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Welcome & buffering

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Welcome & buffering

Paul Meems
I'm new to this mailing list so I do a quick introduction of myself.

I'm Paul Meems. I live in The Netherlands and I am active in the MapWindow GIS community (www.mapwindow.org)
With MapWindow we don't use JTS directly but use NTS and GEOS.
With both libraries we've got issues. Because they both implement JTS I thought the best way to get answers and/or fixes is at the source ;)

The first issue we have is when we create a very large buffer on a shapefile with a projection bases on an ellipsoide (WGS84) we expect a buffer more as an ellipse instead of a circle.

Using these coordinates:
-102,040052163025, 36,9876952244059

a distance of 18 (degrees)

and this projection:
GEOGCS["WGS 84"
,DATUM["WGS_1984"
,SPHEROID["WGS 84"
,6378137,298.257223563
,AUTHORITY["EPSG","7030"]]
,TOWGS84[0,0,0,0,0,0,0]
,AUTHORITY["EPSG","6326"]]
,PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]]
,UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9108"]]
,AUTHORITY["EPSG","4326"]]

This would create a buffer starting at the center of USA and almost as wide as the total width of USA and higher as the total height of the USA.

Is my assumption wrong and should it not create an ellipse? Or should I not use the Buffer method but a different one?

I'm sorry if this is already documented. I tried searching for it but couldn't find it.

Thanks,
Paul


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Re: Welcome & buffering

Martin Davis
Welcome, Paul.

Glad you spoke up - I'll add your products to the list I'm keeping of
clients of JTS/GEOS:  http://tsusiatsoftware.net/jts/jts-links.html

As for your question, at the moment JTS assumes a Cartesian (planar)
coordinate system.  So there is no provision for processing geometries
in  spheroidal coordinate systems, as would be required to produce the
kind of output you're talking about.

If you only need buffers around points, then it would be relatively
straightforward to implement this as a custom function.  You would have
to obtain code to compute distance and azimuth on the spheroid in order
to do this - there is code for this in PostGIS, I believe.

We have been discussing the possibility of extending JTS operations to
allow spherical coordinate systems.  This is feasible, but would involve
a bit of work. If you're interested in discussing this further let me know.

Martin

Paul Meems wrote:

> I'm new to this mailing list so I do a quick introduction of myself.
>
> I'm Paul Meems. I live in The Netherlands and I am active in the
> MapWindow GIS community (www.mapwindow.org <http://www.mapwindow.org>)
> With MapWindow we don't use JTS directly but use NTS and GEOS.
> With both libraries we've got issues. Because they both implement JTS
> I thought the best way to get answers and/or fixes is at the source ;)
>
> The first issue we have is when we create a very large buffer on a
> shapefile with a projection bases on an ellipsoide (WGS84) we expect a
> buffer more as an ellipse instead of a circle.
>
> Using these coordinates:
> -102,040052163025, 36,9876952244059
>
> a distance of 18 (degrees)
>
> and this projection:
> GEOGCS["WGS 84"
> ,DATUM["WGS_1984"
> ,SPHEROID["WGS 84"
> ,6378137,298.257223563
> ,AUTHORITY["EPSG","7030"]]
> ,TOWGS84[0,0,0,0,0,0,0]
> ,AUTHORITY["EPSG","6326"]]
> ,PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]]
> ,UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9108"]]
> ,AUTHORITY["EPSG","4326"]]
>
> This would create a buffer starting at the center of USA and almost as
> wide as the total width of USA and higher as the total height of the USA.
>
> Is my assumption wrong and should it not create an ellipse? Or should
> I not use the Buffer method but a different one?
>
> I'm sorry if this is already documented. I tried searching for it but
> couldn't find it.
>
> Thanks,
> Paul
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> jts-devel mailing list
> [hidden email]
> http://lists.refractions.net/mailman/listinfo/jts-devel
>  

--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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