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Newcomers DBPro Corner / Any Maths gurus?

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BatVink
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Posted: 24th Sep 2003 19:23
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Let's say the earth is 600 units in diameter, or 300 in radius, whichever suits.

I want to pick a point at random on the earths surface, between -60 and +60 degrees of the equator. How do I get that point in 3D space?

r = 300
Angle Y = random value between 0 and 360
Angles X & Z = random values between -60 and +60 (I think, could be wrong here)
Distances X,Y and Z = ?

BatVink (formerly StevieVee)
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Watsisname
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Posted: 24th Sep 2003 19:58
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You only need two angles to get a point on the earth .

angle 1: lat(latitude) = random number between -60.0 and 60.0
angle 2: long(longitude) = random number between 0 and 360.0

Assuming the planet is centred on 0,0,0, and that at lat = 0, long = 0 is at r,0,0, y is the polar axis, then the co-ordinates are

x = r*cos(long)*cos(lat)
z = r*sin(long)*cos(lat)
y = r*sin(lat)

I think that's right off the top off my head. I'll double check when I get home. You should be able to use wrapvalue commands, but sin and cos should do fine if your just placing points on the earth. Hope that helps
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BatVink
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Posted: 24th Sep 2003 20:07
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Thanks, it makes more sense, although I'll just have to believe you on the calculations

I think I confused myself even more by calling my angles X,Y and Z.

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BatVink
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Posted: 25th Sep 2003 02:52 Edited at: 25th Sep 2003 02:52
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It's kinda working... I get random angles between -60 and + 60 for the latitude, but longitude is confined to a 90 degree area. Here's the code based on Watsisname's calcs...



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Watsisname
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Posted: 25th Sep 2003 22:28 Edited at: 25th Sep 2003 22:29
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I think it might be that you're using integer values for everything. You can use integers for the angles and object number (300+n), but everything else, r,x,y,z, should be represented by real values. It seemed to work for me with real variables, r#,x#,y#,z#. Try that!
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BatVink
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Posted: 25th Sep 2003 22:55
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You didn't spot the...er...deliberate mistake.

I started with lat and lon
and went to lat and long

Works fantastically well now...thanks.

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