dead reckoning.

On land, many "primitive" cultures again used time, speed and direction as a way of traveling - e.g. the Taureg through parts of the Sahara.

Now, I'll undermine my own argument by saying that a lot of Inuit navigated by landmarks, as you say, and a lot of coastal travel was done in primitive societies by coast-hugging.

So, it's certainly debatable, and maybe I shouldn't have put it in the most simplistic terms, since it really depends on the environment, the availability of maps, knowledge of landmarks, whether you're on the open ocean and don't have access to a sextant and watch, or it's cloudy, etc.

But, thanks for asking! It's a good point.

John

[EEL]

John:

Might want to make a distinction between dead reckoning and piloting. Believe that would separate getting for A to B based upon watching "landmarks" and estimating position based upon time, speed, course, and point of origin.

Also the distinction between declination and deviation regarding use of compasses.

Without hearing what is said as the slides are shown, it it i hard to comment, but discussing declination in conjunction with taking a bearing on a physical object to which you wish to travel confused me. No need once you have the object in sight to worry about heading being based on magnetic north it seems to me or in selecting intermediate objects. Maybe (or likely) I missed the point.

Ed Lawson

[JohnHuth]

Ed -

Yes, you have to be a bit careful about the words I use (but you couldn't hear). I was trying to develop this part of the course by using hiking/walking as the primary example for the students. I will get into navigation on the seas in later lectures, where currents become important. This just reduces the number of things to worry about in the beginning. So, the term "piloting" doesn't really enter into this particular discussion. I was trying to pare back the concepts to hiking to reduce the amount of stuff they had to deal with in this early lecture.

There's are also some words about "variation" and "deviation" that I wanted to emphasize. "Declination" is a term that's also used for sun, so I don't like "declination". In terms of deviation, I wanted to point out that local magnetic sources can throw off compasses. Declination is typically a land-topo term, and variation is more common in marine applications.

In the idea of sighting objects to go on a heading my point was this - you figure out your heading with a compass and then walk toward that object, like a tree. If you spend your time walking with a hiker's compass in your hand, it's far less accurate than if you sight an object on your heading and then move toward it (from personal experience, I can tell you!). The idea of lining up multiple objects on a heading reduces the need to keep going to the compass.

As it turns out, all humans will walk in a circle without landmarks, or a compass or some kind of direction finding. If you put someone in a flat, unremarkable environment - like a flat pine forest, people will walk in a circle of a radius of between 1/2 and 2-3 miles. I don't know if this would be true of someone paddling in a flat sea in the fog, but I imagine something like that might happen.

[EEL]

As it turns out, all humans will walk in a circle without landmarks, or a compass or some kind of direction finding.

John:

Glad to know I'm normal even if I tend to go in circles even when sitting still.

OK on finding objects with compass and then walking toward them or intermediate points.

Off trail/Cross country stuff can get very interesting depending upon terrain and underbrush.

I found doing so in White Mountains very different from more open forests and tundra out west where things can be more similar to being on ocean in terms of finding ones position and selecting a bearing. Here the intermediate points can get very close, there you can walk for an hour or more between them. I sometimes think many hikers do not practice these skills here because it is so hard to travel off the trails and the trails are so well documented/marked you have to work to get lost. Unfortunate I think. In places like Wind River range in Wyoming, trails can be very vague and keeping track by map and compass can be important.

Ed Lawson

[JohnHuth]

Yes, I think the words I said were important - the bullets were merely aids in the discussion.

I also wanted to make clear the distinction between the following terms:

bearing

heading

declination/variation

deviation

azimuth - both in terms of celestial and terrestrial coordinates

(and the fact that declination has a double meaning).

[JohnHuth]

In case you're interested, I've posted some more presentations. One is a revised version of weather and waves that takes into account Polynesian navigation, and wind directions. Also, there's a presentation on celestial coordinate systems. I'll be adding stuff as the course progresses.

http://huhepl.harvard.edu/~huth/Primitive%20Navigation.html

Of course, the words that go along with the seminars may be more important than the slides.

[EEL]

In case you're interested, I've posted some more presentations.

Thank you for making these available. Already downloaded for repeated study over the winter.

I just glanced through them and wonder if you describe in adequate detail (for a math challenged simpleton) how to determine Lat. and Log. from observation of stars and clock. I have been meaning to do more work with the training sextant I received as a gift and this may spur me on.

The slide on winds from high to low pressure shows arrows, but the arrows are to show the change in velocity and not the direction of the wind. Correct?. Which leads me to ask that if the wind direction is essentially parallel with the isobar lines, then how do you know in which direction the wind is blowing? I assume it is a function flow around the pressure areas, but have never plotted it out.

Ed Lawson

[spuglisi]

In case you're interested, I've posted some more presentations. One is a revised version of weather and waves that takes into account Polynesian navigation, and wind directions. Also, there's a presentation on celestial coordinate systems. I'll be adding stuff as the course progresses.

http://huhepl.harvard.edu/~huth/Primitive%20Navigation.html

Of course, the words that go along with the seminars may be more important than the slides.

John,

Thanks for posting the files. I'm sure that I'm not alone in finding this stuff really interesting and of course, useful.

[JohnHuth]

Latitude, longitude from clock and angles will be in another presentation. I just wanted to give them an overview of celestial coordinates.

[Rob Hazard]

John,

I am enjoying going through all this material you've posted.

The use of the term "declination" always troubles me though. Years ago, maybe in Boy Scouts in the early 60's, I was taught that declination, from the verb decline, meaning to bend down or turn down, actually referred to the vertical component of the earth's magnetic field, that is, that the closer you moved toward the Magnetic North Pole, the more your compass needle would be drawn to point downwards.

Perhaps early explorers of the Northwest Passage found that their compasses would jam if held level, and had to be held at an angle to turn freely...

Now I find that one brand of hiker's compass actually has a variation scale labeled "Declin". It sets my teeth on edge!

Rob

[JohnHuth]

The term "declination" is also used to denote the position of a star above or below the celestial equator - like a celestial latitude. I like to use "magnetic variation" instead of declination for both reasons.

[JohnHuth]

I posted some new stuff on navigation by the sun (mostly) and moon. If any of you want to act as a guinea pig and give me feedback, I'd appreciate it. My seminar is on Thursday.

http://huhepl.harvard.edu/~huth/Primitive%20Navigation.html

[jdkilroy]

The terms Variation and Declination seem to come into some confusion when folks go between maps (usually representing land) and charts (usually representing navigable areas of water). (In deference to John, we'll exclude celestial navigation for now)

Land-based maps use the term "declination" whereas charts will use the term "variation." Both are referring to the difference between magnetic north and the geographic meridian (true north) at a given location. So you will hear land surveyors speak of declination and navigators at sea speak of variation; they are both referring to the same thing.

Hope that helps. Jon

[Bill Gwynn]

In nautical navigation, magnetic declination is divided into two parts, namely Magnetic Variation and Magnetic Deviation. There are also three types of bearings--True, Magnetic, and Compass--which are related by the rules:

Compass Bearing +/- Deviation = Magnetic Bearing

Magnetic Bearing +/- Variation = True Bearing

This relationship (finding what the compass should show when the true course is known) is frequently taught as:

T = true course;

V = variation (of the Earth's magnetic field);

M = magnetic course (what the course would be in the absence of local declination);

D = deviation caused by magnetic material (mostly iron and steel) on the vessel;

C = compass course.

A mnemonic for this is:

"True Virgins Make Dull Company".

To remember that West Variation needs to be added as the "cure" for this condition,

"Add Whiskey," remembering that whiskey stands for west.

If one knows the course shown by the compass and wishes to find the course relative to true north, the steps are inverted and the signs of deviation and variation inverted.

Another method for going the other way (compass bearing to true bearing) is CADET.

C = compass course;

A = add;

D = declination;

E = east;

T = true course.

Variation

Magnetic variation is the difference between True Bearings and Magnetic Bearings and is caused by the different locations of the Geographic North Pole and the Magnetic North Pole plus any local anomalies such as iron deposits. Variation is the same for all compasses in the same location and is usually stated on good quality maps and charts, along with the date it was measured.

Deviation

Magnetic Deviation is the difference between Magnetic Bearings and Compass Bearings. Deviation varies for every compass in the same location and depends on such factors as the magnetic field of the boat, wrist-watches, etc. The value will also vary depending on the orientation of the boat. Magnets and/or iron masses can be used to correct for deviation so that a particular compass will accurately give Magnetic Bearings. More commonly, however, a correction card will be drawn up listing errors for the compass which can then be compensated for arithmetically.

[jdkilroy]

Hi Bill,

I could be mistaken but I'm not aware of any use for the term "Declination" at sea.

There is Variation and Deviation. Compass Error is the sum of those two. On land, (surveyors and topo maps), Variation is substituted with the term Declination with the meaning/definition being identical.

Deviation is something kayakers might have to deal with if you were to pack your boat in such a way as to have metal/electronic objects close enough to your compass to affect it. Even stainless can cause a deflection.

Jon

[JohnHuth]

Yes, declination is commonly used on "land" maps to indicate what mariners call variation.

I just posted some new slides on the website, for a seminar called "planets, airplanes, birds', and there's also mention of a strange phosphorescence phenomenon that was observed by a guy named David Lewis!

John

[JohnHuth]

I posted some new slides on the website. These are on latitude and longitude, using various means (not GPS).

Generally, the sea kayaker isn't interested in such things. But, with a Timex Ironman watch and with a quadrant made from stuff lying around the house, a precision of about 30-40 nm is possible. Without solar tables, I could do maybe 80 nm in latitude, but I was able to still get about 30 nm in longitude, using my watch, and no tables.

It was a fun exercise, although I don't think many of you will be needing to do this any time soon.

John H.