This should be quite simple but I am not getting it.
I have a database of locations with lon/lat specified.
After loading the bing map I get the bounds
var view = map.getBounds();
and then call a webmethod to get all the locations which should be shown (within the bounds of the visible map).
I cannot figure out a query to get the locations (which all have a lon/lat specified) .
This obviously does NOT work as when negative values come into play they mess up the query:
SELECT Location_name, longtitude, latitude FROM location_view WHERE latitude< '40.112' and latitude> '35.783' and longtitude< '28.10453' and longtitude> '19.315'
Is there a normalized way to do this? So the comparison would work?
Your query will work absolutely fine with negative values: a longitude of -130 is still west of a longitude of -120. The only situation in which it won't work is if the bounds of your map crosses the 180th meridian. I.e. the "westmost" longitude is 170 and the "eastmost" latitude is -170.
What database are you using? If you're using SQL Server then you can define each of your locations as a Point using the geography datatype. The geography datatype operates on a round model of the earth, so it will account correctly for crossing the 180th meridian with a query like this:
SELECT Location_name
FROM location_view
WHERE location.STIntersects('POLYGON((19.315 35.783, 28.10453 35.783, 28.10453 40.112, 19.315 40.112, 19.315 35.783))') = 1;
Related
I'm currently storing a lot of data on a Azure SQL for Postgres instance. My table consists of columns like lat, lon, and geometry which is created using PostGis function
ST_SetSRID(ST_MakePoint(lat,lon),4326))
However, when I import the table into QGIS using a filter on an identifier column and between two timestamps the QGIS Query Builder clearly shows a decent number of records being returned. However, QGIS (I've tried Google Map, OpenStreetMap) renders the Geometry points near Africa (see screenshot below). Looking at some lat and lon pairs on maps.google.com that make up the geometry points, the location is completely different from what QGIS shows.
I've changed the CRS of the project, the Layers for the Map and the Coordinates (i.e. geometry points) to 4326. But still all the data points show up near Africa.
Any ideas what may be causing this and how to resolve it?
PostGIS convention is to pass longitude first (as X coordinate) and latitude second (as Y). So try
ST_MakePoint(lon, lat, 4326)
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:
I'm using postgresql as db , i have the table named car_wash with field "point geometry"(use postgis) so in application I'm getting lon lat from user using GOOGLE API, next step I need to create circle around user and check if car_wash inside this circle I use
select *
from car_wash cw
where
ST_DWithin (
cw.lon_lat,
ST_GeomFromText('POINT(54.21 22.54)')
)=false
AND
not cw.was_deleted
Is it corect way? IF you need my srid is 0 according to this query
Select Find_SRID('public', 'car_wash', 'lon_lat')
First - i assume that lat_long is georaphy type column. If it is geometry type column you will have to modify my examples to some other EPSG (propably 3857 metric EPSG for whole world). It is very important because st_dwithin check in meters for geopraphy type , and in map units for geometry (and for EPSG 4326 unit is degree not meter)
Insert your data like this
insert into car_wash values (1,'aaa',st_setsrid(st_makepoint(54.51, 22.54),4326))
I explain why use st_setsrid, st_makepoint, and what the hell is 4326.
- 4326 is EPSG 4326 - it is most known coordinate reference system (where you have lat and long in degrees).
st_makepoint - will create geography point from your lat and long coordinates. It will looks like bytes, but dont worry, if you will need lat and long for some reasons you can get them with st_x() and st_y() or st_astext() functions. Best thing of have geoms or geogs (in this case) is that you can use gist index. Very powerful tool that speed up your geo queries.
st_setsrid - st_makepoint will create point but with srid=0. You have to tell POSTGIS in what EPSG it should read your data. For example if you tell him to read it with 4326 it will be in correct places on google world map, but if you say for example 3857 it will be in completly diffrent place, as 3857 is metric system not degree so it will be around 50 and 50 meters from left down corner (or maybe left up, dont remember)
Create index on geog
create index on car_wash using gist (geog);
We have table, we have data in it and index on it. Now we want to check if your point is close to any of your car washes.
select *
from car_wash cw
where ST_DWithin (cw.geog,ST_GeogFromtext('SRID=4326;POINT(54.21 22.54)'),1000)
AND cw.was_deleted=false
In ST_DWithin third parameters is distance in meters (georpahy) or map units (geometry). So in this case it will show you all car washes that are up to 1000 meters from your user location and are not deleted.
While using ST_DWithin function, your third parameter must be distance.
You can also define srid in ST_GeomFromText
there are two simple example so you can see difference:
select ST_DWithin(
st_geomfromtext('POINT(54.51 22.54)',4326),
st_geomfromtext('POINT(54.21 22.54)',4326),0.5
)
result is true
select ST_DWithin(
st_geomfromtext('POINT(54.51 22.54)',4326),
st_geomfromtext('POINT(54.21 22.54)',4326),0.1
)
result is false
Source:
https://postgis.net/docs/ST_DWithin.html
https://postgis.net/docs/ST_GeomFromText.html
SRID of the ST_GeomFromText('POINT(54.21 22.54)') must be same as the SRID of cw.lon_lat. Suppose SRID of cw.lon_lat is 4326 you can set the other attribute srib by using ST_GeomFromText('POINT(54.21 22.54)',4326).
Secondly, ST_DWithin needs buffer distance as 3rd parameter. So suppose if you want to check if point is within 100 meter buffer it should be like
ST_DWithin (
cw.lon_lat,
ST_GeomFromText('POINT(54.21 22.54)', 3857), 100
)
Buffer value is according to the srid unit. in case of 3857 its meter. You need to convert cw.lon_lat and POINT(54.21 22.54) to the same SRID in order to make this work, using st_setSRID e.g.
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);
I'm migrating all information in a PostGIS system, with full information about thousands of sport spaces. For all this spaces I got latitude and longitude values, and PostGis need a geometry value for that, this column is already in my table "Location".
The SRID of that application is 23030.
I've looked for how to calculate a geometry value from longitude and latitude, and I find that:
update location set point=ST_GeomFromText('POINT('|| longitud || ' ' || latitud ||')',23030);
UPDATE georrepositorio.geometria SET point = ST_SetSRID(ST_Point( longitud, latitud),23030);
UPDATE georrepositorio.geometria SET POINT = ST_SetSRID(ST_MakePoint(longitud,latitud),23030);
I always get a string like: "0101000020F6590000894327550B97114104EA99EA599D4E41"
In the web application which I am building, if I mark the point to locate the space, it insert in table "location" a string like: "0101000020F659000000000020DFB115C00000008053244240" which looks similar to the string I got using those functions.
The problem is that i can't locate each space because there are so many, so I need a massive migration, and using those function to calculate geometry columns doesn't work. Because ok, those functions calculate a geom value, but when you query the application doesn´t show them.
Anyone knows how to calculate geomtry from latitude and longitude, please? anything
First, the geometries are stored as binary, which you see as "0101000020...", which is called well-known binary (WKB). For POINT geometries, you can extract the coordinate ordinals using ST_X(point) for longitude and ST_Y(point) for latitude. You can also use ST_AsText(point) for a human-readable WKT representation.
EPSG:23030 is a projected spatial reference system (SRS), with eastings and northings with units of metres. But if your coordinate data is degrees of latitude and longitude, you need to use a different SRS, such as EPSG:4326. Once stored correctly, the data can be re-projected for the application using ST_Transform(point, 23030).