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TileLayer.TileJSON: point radius in meters?
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Jack Michel  
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 More options May 26 2012, 7:35 am
From: Jack Michel <jackmichel6...@gmail.com>
Date: Sat, 26 May 2012 04:35:45 -0700 (PDT)
Local: Sat, May 26 2012 7:35 am
Subject: TileLayer.TileJSON: point radius in meters?
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Hello!

I'm using the great TileLayer.TileJSON.js extention [1] for leafletjs.
I want to draw points on the map depending on the zoom level and with an
exactly size in meters.

Currently I use this style:

tile.styleFor = function (feature) {

>         var type = feature.geometry.type;
>         var styleColor = feature.properties.style.color;
>         switch (type) {
>             case 'Point':
>                 return {
>                     color: styleColor,
>                     radius: 3 // what's the unit?
>                 };

>         }
>     };

The radius value is used in the TileJSON.js the following:

g.arc(p.x, p.y, style.radius, 0, Math.PI * 2);


I'm wondering which unit is the radius value? Are these meters? If not, how
can I set this value to meters?
I'm using the default Openstreetmap projection.

Thank you for your help!

Best regards,
Jack

[1] https://gist.github.com/2479540


 
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Diego Guidi  
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 More options May 26 2012, 8:37 am
From: Diego Guidi <diegogu...@gmail.com>
Date: Sat, 26 May 2012 14:37:20 +0200
Local: Sat, May 26 2012 8:37 am
Subject: Re: [Leaflet] TileLayer.TileJSON: point radius in meters?
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> I'm using the great TileLayer.TileJSON.js extention [1] for leafletjs.

happy to read that you like it :)

> The radius value is used in the TileJSON.js the following:
>> g.arc(p.x, p.y, style.radius, 0, Math.PI * 2);
> I'm wondering which unit is the radius value? Are these meters? If not, how
> can I set this value to meters?
> I'm using the default Openstreetmap projection.

the unit here is in pixels, and is completely indipendent of
projection or even zoom factor.
If you want to draw a point that has a radius of 50 meters, you must
evaluate some points.
I'm not sure about this, so feel free to verify and to check better
what I write.
If you use spherical mercator projection (the same used by google and bing)
a pixel at the maximum zoom is a meter, so you simply must set radius
value as 50 / (2 * currentzoom).

Some useful references:
http://www.sharpgis.net/post/2007/05/spatial-references2c-coordinate-...
http://www.sharpgis.net/post/2007/07/The-Microsoft-Live-Maps-and-Goog...
http://msdn.microsoft.com/en-us/library/bb259689.aspx


 
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Jack Michel  
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 More options May 27 2012, 5:46 am
From: Jack Michel <jackmichel6...@gmail.com>
Date: Sun, 27 May 2012 02:46:18 -0700 (PDT)
Local: Sun, May 27 2012 5:46 am
Subject: Re: [Leaflet] TileLayer.TileJSON: point radius in meters?
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Hello Diego!

Am Samstag, 26. Mai 2012 14:37:20 UTC+2 schrieb Diego Guidi:

> If you use spherical mercator projection (the same used by google and
> bing)
> a pixel at the maximum zoom is a meter, so you simply must set radius
> value as 50 / (2 * currentzoom).

Do you mean with "currentzoom" the zoomlevel like 17? If so, the pixel
value for a 50m point would be 1,47.
I tried this, but comparing to the elements on my map (streets, buildings)
in zoom level 17, the point looks too small.

I found on the Openstreetmap wiki some information about "meters per pixel"
[1]. This document says that I have at zoom level 17 about 1,193 meter per
pixel. Therefore the radius value (in pixel) for a 50m point would be
(50m/1,193 pixel)/2 =  20.95

I'm not sure, if this is calculation is right. The point size on level 17
looks good, I think.
Is there a better - more scientific - solution?

[1] http://wiki.openstreetmap.org/wiki/Zoom_levels


 
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Diego Guidi  
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 More options May 27 2012, 5:57 am
From: Diego Guidi <diegogu...@gmail.com>
Date: Sun, 27 May 2012 11:57:42 +0200
Local: Sun, May 27 2012 5:57 am
Subject: Re: [Leaflet] TileLayer.TileJSON: point radius in meters?
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> I found on the Openstreetmap wiki some information about "meters per pixel"
> [1]. This document says that I have at zoom level 17 about 1,193 meter per
> pixel. Therefore the radius value (in pixel) for a 50m point would be
> (50m/1,193 pixel)/2 =  20.95

I'm sure this is right. Looks strange that a "actual" size in meters
of a pixel depends on current zoom, but maybe you can search for this
and other explainations also in google/openlayers forum.
Please share results of your experiments :)

 
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