How to Draw a Circle in Google Earth
- #1
When you lot use the "ruler" tool to draw a line on Google World you become a altitude and a heading, for case from the southern tip of New Zealand to Southern Chile is virtually 4000nms.
Is this a rhumb line or smashing circle ?
If you lot rotate the world the line looks very like the arc of a circle.
Thanks
- #two
Probably neither. Information technology volition be the distance according to the projection used by GE, which will be fine for short distances but completely inaccurate for long distances. No map projection (the machinery for drawing a spherical earth on a flat piece of paper) can preserve distance, so whatsoever measurement washed on a map (even a clever one like GE) will be inaccurate.
Yous can cheque for Great Circle distances using spider web-sites similar this. Rhumb line distances are here.
- #3
Information technology appears google is using a non-spherical (i.due east. authentic) model: http://quezi.com/11698
One can play with Google Globe to see that the great circle road between the English Channel and New York (the shortest distance 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 due north and then south over the 50th latitude, curving north of the straight line betwixt the two on the Mercator projection image.
So yous could see if you're really using shortest distance by comparing the line to the longitude lines..
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- #4
Interesting
Point A to Signal B, 4,019nms, 149degs according to Google World
Point A to Point B, 4,023nms, 149degs according to this Great Circle estimator
Question answered
Thanks
- #5
alan_d
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I idea that i of the things about bang-up circle courses was that you could not sail on a abiding bearing (except along the equator), so what is the 149 degs? (Couldn't get the link to work to check.)
- #6
mortehoe
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Neither ... information technology's all downwardly to a schoolhouse earth, bluetack and a piece of string .....
When you use the "ruler" tool to draw a line on Google Earth you get a distance and a heading, for instance from the southern tip of New Zealand to Southern Chile is near 4000nms.
Is this a rhumb line or not bad circle ?
If you rotate the globe the line looks very like the arc of a circle.
Thanks
There was a program yonks agone on the TV and DHL. It was nigh how they (DHL) calculated their fuel load for flying from (say) gamma to delta ....
.... a not-DHL engineer stood upward and said "If you have a school-size globe, some cord, some bluetack and a pair of scissors a) you will find the distance to the nearest 100km and b) from the previous yous will already accept on you a cellphone with a computer that will have already calculated the amount of fuel that you need for this flight .....
.... and the answer is "Great Circumvolve" .... and if you want to get to the root of the 3D trig then y'all take to know about haversines 
North
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- #vii
Hither is the link again
I was'nt sure about the abiding bearing either.
Perhaps somebody who understands peachy circle navigation may intendance to enlighten us.
- #8
That is right - the bearing continually changes. The bearings given will be initial bearings, I imagine.
You don't need haversines; the cosine rule of spherical trigonometry is the fundamental equation. Haversines were introduced to brand the computation simpler and more reliable, but which foul upwardly the maths horribly!
Software to handle these computations very accurately is "geod"; not the friendliest bit of software ever, but very accurate once y'all've learnt to handle it!
- #9
I trust Google to give authentic NM'south every bit a GC rail.
(I am a airplane pilot and depend on it to verify new route distances)
- #10
I use spherical geometry for distance and form calculations in my software and , equally you say, the bearing changes continually along the road (otherwise it would exist a rhumbline!) although over brusk distances a more immediate consideration, if using a magnetic compass, is changes in variation.
Although spherical geometry works well for modest sections of the globe I guess the oblateness of the planet would demand to exist taken into account when doing accurate measurements over very big distances. I wonder if this is the reason for the difference noted in the before mail past Fascadale.
- #11
Aye, it does. Simply the fault is small, and will produce errors much less than 1% of the distance. A geodesic calculation program like geod will produce results that are expert to millimetres, and has to use oblong calculations to do and then. Simply results practiced to meliorate than a nautical mile will be obtained from spherical calculations. I practise geographic calculations every bit role of the day job; for nearly all purposes the Cosine Rule is plenty good enough!
Oh, and technically it is a geodesic on an ellipsoid, not a Great Circumvolve.
I produced an illustration like this a while agone.
The globe is actually a rather irregular figure chosen the geoid (which is the surface of gravitational equipotential, for the geeks among us). The Geoid can exist modelled by a sphere with balance differences of about xi km between the "best fit" sphere and the geoid at the poles and the equator. If you model it by an oblate spheroid, the maximum errors drib to +ninety and -110 m. So, the oblate spheroid is a very skilful model. All the same, the 11 km is on a sphere of radius approximately 6350 km, so in fact the sphere accounts for the vast majority of the shape of the earth, and can be used equally an approximation proficient enough for well-nigh purposes.
I believe that just spherical trigonometry is used in navigation; the sphere is a good enough approximation until the error drops to less than 100m.
- #12
Every bit per AtlanticPilot, information technology volition be the initial bearing. Y'all deceit sail an exact GC Course, then you lot canvass a series of rhumb lines.
In the Southern Hemisphere, (NZ to Chile), the initial course will always be South of East/West, and the last course will exist North of East/West. (I judge there volition be some GCs where the terminal course could be East or West, and there may be some where the initial form is Due south, (in the Southern Hemisphere).
In general, yous demand to be travelling long distances, (like crossing oceans), to brand a GC road worthwhile in terms of distance savings. Y'all also need to be aware that they take y'all into colder climes, and potentially bad conditions.
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- #xiii
alan_d
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A GC grade would therefore comprise an space number of infinitely short rhumb lines and would accept an infinite time to complete ...
(I think I should prevarication down, I feel Zeno's paradox coming on ...)
- #14
A GC course would therefore incorporate an infinite number of infinitely curt rhumb lines and would take an space time to complete ...
(I call back I should prevarication down, I experience Zeno's paradox coming on ...)
Hither it is:
If the infinite number of rhumb lines are each infinitely brusk, they would each take no time to sheet, and then y'all would arrive the moment yous ready off
Source: https://forums.ybw.com/index.php?threads/google-earth-great-circle.232893/
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