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/
Related
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´ve got a job offer to work with postgres and I have not much idea of it. The guy told me to build a simple data base which automatically calculates the distance to my house from a list of some other places (bars, pharmacies, museums, whatever...) everything given in geocoordinates.
I have already installed postgres, also postgis and create a data base. May you give me some hints about how I should do this task? Is there any tutorial or resource I could use to make this tasks easier? Should I use postgis?
Thank you.
PostGIS will do this easily. Boundless Geo have an excellent PostGIS tutorial. I also recommend you are familiar with Ch3 & 4 of the PostGIS manual.
I strongly advise you to learn & understand the difference between projected and unprojected coordinates if you're going to be working with spatial data. Projected means the coordinates have been taken from a 'round earth' and adjusted or projected onto to a flat map page (with coordinates normally given in feet or metres depending on the properties of the projection used). This enables computationally efficient normal cartesian calculations to be done for distance, area, direction, intersection, contains etc. There are trade offs for using projections- You can't preserve all of area, distance, shape, direction when you project a curved line or surface onto a flat map. Different projections are optimised for different trade-offs. Calculations on projected data are only accurate over a relatively small portion of the earth's surface. There are many map projections available to suit various needs and localities. If you are going to be working with projected data, you need to pick a projection that suits your purposes and location. If you don't understand projections, your queries can easily produce garbage without realising it.
Unprojected data (ie, raw Lat/Lon coordinates which is what you have) involve much more complicated calculations as they are done on the curved surface of the spheroid representing the earth. There are a number of reference coordinate systems that are used to express Lat/Lon, however the most common is "WGS84" (which is what "GPS" coordinates are expressed in).
PostGIS objects (in the form of "simple features" as defined by the OGC) can be stored as either "geometry" types (projected coordinates) or "geography" types (unprojected Lon/Lat in WGS84: note the order, a common source of confusion!). As a bit of a wrinkle, Lon/Lat (order again!) can also be stored as a "pseudo projected" geometry type (typically with a projection SRID of '4326' for WGS84 Lon/Lat).
The method you use to calculate distance will depend on how you choose to store your points ('geometry' or 'geography').
See ST_Distance from the PostGIS docs for excellent examples of measuring distance using both geometry and geography points. Note, if you wish to calculate projected map distances you will need to pick an appropriate map projection and use ST_Transform to project your points to the appropriate spatial reference system (currently in SRID 4326- "GPS" coordinates). For only a few points the difference won't be at all noticeable, but once you start doing lots of more complex spatial queries, the difference can be significant. PostGIS has a lot more functions for geometry types than for geography types which may influence your decision. Also see ST_DistanceSpheroid for another possibility for calculating distance from Lon/Lat coordinates.
To start with, I'd store your points as 'geography' to simplify your experiments. Your distances will then be 'great circle' calcs in metres and you won't have to worry about projections initially.
Google Maps has the function isLocationOnEdge(point, polyline, tolerance) that takes a tolerance value in degrees and uses it to determine whether a point falls near a polyline.
Is there anything similar in Leaflet(or some plug-in) that does the same thing?
A handful library for such operation is Turf.
For your case, a simple approach would be to:
Create a polygon out of your polyline using turf.buffer with appropriate "tolerance" (Turf takes a distance at Earth surface, or degrees).
Check whether your point is within that polygon or not using turf.inside.
Unfortunately, turf.buffer is only an approximation, it does not takes geodesy into account… therefore for big tolerance you will have a deformed shape.
An exact method could be to:
Use instead turf.pointOnLine to find the nearest point of the polyline.
turf.distance to measure the distance between those 2 points, and compare with your tolerance (or even just Leaflet latLng.distanceTo, but you would have to convert GeoJSON points back to Leaflet LatLngs).
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'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.