I would like to find out what is the pragmatic meaning of SRID (spatial reference id) in postgis.
I really do not understand what it is for. Can anyone throw some light on the matter?
For instance I noticed that the postigs function ST_GeomFromText(text WKT, integer srid) accept such an (optional) param as second argument. Why would I need to pass it in the get postigs to turn the text representation into a binary one? What is the value it adds?
Thanks
Spatial reference ID refers to the spatial reference system being employed -- this is important when going from a a geographic view of the world to a projected view of the world, ie, what you see when you look at a 2 dimensional paper map.
Spatial reference systems contain a couple of elements.
Firstly, the geoid, is a model of the shape of the earth -- the earth is not a sphere (sh, don't tell Google), it is in fact an oblate spheroid. The geoid shape used for GPS is known as WGS84, which is a model that works faily well globally. National mapping agencies use other geoids, that might be a better fit to local geographies.
Secondly, the projection type. This is essentially the mathematical model used to go from a 3D to a 2D representation of the world. Types include Mercator, Transverse Mercator, (both cylindical), Azimuthal, Conic, etc. All of these have trade-offs between accurately measuring distance, area or direction -- you can't preserve all three.
So, essentially when you declare a SRID in Postgis you are saying use this geoid and this projection model. Under the hood, Postgis uses a library called Proj.4, and based on the SRID information, it can convert from one coordinate system to another.
So, for example, to convert from lat/lon, which is know as 4326 in SRID terms to 900913, which is spherical Mercator, as used by Google/Bing maps, and other web mapping frameworks, you could run something like:
select st_astext(st_transform(st_setsrid(st_makepoint(-.5,52),4326),900913));
This is an example of a query I use. It uses the Lambert azimuthal equal-area projection (ETRS89-LAEA, srid = 3035).
ST_GeomFromText('POINT(2843711.1098048678, 2279498.6551480694)', 3035);
If you don't pass the srid, postgis will not know which spatial reference system to use.
Related
I dont't have any experience with the geographics databases so I hope that you be patiente with me, the thing is: I know that the datatype geography is used if all you care about is measuring distances and lengths and you have data from all over the world and the datatype geometry is used is best if you are pretty comfortable with spatial reference systems or you are dealing with localized data where
all your data fits in a single spatial reference system (SRID), or you need to do a lot of spatial processing (that's what I read in chapter 3 of the documentation of postgis) and I saw on the link https://www.tutorialspoint.com/postgresql/postgresql_data_types.htm the following datatypes: point, line, lseg, box, path, path, polygon and circle. And then I saw the documentation again of the postgis on the section 4.1.3 SQL-MM Part 3 about the following types with the extension well-know text: circularString, compoundCurve, curvePolygon, multiCurve, multiSurface. What I don't understand is: what is the difference between these datatypes?
I would like to buffer the warning polygon by two miles can anyone help me with
this so if ema personal are with in to miles of the warning the are listed, I've been trying to use ST Buffer (to expand the polygon coverage for the search) but cant seem to get it right? Is it in Meters (3218.69)? I'm using the latest opengeo suite.
SELECT DISTINCT ON (ema.name)
ST_X(ema.geom),ST_Y(ema.geom),ema."name", torpoly.expire
FROM ema INNER JOIN torpoly ON ST_Within(ema.geom, ST_BUFFER(torpoly.geom)
ORDER BY ema."name"
Your options are either:
Use an appropriate projected coordinate system for the region that uses linear units in metres or feet (UTM, State plane, etc.). All distance calculations on geometry types use a Cartesian coordinate system, which is quick and simple.
Use the geography type, which does distance calculations on objects with EPSG:4326 (lat/lon) with distance units in metres. If you don't want to change the data types, you can use a geom::geography cast, and maybe make an index on that cast.
And never do ST_Within(.., ST_Buffer()) for this type of analysis. It is slower and imperfect. Instead, use ST_DWithin, which finds all geometry/geography objects within a distance threshold of each other, which is just like a buffer. This function may use a spatial GiST index, if present.
Assuming that I have a table of postal codes, with a Geography column that was populated with Global - WGS84 (SRID 4326), can I accurately compare them (using STDistance) with a Geography point that has been populated with North America – NAD83 (SRID 4269)?
In short, No.
SQL Server requires for spatial functions that all items' have the same SRID. This is because the SRID provides other information in the background used to calculate distances and such on an ellipsoidal model.
That said, you could have a second column which is calculated to have a common SRID and use that for distance calculations. It's as simple as:
Declare #commonSrid geography = geography::STGeomFromWKB(<existing column>.STAsBinary(), 4326);
In doing this, you must be sure that all SRIDs are based from latitude and longitude decimal coordinates, and not for example grid references. Also because you're not doing a proper conversion between them, you may find distances are not 100% accurate - but they will be very very close.
I've recently start working on a personal project involving geo locations, maps (Google Maps V3) etc.
The project is developed in Python and is intended to run on Google App Engine.
I've learned that in order to find markers/position close to a position one can use to geohash algorithm (which is pretty cool).
What I don't understand is this: lets say I have all my locations in the data store (along with a latitude, longitude and a geohash (with high precision) of each location.)
I know that I should use the prefix of the geohash (to match locations within), but how do I calculate a geohash of a bounding box? Considering the bounding box is made up of two points, North-East and South-West, I do not understand how to go about doing this..
In order for me to querying which locations should be returned for the currently visible bounding box, I need the geohash of the visible/viewable bounding box - Now I know I can geohash the center location on the viewable map, but I do not know how many letters to cut off (to reduce precision) to achieve 'a fit' to the actual bounding box. (Or maybe that isn't the way...?)
What do you do when the bounding box container to geohashes? (like in the middle of the viewable area it splits between 'dqcjr0' and 'dqcjqb')
Also, lets assume I have a 5 letter geohash, how can I convert that back into a viewable bounding box? or in other words, how do I know what is 'included' the hash, and what is in adjacent hashes?
Thanks in advance for your help,
Ken.
I used geohash with google app engine data types ie db.GeoPt a lot and I used to keep a geohash which I found was inferior to combine the db.GeoPt with the very good but a bit slow library called geomodel Geomodel can do bounding-box and radius mappings and I suggest that you try with the bounding-box since it is not as expensive as the radius. I can perform a bounding-box query like this:
articles = Article.bounding_box_fetch(Article.all().filter('modified >',
timeline).filter('published =',
True).filter('modified <=',
bookmark).order('-modified'),
bounds,
max_results=PAGESIZE + 1)
So even if I stored geohash for every article, using geomodel was much better in my case. Maybe you already evaluated geomodel and found that it didn't suit your purpose and that you absolutely must use geohashes I suggest that we agree on a common library for the geohash so that our coordinates hash to the same value. I do keep a version of the geohash library I used somewhere but it is probably outdated and the recent articles about geospatial queries also metion geomodel, so if you didn't look at geomodel yet, I really propose you look at the geomodel library to perform your geospatial queries.
Ken
You may want to update your question stating whether or not you're using django / django-nonrel?
I'm just about to try this (currently archived) port of Geomodel to django:
https://bitbucket.org/scotch/django-geomodel/
Kyle suggests that the upcoming Google "full text search" would replace his Geomodel implmentation. Nonetheless, I need it working within the next few days.
(My current conversation re: this topic:
https://groups.google.com/forum/#!topic/django-non-relational/WCxFjkUzw18
)
Jon
I'm playing with the new geography column in SQL Server 2008 and the STGeomFromText function. Here is my code (works with AdventureWorks2008)
DECLARE #region geography;
set #region = geography::STGeomFromText('POLYGON((
-80.0 50.0, -90.0 50.0,
-90.0 25.0, -80.0 25.0,
-80.0 50.0))', 4326);
SELECT #region;
My question is about the 4326 in the code. It is supposed to be a spacial Reference ID. When I go to MSDN there isn't a lot on it. If I change the value to 56 I get an error telling me the value must be in the sys.spatial_reference_systems table.
You can look at that table by executing:
select * from sys.spatial_reference_systems
There is a well_known_text column in that table, but it doesn't tell me much. The value for 4326 is:
GEOGCS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84", 6378137, 298.257223563]], PRIMEM["Greenwich", 0], UNIT["Degree", 0.0174532925199433]]
Can anyone explain this mystery to me? What is the SRID?
So I ended up talking with an ex-military guy yesterday who was a radar/mapping specialist.
Basically, he knew exactly what that number (4326) was, where it came from, and why it is there.
It is an industry standard for computing geography. The problem is that the earth is not a perfect sphere (it bulges in the middle), and SRID 4326 accounts for that.
As I stated, the table sys.spatial_reference_systems lists all of the code and what they are. But the short version is that you are really only going to use 4326 unless you have a very specific reason to use something different.
SRID = Spatial Reference IDentifier
coordinates must use the same SRID to be comparable. otherwise you'd end up comparing kilometeres and miles. or something similar.
There are a lot of systems to map the earth. For example you want to map some state in USA. You can set the most south-east point as 0,0 and map all other spatial coordinates according to this point. On the other hand you may want to map some spatial data that span all over the map. In any case you must choose some point as 0,0. In addition you must select some sort of measurement unit: miles/kilometers/degrees/some other magical unit that suits you better. Over the years a lot of such systems where developed. Each has its own zero point, its own coordinates, its own rules about if the earth is flat or not. SRID or SRS is the id of such system. Using this id you can map point expressed in one system to another system, although sometimes it involves some pretty complex math.
And about 4326 SRID. It also called "WGS 84"
(http://en.wikipedia.org/wiki/World_Geodetic_System) system. It's the most common system to represent point on spherical(not flat) earth. It uses degree,minute,second notation and its x and y coordinates are usually called latitude and longitude.
Most used non-spherical earth projection is called UTM. You can read about it here: http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system
Anyway, as long you are not doing any spatial conversions from one system to other, you don't really care about the system that you data uses.
I have found this website: http://spatialreference.org/ref/epsg/4326/ quite helpful in understanding the SRID you intend to use. It provides a handy map, some bounding box information and other links.
For other SRIDs simply change the digits at the end of the URL to what you are after.
The distance returned depends on the "Spatial Reference Identifier (SRID)" you define for your geography types.
In the example below, the default SRID of 4336 is used, see the second argument of STGeomFromText. This means the distance returned is in meters, you find this via querying the catalog view spatial_reference_systems i.e. select srs.unit_of_measure from sys.spatial_reference_systems as srs where srs.spatial_reference_id = 4326
As an alternative to STGeomFromText, you can use parse which assumes a SRID of 4326 and you don't have to specify one explicitly.
When calculating the distance between two points, you must use the same SRID for both geography types else you get an error. Example:
DECLARE #address1 GEOGRAPHY
DECLARE #address2 GEOGRAPHY
DECLARE #distance float
SET #address1 = GEOGRAPHY::STGeomFromText ('point(53.046908 -2.991673)',4326)
SET #address2 = GEOGRAPHY::STGeomFromText ('point(51.500152 -0.126236)',4326)
SET #distance = #address1.STDistance(#address2)
SELECT #distance --this is the distance in meters