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:
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 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 am storing a number of point features inside a SQLite database for display inside of MKMapView and I would like to precompute and store coordinates in the the projection of the map instead of in lat/lon (for index/performance reasons). However, I am creating this database externally (in Python), so I cannot simply use the MKMapPointForCoordinate() function. What is the projection used by MKMapView? The documentation states that it is a mercator projection but it does not appear to be the web mercator projection that I expected. How can I compute an X/Y coordinate from a Lat/Lon ?
Personally I feel you're approaching the problem from the wrong angle - you say don't want to store latitude/longitude co-ordinates for 'index/performance reasons'. What are these reasons?
Storage, indexing, and querying of geographic coordinates inside of a database is not an unusual problem. They are simply two decimal numbers, making indexing and querying very straightforward.
Apple also tell you only to save co-ordinates rather than MKMapPoints in their own documentation:
When saving map-related data to a file, you should always save coordinate values (latitude and longitude) and not map points.
Perhaps you could share some of the issues you're having saving co-ordinates to a database?
I'm pretty sure it is webmercator because the overlay tiles I have produced are in that projection and they fit. But like other conmenters said, you're advised to store them in lat/long. If you want to query results in a grid lat/long provide a pretty good starting point. The grid isn't square as you leave the equator but it is rectangular.
I would like to add placemarks to parts of collada geometry in google earth. To do this I need to translate the geometry coordinates in the xml to match the transformation to the model in google earth. Given longitude latitude and orientation how do I translate geometry coordinate to match the google earth transformation?
I believe models are located on the earth using only 1 coordinate which is used to position the 'center' of the model. How that is determined I am not sure, and might even depend on the kind/shape of the model.
The coordinate is easily found in the kml structure for the model which is referenced here
http://code.google.com/apis/kml/documentation/models.html
Determining how far a certain part of your geometry is from the middle might be very complex but here is an example of some code that will provide you the distance between two points.
http://earth-api-utility-library.googlecode.com/svn/trunk/extensions/examples/ruler.html
It is based upon the google earth extensions (gex) library
http://code.google.com/p/earth-api-utility-library/
Perhaps you could use SketchUp to help determine the location of your points of interest within the geometry?
http://sketchup.google.com/
I'm using Geoserver version 2.1.1, Postgres 9 and PostGIS 2.0
What I want to achieve should (i think!) be quite straight forward. I want to render on a map a line that represents the Great Circle between two cities on the earths surface.
My database contains the city locations represented as geography points defined as latitude and lonfitude pairs.
I have a layer defining an SQL view in Geoserver which retrieves a linestring (st_makeline) from the two coordinates for the specified cities. I'm having to type cast the geographies to geometries to get this to work.
But when I draw the returned line on a map what i get is a straight line and not the curved line that I am expecting.
Can someone tell me how I should be going about this?
Thanks!
PostGIS offers mainly "constructors" of the base geometries point, linestring and polygone, like ST_MakeLine.
And what yo uwant to do depends also on the coordinate reference system you use when displaying your map layers.
Here's a nice trick about great circles or parts of:
https://gis.stackexchange.com/questions/5204/curved-point-to-point-route-maps
Yours, Stefan
P.S. Here's some related stuff:
Drawing circles on a sphere
And here's some math:
http://www.mathworks.ch/matlabcentral/newsreader/view_thread/277881
I had a similar problem in cartodb (which also uses PostGIS); I wanted to get curved lines from straight lines. Maybe this post can help.