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I have the center of a circle and it's radius and I need to place variable amount of markers on the outline/stroke of the circle. The distance between this markers should kept same, so if I have 8 markers to put on the orbit, each of them has 45 degree space (360 / 8) and also it should not be hard-coded as the number of markers is variable.

I wanted to know what is the right way for calculating the Geo-coordinates of each marker?

here you can see an example, the outer marker should move on, however I can't calculate the proper LatLng values.

enter image description here


I finally came up with this formula:

var dg = 90;
var lat = Math.sin(dg * Math.PI / 180) * r + center_geo_lat;
var lng = Math.cos(dg * Math.PI / 180) * r + center_geo_lng;

If you look at snapshots you can see that markers at 0 and 180 degree has placed quite fine, but it seems there is something wrong with other markers. It's gonna works fine for me, because I don't need such a perfect accuracy, however I'm wondering what's cause the problem?

enter image description here

enter image description here

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Please see this Question and Answers for the info on the stretch in a North/South direction gis.stackexchange.com/questions/6822/… –  Mapperz Oct 26 '12 at 13:10
    
@Mapperz thanks! I'm reading that now! :) –  Mahdi Oct 26 '12 at 13:19
    
I merged your questions since the second one was following up on your first attempt at solving the question posed in the first one. –  underdark Oct 26 '12 at 13:32
    
@underdark thanks, it would be better like now :) –  Mahdi Oct 26 '12 at 13:33
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1 Answer

up vote 3 down vote accepted

I think this is all down to the projection you're using. Looks like your code is correct, but basically you can't assume that by travelling one unit longitudinally on a map will look the same as the same distance latitudinally.

This has a nice explanation of the differences in projections (see the bit under Tissot's indicatrix):

http://geokov.com/education/map-projection.aspx

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hey, thanks! I'm reading that now, but is there any simple solution for that or not? –  Mahdi Oct 26 '12 at 13:20
1  
You might also need to consider whether the circles you have plotted are showing you what you really want - or whether these should be ellipses. –  pecoanddeco Oct 26 '12 at 13:21
    
Simple solution is to choose a projection which preserves the shape of areas on the map, i.e. one that is conformal. –  pecoanddeco Oct 26 '12 at 13:29
    
I need circles, i guess ... –  Mahdi Oct 26 '12 at 13:29
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