[postgis-users] radius of buffer parameter issue at st_buffer function...
I am using postgis 1.5 and use the st_buffer function with my spatial queries. I have set the srid to 3308 for my spatial data which the measurement unit is meter, but when I use the st_buffer function and set the radius of buffer in meter. The spatial queries return far more rows than what I expected. The measurement unit of radius of buffer in st_buffer function does not seem to me to be in meter. Instead of using 10 for a radius of 10 meters, I have to use a value of 10*1.7453292519943283E-5 instead. What could be the problem with this st_buffer function? I do not mind to use this value but it is not what postgis documented. It would be appreciated if you could help on this. Regards, Gorton ZHAO Traffic Management T 02 9319 9431 F 02 9319 2015 http://www.scats.com.au http://www.rmservices.nsw.gov.au Roads and Maritime Services 2 Locomotive Street, Eveleigh NSW 2015 Before printing, please consider the environment IMPORTANT NOTICE: This e-mail and any attachment to it are intended only to be read or used by the named addressee. It is confidential and may contain legally privileged information. No confidentiality or privilege is waived or lost by any mistaken transmission to you. Roads and Maritime Services (RMS) is not responsible for any unauthorised alterations to this e-mail or attachment to it. Views expressed in this message are those of the individual sender, and are not necessarily the views of RMS. If you receive this e-mail in error, please immediately delete it from your system and notify the sender. You must not disclose, copy or use any part of this e-mail if you are not the intended recipient. ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
Re: [postgis-users] radius of buffer parameter issue at st_buffer function...
Are you sure your coordinates are projected? Could you get an example EWKT from you dataset using: SELECT ST_AsEWKT(geom) FROM mytable LIMIT 1; If you are seeing something like POINT(147.65625 -33.046875), which has linear units of degrees (not metres), then you need to properly set the SRID and transform them: SELECT ST_AsEWKT(ST_Transform(ST_SetSRID(ST_geom, 4326), 3308)) FROM mytable LIMIT 1; Note: I'm only guessing 4326 for WGS84, but your data could use a different datum. If your geometries are indeed projected, and the linear distances make sense, make sure your geometries are valid: SELECT * FROM mytable WHERE NOT ST_IsValid(geom); Invalid geometries cause havoc with some operations. -Mike On 18 January 2012 12:45, ZHAO Gorton gorton.z...@rms.nsw.gov.au wrote: I am using postgis 1.5 and use the st_buffer function with my spatial queries. I have set the srid to 3308 for my spatial data which the measurement unit is meter, but when I use the st_buffer function and set the radius of buffer in meter. The spatial queries return far more rows than what I expected. The measurement unit of radius of buffer in st_buffer function does not seem to me to be in meter. Instead of using 10 for a radius of 10 meters, I have to use a value of 10*1.7453292519943283E-5 instead. What could be the problem with this st_buffer function? I do not mind to use this value but it is not what postgis documented. It would be appreciated if you could help on this. Regards, Gorton ZHAO ___ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
