Draw a Great Circle on Google Maps
- #1
When you use the "ruler" tool to draw a line on Google Earth you lot go a altitude and a heading, for instance from the southern tip of New Zealand to Southern Chile is well-nigh 4000nms.
Is this a rhumb line or nifty circumvolve ?
If yous rotate the globe the line looks very like the arc of a circle.
Thank you
- #two
Probably neither. It will be the distance according to the projection used by GE, which volition be fine for short distances but completely inaccurate for long distances. No map projection (the mechanism for drawing a spherical earth on a flat piece of paper) tin can preserve altitude, and then whatever measurement done on a map (fifty-fifty a clever 1 like GE) will be inaccurate.
You can check for Neat Circle distances using web-sites like this. Rhumb line distances are here.
- #iii
Information technology appears google is using a non-spherical (i.due east. accurate) model: http://quezi.com/11698
One tin play with Google Earth to run across that the great circle route betwixt the English Channel and New York (the shortest altitude on a sphere) passes over Newfoundland and Nova Scotia. Click on "Tools" and "Linear" and draw a line from the Channel to New York. This line goes north and and so southward over the 50th latitude, curving due north of the direct line betwixt the 2 on the Mercator projection image.
So you could see if yous're actually using shortest altitude past comparing the line to the longitude lines..
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- #4
Interesting
Point A to Point B, 4,019nms, 149degs according to Google World
Betoken A to Point B, 4,023nms, 149degs according to this Great Circle calculator
Question answered
Cheers
- #5
alan_d
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I thought that i of the things about bang-up circle courses was that you could non sail on a constant bearing (except along the equator), so what is the 149 degs? (Couldn't get the link to work to bank check.)
- #6
mortehoe
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Neither ... it's all downward to a school globe, bluetack and a piece of string .....
When you utilize the "ruler" tool to draw a line on Google World y'all get a distance and a heading, for instance from the southern tip of New Zealand to Southern Republic of chile is about 4000nms.
Is this a rhumb line or great circle ?
If you rotate the earth the line looks very like the arc of a circle.
Thanks
At that place was a program yonks ago on the Television and DHL. It was about how they (DHL) calculated their fuel load for flying from (say) gamma to delta ....
.... a non-DHL engineer stood up and said "If you have a schoolhouse-size globe, some string, some bluetack and a pair of scissors a) you will find the altitude to the nearest 100km and b) from the previous y'all volition already have on yous a cellphone with a estimator that will have already calculated the amount of fuel that you lot need for this flight .....
.... and the answer is "Neat Circle" .... and if you want to get to the root of the 3D trig and then you accept to know nearly haversines 
N
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- #7
Here is the link again
I was'nt sure about the constant bearing either.
Perhaps somebody who understands great circumvolve navigation may care to enlighten us.
- #eight
That is correct - the begetting continually changes. The bearings given will be initial bearings, I imagine.
You lot don't need haversines; the cosine rule of spherical trigonometry is the key equation. Haversines were introduced to brand the computation simpler and more reliable, but which foul up the maths horribly!
Software to handle these computations very accurately is "geod"; not the friendliest bit of software ever, merely very accurate in one case y'all've learnt to handle it!
- #ix
I trust Google to give accurate NM'southward equally a GC rails.
(I am a pilot and depend on information technology to verify new road distances)
- #10
I employ spherical geometry for distance and grade calculations in my software and , as you say, the bearing changes continually along the road (otherwise it would exist a rhumbline!) although over short distances a more than immediate consideration, if using a magnetic compass, is changes in variation.
Although spherical geometry works well for small sections of the globe I judge the oblateness of the planet would need to be taken into account when doing authentic measurements over very large distances. I wonder if this is the reason for the departure noted in the earlier post past Fascadale.
- #xi
Yes, it does. Merely the error is pocket-sized, and will produce errors much less than 1% of the altitude. A geodesic calculation program like geod will produce results that are good to millimetres, and has to utilize ellipsoidal calculations to do so. But results proficient to improve than a nautical mile will be obtained from spherical calculations. I do geographic calculations as part of the twenty-four hours job; for nearly all purposes the Cosine Rule is plenty adept enough!
Oh, and technically information technology is a geodesic on an ellipsoid, not a Great Circumvolve.
I produced an illustration like this a while ago.
The earth is actually a rather irregular figure chosen the geoid (which is the surface of gravitational equipotential, for the geeks among united states of america). The Geoid can be modelled past a sphere with residual differences of nearly 11 km between the "best fit" sphere and the geoid at the poles and the equator. If y'all model information technology by an oblate spheroid, the maximum errors drib to +xc and -110 1000. And then, the oblate spheroid is a very proficient model. However, the 11 km is on a sphere of radius approximately 6350 km, then in fact the sphere accounts for the vast majority of the shape of the earth, and tin can be used as an approximation good plenty for most purposes.
I believe that only spherical trigonometry is used in navigation; the sphere is a good plenty approximation until the error drops to less than 100m.
- #12
Equally per AtlanticPilot, it will exist the initial bearing. You cant sail an exact GC Class, so you sail a series of rhumb lines.
In the Southern Hemisphere, (NZ to Chile), the initial course will always be S of East/W, and the final course volition be Due north of East/W. (I guess there will be some GCs where the final class could be East or W, and in that location may be some where the initial course is Southward, (in the Southern Hemisphere).
In general, you demand to be travelling long distances, (like crossing oceans), to make a GC route worthwhile in terms of distance savings. You besides demand to be aware that they take you into colder climes, and potentially bad atmospheric condition.
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- #13
alan_d
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A GC course would therefore incorporate an infinite number of infinitely curt rhumb lines and would have an space time to complete ...
(I think I should lie down, I feel Zeno's paradox coming on ...)
- #xiv
A GC course would therefore incorporate an infinite number of infinitely short rhumb lines and would take an infinite time to complete ...
(I call up I should lie downward, I feel Zeno's paradox coming on ...)
Here it is:
If the space number of rhumb lines are each infinitely brusque, they would each take no time to canvas, so you would arrive the moment y'all set off
Source: https://forums.ybw.com/index.php?threads/google-earth-great-circle.232893/
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