BA
Topic Author
Posts: 10133
Joined: Fri May 19, 2000 11:06 am

Great Circle Mapper

Sun Jul 04, 2004 11:12 pm

I'm sure many of you know the Great Circle Mapper website:
http://gc.kls2.com

I was wondering if this excellent program takes into account the fact that the Earth is an oblate spheroid? Or does it consider the Earth to be a perfect sphere?

Just wondering to get an idea about its accuracy.

Regards
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
scottysair
Posts: 6442
Joined: Tue Mar 28, 2000 4:07 pm

RE: Great Circle Mapper

Sun Jul 04, 2004 11:13 pm

BA,

This is very good website of the Great Circle Mapper. I am sure you can do it for yourself. Sometimes, when I am use with their website on The Gret Circle Mapper.
 
VirginFlyer
Posts: 3877
Joined: Sun Sep 10, 2000 12:27 pm

RE: Great Circle Mapper

Thu Jul 08, 2004 8:54 am

BA - perhaps it would be worth email the site's owner and asking - his email address is given on the bottom of the page...

V/F
"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh
 
ly7e7
Posts: 2222
Joined: Tue Jun 29, 2004 3:15 am

RE: Great Circle Mapper

Thu Jul 08, 2004 4:47 pm

was wondering if this excellent program takes into account the fact that the Earth is an oblate spheroid?

Although that is a great software I really doubt that that is the case. In order to take in account the Geoid (i.e. using a certain datum for the calculations) there is a need for a professional GIS infrastructure. If such was used I guesss we all would be paying for the GCM.

Still , a very good application, I use it a lot.

Regards.
2 things are endless: ignorance and space
 
timz
Posts: 6081
Joined: Fri Sep 17, 1999 7:43 am

RE: Great Circle Mapper

Fri Jul 09, 2004 7:29 am

As he explains, the Mapper uses the Clarke 1866 spheroid, which isn't the right one nowadays, but it seems to give distances correct within a small fraction of a kilometer. If he used a sphere the calculated distances would occasionally be off by 10 km or more.
 
Arcano
Posts: 2299
Joined: Mon Mar 22, 2004 4:34 am

RE: Great Circle Mapper

Fri Jul 09, 2004 11:06 am

Look how it draws the route SCL-SYD:



If it considered the Earth as a perfect sphere, why going down instead of suggesting a stright line across the pacific?
The software recognize thant the best route is going down, so it realizes the Earth is not a sphere!



Regards
in order: 721,146,732,763,722,343,733,320,772,319,752,321,88,83,744,332,100,738, 333, 318, 77W, 78, 773, 380, 73G, 788, 789, 346
 
BA
Topic Author
Posts: 10133
Joined: Fri May 19, 2000 11:06 am

RE: Great Circle Mapper

Fri Jul 09, 2004 6:11 pm

Thanks for all the replies.

If it considered the Earth as a perfect sphere, why going down instead of suggesting a stright line across the pacific?

A straight line across the Pacific would be the shortest distance if the Earth were perfectly flat.

However, since the Earth is a sphere, you cannot truly get an accurate drawing unless you draw it on a 3D model of the Earth. However, to simulate the sphere of the Earth, we draw the line curved, following the true path.

Take an orange, draw a line across it somewhere. Peel the orange carefully, and lay the peel flat and you will see that your line appears flat, taking a southernly or northernly course.

My suggestion also is you take a globe, and use a string and connect two points and experiment around with different routings, you will see what the shortest route is.

Regards
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
timz
Posts: 6081
Joined: Fri Sep 17, 1999 7:43 am

RE: Great Circle Mapper

Sat Jul 10, 2004 3:07 am

Take this example: use the Mapper to get the distance from ANC to LED; it says 6587 km. If they were using a sphere, that would imply a radius of 6394 km, based on the lat-lons they give. But if you try HNL-RAR, they give a distance of 4708 km, which implies a radius of 6338 km. So they're not using a sphere.
 
toltommy
Posts: 2462
Joined: Tue Dec 09, 2003 9:04 am

RE: Great Circle Mapper

Sat Jul 10, 2004 4:53 am

"I was wondering if this excellent program takes into account the fact that the Earth is an oblate spheroid?"

I'm pretty sure it did, but the name "Great Oblate Spheriod Mapper" just didn't have the same sex appeal. Darn those marketing people!  Nuts

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