[postgis-users] st_distance accuracy?

[Sufficool, Stanley]

I'm walking each vertex in the line segment to find the linear distance
along the line that matches with the percentage along the line .
 
Then extracting the last point index that matches and the subsequent
point to get the vertex that matches the requested percentage.
 
Then calculating the delta x/y of the vertex to offset the interpolated
point to the left or right side of the matching vertex  by the requested
offset.
 
The function should perform the same as st_line_interpolate_point(geom,
double) but with the added function of translating the point by the
appropriate angle to the left or right side of the line segment.

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
nicklas.aven at jordogskog.no
Sent: Tuesday, October 20, 2009 2:00 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] st_distance accuracy?


I'm not sure I understand what you are trying to do. But as I read it
you are comparing the length of the line with the 
straight distance from the first to one and each of the following
vertexes. 
 
Is that really what you want?
 
Nothing says that the last vertex is the one most far away from the
first and the only case when that distance is the same as the length of
the line is if the line is straight.

 
Or am I missunderstanding you?
 
/Nicklas
 
 

2009-10-20 Sufficool Stanley wrote:

I am attempting to get the last vertex index of a line for a given
>measure using the following:
>
>idx := max(np) from generate_series(1,st_npoints(m_geom)) as np where
>st_distance(st_pointn(m_geom,1), st_pointn(m_geom,np)) <= m_distance;
>
>However, some of my queries are returning the last point on the
>linestring when "m_distance" is less than the st_length() of the given
>geometry.
>
>Is this an issue with st_distance? I don't get the same issue when
using
>Lat/Lon and st_distance_spheroid().
>
>select postgis_version()
>1.4 USE_GEOS=1 USE_PROJ=1 USE_STATS=1
>
>select version()
>PostgreSQL 8.4.0, compiled by Visual C++ build 1400, 32-bit
>
>
>FULL FUNCTION TEXT
>------------------------
>CREATE OR REPLACE FUNCTION st_line_interpolate_point(m_geom geometry,
>m_percent double precision, m_offset double precision)
> RETURNS geometry AS
>$BODY$
>
>declare idx int;
>declare m_distance double precision;
>declare p1 geometry;
>declare p2 geometry;
>
>BEGIN
>
>IF (m_distance > 1 or m_distance < 0) THEN
> raise exception 'Percentage must be between 0 and 1';
>END IF;
>
>/* Convert percentage to actual distance measure */
>m_distance := st_length(m_geom) * m_percent;
>
>/* Get the last point number on the line that is less than the
requested
>distance */
>
>/* For Feet (NAD1983) */
>idx := max(np) from generate_series(1,st_npoints(m_geom)) as np where
>st_distance(st_pointn(m_geom,1), st_pointn(m_geom,np)) <= m_distance;
>
>/* For Lat/Long (TIGER Lines) */
>--idx := max(np) from generate_series(1,st_npoints(m_geom)) as np where
>st_distance_sphere(st_pointn(m_geom,1), st_pointn(m_geom,np)) <
>m_distance;
>
>/* Past the end of the line segment return exception (this should never
>happen) */
>if idx = st_npoints(m_geom) then 
> raise exception 'st_line_interpolate_point(geom, dbl, dbl): Attempt to
>seek beyond end of line.';
>end if;
>
>/* Grab the 2 points of the matching line segment */
>p1:=st_pointn(m_geom,idx);
>p2:=st_pointn(m_geom,idx + 1);
>
>/* get the delta of the line segment and offset the interpolated point
>*/
>/* Rotate deltas by 90 degrees (invert dx/dy ) */
>/* spheroid projections will need the offset to vary based on lat/lon
*/
>return st_translate(
> line_interpolate_point(m_geom, m_percent), 
> ((st_y(p2) - st_y(p1)) / st_distance(p1,p2)) * m_offset,
> ((st_x(p2) - st_x(p1)) / st_distance(p1,p2)) * (-m_offset)
>);
>
>END;
>
>$BODY$
> LANGUAGE 'plpgsql' VOLATILE
> COST 100;
>ALTER FUNCTION st_line_interpolate_point(geometry, double precision,
>double precision) OWNER TO postgres;
>
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