Kevin,
I have played with these functions and created a query however it
does not seem to be producing the correct results.
What I have done is the following (just so you understand the data
that I am working with).
I have downloaded the World shapefile from http://www.cipotato.org/
diva/data/MoreData.htm
I used shp2pgsql to create the inserts for PostGIS into a table
called world_countries with the following definition:
gid - integer
name - character varying(40)
gmi_cntry - character varying(3)
region - character varying(25)
the_geom - geometry
I selected a point close to the canadian border in Montana (lat:
48.400032 long: -107.753906) and ran the following queries
1)
select name, gmi_cntry,
distance(the_geom,
GeomFromText('Point(-107.753906 48.400032)', srid(the_geom))) *
69.046767 as distance
from worl_countries order by distance limit 5;
2)
select name, gmi_cntry,
distance_sphere(
line_interpolate_point(
ExteriorRing(GeometryN(the_geom, 1)),
line_locate_point(
ExteriorRing(GeometryN(the_geom, 1)),
GeomFromText('Point(-107.753906 48.400032)',
srid(the_geom))
)
),
GeomFromText('Point(-107.753906 48.400032)', srid(the_geom))
) / 1609.344 as distance
from world_countries order by distance limit 5;
Results of query 1:
United States | USA | 0
Canada | CAN | 41.42...
Mexico | MEX | 1147.37...
Guatemala | GTM | 2408.14...
Cuba | CUB | 2413.21...
Results of query 2:
United States | USA | 1277.33...
Mexico | MEX | 1336.31...
Canada | CAN | 1509.44...
Cuba | CUB | 2283.77...
Guatemala | GTM | 2313.55...
As you can see from the results of the 2 queries, they are not even
close. I understand that the first query is not going to be all that
accuate, but I tthought that the two would at least be in the same
ballpark. The results from query 1 are more like what I need, I was
just trying to get a little more accurate. Query 2 seems to be
better when the Point that I am using in the query is not inside of a
country already.
Any ideas? suggestions? Anything would be appreciated.
thank you very much,
Doug
On Sep 10, 2008, at 11:34 AM, Kevin Neufeld wrote:
Yes. Experiment with
- ST_Line_Locate_Point(linestring, Point) - which returns a
percentage along the linestring the point occurs.
- ST_Line_Interpolate_Point(linestring, location) - which accepts a
"percentage along" and returns the interpolated point along the
linestring.
Since these functions accept a linestring, you could extract the
exterior ring of your country polygon.
Something like:
SELECT
ST_Line_Interpolate_Point(
ST_Exterior_Ring(the_geom),
ST_Line_Locate_Point(ST_Exterior_Ring(the_geom), gps_pt)
)
FROM ...
Cheers,
Kevin
Doug Fischer wrote:
...
Is there some way to determine the closest point in the border of
the country to the point collected by the GPS in order to use the
distance_sphere() or distance_spheroid() functions for a more
precise measurement?
Thank you very much for any help.
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