Re: [jts-devel] Projection question.

[Michael Bedward]

thanks for posting that link Jeff - definitely one for my handy
snippets collection

Michael

2009/2/17 Jeff Adams :
> I wound up just doing the math in lat/lon, which works (different math than
> in a cartesian coord system of course, but I found a couple examples here:
> http://forum.worldwindcentral.com/showthread.php?t=20688).
>
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[jts-devel] Projection question

[Stefan Steiniger]

Jeff,

why spherical mercator? What is spherical mercator? A mercator 
projection on the sphere?
- how about a (Lambert) azimuthal projection with projection center in 
the circle center?

http://en.wikipedia.org/wiki/Lambert_azimuthal_equal-area_projection

Btw. if you have only 100mile.. that is not so much. So the question 
rather is: What accuracy to you need? [1 m or 100m?] Unfortunately I 
forgot how much distortion you would get for 100miles in UTM (which 
depends of course on the latitude).


btw. a tip: along the touching meridian and in equator direction one 
degree is approximately 111km (derived for a sphere of radius ?6373km?). 
This relationship could be helpful for conversions.


stefan

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Re: [jts-devel] Projection question.

[Jeff Adams]

I wound up just doing the math in lat/lon, which works (different math than
in a cartesian coord system of course, but I found a couple examples here:
http://forum.worldwindcentral.com/showthread.php?t=20688).

On Mon, Feb 16, 2009 at 11:03 AM, Jeff Adams  wrote:

> At the moment the largest circle we'll draw is a 50-mile radius.  That
> might go up but I doubt it would go much beyond 100 mile radius.  The
> smallest is 1-mile radius.
>
> The location is North America.
>
>
> On Mon, Feb 16, 2009 at 10:53 AM, Paul Uszak  wrote:
>
>> Jeff,
>>
>> what size (roughly) is your circle?  And, where in the world can it be?
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>
>
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Re: [jts-devel] Projection question.

[Jeff Adams]

At the moment the largest circle we'll draw is a 50-mile radius.  That might
go up but I doubt it would go much beyond 100 mile radius.  The smallest is
1-mile radius.

The location is North America.

On Mon, Feb 16, 2009 at 10:53 AM, Paul Uszak  wrote:

> Jeff,
>
> what size (roughly) is your circle?  And, where in the world can it be?
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Re: [jts-devel] Projection question.

[Paul Uszak]

Jeff,

what size (roughly) is your circle?  And, where in the world can it be?
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[jts-devel] Projection question.

[Jeff Adams]

This isn't JTS-related (except inasmuch as we're using it for handling our
geometries) but you guys are pretty knowledgeable so maybe you can point out
what I'm doing wrong.

I earlier had the question on how to find distance in lat/lon.  The solution
we've decided to use (because it's pretty fast and easy) is just to
construct a "circular" polygon (a 60-point polygon is close enough to a
circle for our purposes) and query ArcGIS Server for all intersecting
polygons.

The problem is how to construct the circle, since we're working in lat/lon.
At first we though it would be easy to just reproject the center point to
spherical mercator, draw a circle, and reproject the circle points back to
lat/lon.  The problem is that doesn't keep the bounds of the circle "correct
for what we expect to see".

We're viewing the map stretched flat, using web mercator, so vertical
distance (in degrees latitude) is a constant (in screen pixels), but
Antarctica and Greenland are of course stretched to rediculous widths.  What
the above technique does is keeps the ratio between width and height
correct, but the "wrong" one gets scaled (for what we want).  So as you move
the circle north from the equator, it gets shorter instead of wider.

Would I be better off just writing a function to generate circle points in
lat/lon, or did I just pick the wrong projection to use to draw the circle
in?

Thanks,
Jeff
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