Hi,
On Tue, May 28, 2013 at 9:13 PM, Casey Gum casey.gum@gmail.com wrote:
In the docs about the geospatial extension, it mentions a buffer function:
CREATE FUNCTION Buffer(a Geometry, distance FLOAT) RETURNS Geometry
Does anyone know what a good value would be to use for distance if I wanted to buffer a point, line, etc. X number of meters? Or does anyone have any ideas for what would be a good approach for this problem - where you want the user to be able to specify how close two things can be to one another to be a match?
Actually MonetDB assumes that all geometry objects are internally represented in a specific spatial reference system and in particular EPSG 4326 (http://spatialreference.org/ref/epsg/4326/) aka WGS84. Since the units of measurement for such a reference system is degrees, then whenever you need to specify a distance value, you have to specify it in degrees. If I am not mistaken, a good value for a distance of 2 to 10 kilometers might be around to 0.002 degrees or something like that, but take this with a grain of salt. Generally speaking, there is no single answer and 1-1 transformation of meters to degrees since in different geographical areas this correspondence variates (for example by moving from the equator to a pole). And this has to do with the ellipsoid approximation of the Earth made by WGS84. See for example this wikipedia entry http://en.wikipedia.org/wiki/Decimal_degrees. Thus, if you are interested in a specific area, you might be able to "tune" that value.
Hope that helps, Babis
Thanks a lot,
Casey
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