I'm new to PostGIS so my question might sound silly.
Currently working with Points to represent geographical locations (with latitude and longitude only) and was wondering how I can use [ST_DWithin][3] with meters as a unit.
I can't find the right way and been lost in documentation trying to solve this issue.
You need to make sure that your points are geography rather than geometry, as explained in the manual.
So you want a query like:
SELECT s.gid, s.school_name
FROM schools s
LEFT JOIN hospitals h ON ST_DWithin(s.geom::geography, h.geom::geography, 3000)
WHERE h.gid IS NULL;
Related
I store GPS tracks in Postgres with PostGIS and would like to find intersections between them not just in space but also in time. I'm thinking of using 3D geometry but treat 3rd coordinate as time. Basically as soon as new data comes I insert POINT(lat, lon, t) into a GPS track. To find intersections I make a query to find nearby objects along the path (a LINESTRING).
Based on what I know about PostGIS and spatial indexes it should work just fine. From the other side this kind of unorthodox usage of a spatial coordinate makes me a bit nervous. Is it ok to use PostGIS this way or there are better way to do what I want?
I try to figure out how to come from a single given coordinate (lat/lon) to the nearest bounds which enclose this coordinate on a map e.g. streets or sea.
Here two examples to give you a better understanding of what I mean:
What i tried already or thought about:
Setting up a Nominatim server and search for the given coordinate via the reverse-function to get the bbox and/or the geojson polygon of this coordinate. -> this only works when the given coordinate is within a POI or for example directly on a street.
Writing an algorithm to walk in all 4 or 8 directions (n/e/s/w) and 'stop' when the map layer/surface changes (change = stop for this direction and mark a bounding-point)
Building up an image-recognition system using TensorFlow to detect the different colors and 'draw' the polygon. Worked with TensorFlow a couple of times but this seems to be the most tricky solution to implement (but at my current understanding the most precise one)
Does someone of you have any other ideas to get a solution for this problem? Would appreciate any kind of approaches
Cheers!
If I got your question right, you might wanna first select all polygons in which the given point is inside of using ST_Contains, and then compute the distance to this point using ST_Distance. If you ORDER BY distance and LIMIT to 1 result you'll get the nearest polygon, e.g.
Data Sample
CREATE TABLE t (gid int, geom geometry);
INSERT INTO t VALUES
(1,'POLYGON((-4.47 54.26,-4.44 54.28,-4.41 54.24,-4.46 54.23,-4.47 54.26))'),
(2,'POLYGON((-4.48 54.25,-4.40 54.25,-4.41 54.23,-4.48 54.23,-4.48 54.25))'),
(3,'POLYGON((-4.53 54.23,-4.44 54.29,-4.38 54.22,-4.53 54.23))');
Query
SELECT gid,ST_AsText(geom) FROM t
WHERE ST_Contains(geom,ST_MakePoint(-4.45, 54.25))
ORDER BY ST_Distance(geom,ST_MakePoint(-4.45, 54.25))
LIMIT 1;
gid | st_astext
-----+------------------------------------------------------------------------
1 | POLYGON((-4.47 54.26,-4.44 54.28,-4.41 54.24,-4.46 54.23,-4.47 54.26))
(1 Zeile)
I am new to PostGIS, am not getting the area of polygon right, my sample data is from Google maps, I know the area of the polygon is 11 acres, but the area returned by st_area doesn't match,
I already referred to a few links like below, but unable to resolve the issue, Internet says google follows 4326 Spatial references, I tried a lot, can you please help, Image attached is the polygon from google maps.
I am expecting an array of such coordinates from the user, I have to calculate the area from PostGIS and give an error back to the user if the area entered is not approximated to calculated area.
https://gis.stackexchange.com/questions/169422/how-does-st-area-in-postgis-work
How do I convert a latitude/longitude pair into a PostGIS geography type?
https://gis.stackexchange.com/questions/56862/what-spatial-reference-system-do-i-store-google-maps-lat-lng-in/56925
17.475197 78.389024
17.4771 78.39044
17.475657 78.391652
17.474408 78.390847
17.475197 78.389024
l_polygon_text='MULTIPOLYGON(((
17.4771000000000001 78.3904399999999981,
17.4751970000000014 78.3890240000000063,
17.4756570000000018 78.3916519999999934,
17.4751970000000014 78.3890240000000063,
17.4744080000000004 78.3908469999999937,
17.4771000000000001 78.3904399999999981)))';
st_area(ST_GeometryFromText(l_polygon_text,4326))
st_area(ST_GeometryFromText(l_polygon_text,2163));
st_area(ST_GeometryFromText(l_polygon_text,2249));
st_area(ST_GeometryFromText(l_polygon_text,3859));
ST_AREA(ST_Transform(ST_GeomFromText(l_polygon_text,4326),31467));
ST_Area(ST_Transform(ST_SetSRID(ST_GeomFromText(l_polygon_text),4326),900913));
polygon
In PostGIS, coordinates must be expressed as longitude first, then latitude. Google uses the opposite.
After swapping the coordinates to the proper order, you can't directly call st_area, else you would get an area in "square degrees" which is meaningless. You would have to project to a suitable local coordinate system, or you can use the geography type which will return an area in m2.
select st_area(st_geogFromText('MULTIPOLYGON(((78.3904399999999981 17.4771000000000001, 78.3890240000000063 17.4751970000000014,78.3916519999999934 17.4756570000000018,78.3890240000000063 17.4751970000000014,78.3908469999999937 17.4744080000000004,78.3904399999999981 17.4771000000000001)))'));
st_area
--------------------
26956.897848576307
That being said, the example you have provided is about 6.5 acres, not 11, because the polygon is not properly defined:
long,lat,time
10,11,0
11,12,1
12,13,2
I have a simple table with longitudine, latitude and time and i want know a function that calculate the total distance of points in postgres.
If you want to calculate the distance you can use the extention earthdistance and do something like :
SELECT earth_distance(lat, long)
FROM your_table;
The earthdistance module provides two different approaches to calculating great circle distances on the surface of the Earth. The one described first depends on the cube package (which must be installed before earthdistance can be installed). The second one is based on the built-in point datatype, using longitude and latitude for the coordinates.
In this module, the Earth is assumed to be perfectly spherical. (If that's too inaccurate for you, you might want to look at the PostGIS project.)
I have a question about the use of postgreSQL/postGIS.
I would like to display markers on a map (stored in a database) which are some distance away from the user (coordinates given to the request).
The type of the field of the markers is POINT (I store lat/long).
The user position is detetermined by the Google Map API.
Here is the actual request :
SELECT * FROM geo_points WHERE ST_distance(ST_SetSRID(geo_points.coords::geometry,4326),ST_GeomFromEWKT('SRID=4326;POINT(45.0653944 4.859764599999996)')) > 65
I know (after some research on internet) that the function ST_distance gives me the distance in degree between markers and the user position and that I test the distance in km.
I think I have to use the function ST_tranform to transform the points in metric coordinates.
So my questions are :
- what is the SRID for France
- how can I make this dynamically for the entire world according to the user position ?
I also kow that the function ST_within exists and that could do this. But I anticipate the fact that later, I could need the distance.
Any help would be greatly appreciated
ps: there are maybe solutions in other post, but all the answers I have found during my researches were not really meeting my needs.
Firstly, pay attention to the axis order of coordinates used by PostGIS, it should be long/lat. Currently you are searching in Somalia. Swapping to the coordinates, you would be searching in France.
You can use a geodesic calculation with the geography type, or use geodesic functions like ST_Distance_Spheroid. With the geography type, you may want to use ST_DWithin for higher performance.
Here are geo_points 65 m away or less from the point of interest in France (not Somalia):
SELECT * FROM geo_points
WHERE ST_Distance_Spheroid(
ST_Transform(geo_points.coords::geometry, 4326),
ST_SetSRID(ST_MakePoint(4.859764599999996, 45.0653944), 4326),
'SPHEROID["WGS 84",6378137,298.257223563]') < 65.0;
However, it will be very slow, since it needs to find the distance to every geo_points, so only do this if you don't care about performance and have less than a few thousand points.
If you change and transform geo_points.coords to store lon/lat (WGS84) as a geography type:
SELECT * FROM geo_points
WHERE ST_DWithin(
geo_points::geography,
ST_SetSRID(ST_MakePoint(4.859764599999996, 45.0653944), 4326)::geography,
65.0);