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  #1  
Old 11-24-2006, 10:31 AM
Ficer67 Ficer67 is offline
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Land Surveyors: How do you determine true north from solar observations?

Can someone direct me to a web page that details the procedure for determining true north by solar observations?



Thanks in advance

Ficer67
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  #2  
Old 11-24-2006, 11:37 AM
David Simmons David Simmons is offline
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I haven't done any surveying for years and never had to find true north when I did survey. There are a number of methods for finding true north. For most purposes I would suspect that surveyors use a compass and a chart of magnetic deviation (or is it called variation these days?) for the area.
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Old 11-24-2006, 03:09 PM
LSLGuy LSLGuy is offline
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Variation is the local difference between true & magnetic north.
Deviation is the instrumental calibration difference between where your compass should point (ie mag North) & where it does point (ie 1.5 degrees West of mag North becasue it's imperfect).
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Old 11-24-2006, 03:22 PM
Chronos Chronos is offline
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I'm not a surveyor, but I am an astronomer. The simplest way to get a precise answer would be to mark the position of the Sun at rising at setting, and take a line directly between those two directions. This will work best near the solstices: Near the equinoces, the azimuth will be changing relatively rapidly, and might be significantly different from morning to evening. Of course, there's also a problem if your eastern and western horizons aren't both flat, but you don't actually have to use rising and setting; any two points with the same altitude (which can be determined using a sextant or other astrolabe and an "artificial horizon", a reflecting pool of liquid) will work.

If you don't want to spend all day at it, a less accurate method is to just take a sighting of the Sun at one time. You can calculate or look up in tables the position of the Sun in the sky, for a given time of day, date, latitude, and longitude. This assumes, of course, that you already know your latitude, longitude, and time to high precision. And an even quicker and dirtier method is to note the position of the Sun at exactly noon, which will be due South (or North, if you're in the Southern Hemisphere).
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Old 11-24-2006, 04:15 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by LSLGuy
Variation is the local difference between true & magnetic north.
Deviation is the instrumental calibration difference between where your compass should point (ie mag North) & where it does point (ie 1.5 degrees West of mag North becasue it's imperfect).
I know. That's the way I was taught but I've read various sources who say that the terminology has changes.

The mnemonic I learned is "Can Ducks Make Vertical Turns?" C = compass reading. Apply D = deviation, to get M = magnetic heading and V = variation to get T = true heading.
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Old 11-24-2006, 04:36 PM
LSLGuy LSLGuy is offline
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I apologize for stating the obvious. I didn't notice it was you, somebody who'd know that inside out.

Well I'll be. I've not heard of a terminology change. Not to say there hasn't been one, or that surveying and navigation don't use different terms for the same thing.

The mnemonic we used was True Virgins Make Dull Company, ie True +/- Variation gives Magnetic +/- Deviation gives Compass.

I guess the better mnemonic depends on whether you're trying to go from chart to instrument, ie planning, or from instrument to chart, ie enroute.


I wonder what bland child-safe terminology the gender-neutral military uses now? Most of the female pilots I've met over the years would agree with the sentiment of "True Virgins ...", they'd just be referring to the wimpy guys they have to put up with.
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Old 11-24-2006, 07:47 PM
Bernard Marx Bernard Marx is offline
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Quote:
Originally Posted by LSLGuy
I wonder what bland child-safe terminology the gender-neutral military uses now? Most of the female pilots I've met over the years would agree with the sentiment of "True Virgins ...", they'd just be referring to the wimpy guys they have to put up with.
Here in Aust we still use the "True Virgins" mnemonic, although "Go Rub Your Balls With Grease" has recently been revised to "Go Rub Your Back With Grease"...to the disgust of many of the more "traditional" naval officers
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Old 11-24-2006, 09:54 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by LSLGuy
I apologize for stating the obvious. I didn't notice it was you, somebody who'd know that inside out.
No problem. It's been 61 years and I have been know to forget, incredible as that may sound.
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  #9  
Old 11-24-2006, 11:04 PM
spingears spingears is offline
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Quote:
Originally Posted by Ficer67
Can someone direct me to a web page that details the procedure for determining true north by solar observations? Thanks in advance Ficer67
As usual it all depends on the degree of accuracy desired or needed.
Magnetic declination is used for ordinary everyday applications. It varies over time and is not dependable for high precision observations.
High precision requires observations of Polaris, accounting for elongination and culmination differences.
For precision solar observations an accurate time piece and an astronomical almanac are necessary.
Last and not least is the know how.
Check out the "Bad Astronomy" web site for all things gastto....... err Astronomical.
Also Wikipedia.
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Old 11-25-2006, 01:09 AM
Richard Pearse Richard Pearse is offline
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Another quick dirty method involves the use of an analogue watch.

Point the hour hand at the sun, split the dfference between the hour hand and the 12 on the watch and that is north in the southern hemisphere.

I learned the "True Virgins Make Dull Company" one. The terminology has remained the same as far as I know.
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Old 11-25-2006, 05:57 AM
intention intention is offline
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Quote:
Originally Posted by 1920s Style "Death Ray"
Another quick dirty method involves the use of an analogue watch.

Point the hour hand at the sun, split the dfference between the hour hand and the 12 on the watch and that is north in the southern hemisphere.

I learned the "True Virgins Make Dull Company" one. The terminology has remained the same as far as I know.
Actually a very neat trick, but remember that it gives south in the northern hemisphere ...

w.
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  #12  
Old 11-25-2006, 01:16 PM
GusNSpot GusNSpot is offline
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Not much need to do this anymore. But if you have a lack of equipment and are on some deserted island that has never been surveyed and so have no 'known' points to tie to because you lack a goos GPS set up, just rent a little hand held GPS and pick a spot, then go to the same longitude at at a distance North to give you the base line you need and drag your chain and 'dumpy' level all over the place and have fun.

If you are just playing, use the wrist watch.

YMMV
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Old 11-25-2006, 02:06 PM
The Flying Dutchman The Flying Dutchman is offline
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Quote:
Originally Posted by Chronos
And an even quicker and dirtier method is to note the position of the Sun at exactly noon, which will be due South (or North, if you're in the Southern Hemisphere).
That would work pretty well in the time zones of the lower 48 states, give or take 8 degrees or so, but you would get a far greater error in Alaska.

Come to think of it, navigation and orientation must be quite difficult in Alaska .
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Old 11-25-2006, 04:01 PM
Quartz Quartz is offline
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Quote:
Originally Posted by Chronos
And an even quicker and dirtier method is to note the position of the Sun at exactly noon, which will be due South (or North, if you're in the Southern Hemisphere).
My brother used to work in Brunei (4 degrees N) and he said that, due to the tilt of the Earth, depending on the time of year, if you're sufficiently close to the equator, you can be north of the equator and the noonday sun will be due north.
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  #15  
Old 11-25-2006, 04:14 PM
Chronos Chronos is offline
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Quote:
My brother used to work in Brunei (4 degrees N) and he said that, due to the tilt of the Earth, depending on the time of year, if you're sufficiently close to the equator, you can be north of the equator and the noonday sun will be due north.
True, but that was more detail than I thought was needed. If you're between the Tropics (23.5 degrees above or below the Equator), then there will be two days a year (called "lahaina noon", in Hawaii) when the Sun is directly overhead at noon, and days in between when the Sun will be on the "wrong" side of directly overhead. At true noon, the Sun will always be somewhere on the north-south line in the sky (called the "meridian", hence AM and PM), though, regardless of date or latitude, so this method is in principle usable anywhere, at almost any time.

The problems are that, first, true noon is not generally at exactly 12:00, but will vary based on where you are in your time zone and on the date (and, of course, on things like Daylight Saving Time). And second, it can be hard to determine the exact azimuth of the Sun, especially when it's very near the zenith. So this quick-and-dirty method is less quick and more dirty, close to the equator.
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Old 11-26-2006, 02:06 AM
Ficer67 Ficer67 is offline
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Ok, well these methods are great for astronomers, the only method that has any merit for land surveyors is the second method mentioned by chronos, which I quote here:

If you don't want to spend all day at it, a less accurate method is to just take a sighting of the Sun at one time. You can calculate or look up in tables the position of the Sun in the sky, for a given time of day, date, latitude, and longitude. This assumes, of course, that you already know your latitude, longitude, and time to high precision. And an even quicker and dirtier method is to note the position of the Sun at exactly noon, which will be due South (or North, if you're in the Southern Hemisphere).

So how do I go about this? Is there a web page with these tables, or a detailed explanation?

Note: Land Surveyors cannot spend all day waiting for the sun to rise and set, nor can they wait until noon.
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  #17  
Old 11-26-2006, 05:48 AM
Quartz Quartz is offline
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Quote:
Originally Posted by Ficer67
Note: Land Surveyors cannot spend all day waiting for the sun to rise and set, nor can they wait until noon.
So they just hire a labourer to do it.
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  #18  
Old 11-28-2006, 01:59 PM
Ficer67 Ficer67 is offline
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Mr. Simmons, I don't know why I went right past your link, but it details exactly what I need to know. Thanks for everything

Ficer67
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  #19  
Old 11-28-2006, 04:19 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Ficer67
Mr. Simmons, I don't know why I went right past your link,
I'm used to it.
Quote:
but it details exactly what I need to know. Thanks for everything

Ficer67
You're welcome.
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  #20  
Old 11-28-2006, 04:43 PM
The Flying Dutchman The Flying Dutchman is offline
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Quote:
Originally Posted by Quartz
So they just hire a labourer to do it.
Surveyors usually have a "chainman" on the crew that does the grunt work.
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  #21  
Old 11-29-2006, 10:20 AM
Fir na tine Fir na tine is offline
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Originally Posted by Bernard Marx
Here in Aust we still use the "True Virgins" mnemonic, although "Go Rub Your Balls With Grease" has recently been revised to "Go Rub Your Back With Grease"...to the disgust of many of the more "traditional" naval officers

Well, I've been waiting 4 days for someone else to ask, guess I'll have to step up to the plate.

What in Holy Hell is "Go Rub Your Balls With Grease" a mnemonic for? I'd love to work in a profession where that is an acceptable answer to any question.

"Hey Smith, how do you frizzen stop the whistle duck?"

"Go Rub Your Balls With Grease!"

"Thanks"
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  #22  
Old 11-29-2006, 10:45 AM
The Flying Dutchman The Flying Dutchman is offline
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British Time Zone

Why do the British not join the rest of western Europe and adjust their time zone according. How long has it been this way?
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