I am working with Zillow neighborhood data provided freely at http://www.zillow.com/howto/api/neighborhood-boundaries.htm . I have successfully Imported the data with SRID 4120. Now I am trying to find out the neighborhoods by giving a coordinate(lat,long) and a radius. Finding a neighborhood in which my point exists is easy and is done through STIntersect method. I am actually confused with STDistance. For complete WA state data, It is giving me a maximum distaince of 4.xxx relative to any point in the wa. My question is what is the good way to find the points which are in a given radius and what is the unit.
thanx
zAfar
Got it, I was importing geography data into geometry column.
Related
I have a PostGreSQL table that has a geometry type column, in which different simple polygons (possibly intersecting) are stored. The polygons are are all areas within a city. I receive an input of a point (latitude-longitude pair) and need to find the list of polygons that contain the given point. What I have currently:
Unclustered GiST index defined on the polygon column.
Use ST_Contains(#param_Point, table.Polygon) on the whole table.
It is quite slow, so I am looking for a more performant in-memory alternative. I have the following ideas:
Maintain dictionary of polygons in Redis, keyed by their geohash. Polygons with same geohash would be saved as a list. When I receive the point, calculate its geohash and trim to a desired level. Then search in the Redis map and keep trimming the point's geohash until I find the first result (or enough results).
Have a trie of geohashes loaded from the database. Update the trie periodically or by receiving update events. Calculate the point's geohash, search in the trie until I find enough results. I prefer this because the map may have long lists for a geohash, given the nature of the polygons.
Any other approaches?
I have read about libraries like GeoTrie and Polygon Geohasher but can't seem to integrate them with the database and the above ideas.
Any cues or starting points, please?
Have you tried using ST_Within? Not sure if it meets your criteria but I believe it is meant to be faster than st_contains
I am using PostGis for Location based calculations in my Application. In a Table i have a column called 'location' in geography type(Point(lon lat))...Like this number of rows present in the Table.
I want to pass a point(Point(lon lat)) and check distance between this point(i passed) and location column in all rows....and if distance is less than 5 m....it will return the name of the point.How to query this.
Assuming that your srid of your data is 4326 the query you are looking for is:
SELECT the_geom FROM mytable WHERE ST_DWithin(the_geom,ST_GeomFromEWKT("srid=4326;POINT(lon lat)"), 0.0008);
Note that the units(0.0008) in ST_DWithin are in the same units of your projection, in the 4326 case they are degrees. If your projection data is in meters, you will be able to use meters.
For a production application you should use geometry types, is faster. From a stackoverflow previous question:
Short Answer: geography is a new data type that supports long range
distances measurements. If you use geography -- you don't need to
learn much about planar coordinate systems. Geography is generally
best if all you care about is measuring distances and lengths and you
have data from all over the world. Geometry datatype is an older data
type that has many functions supporting it and enjoys great support
from third party tools. Its 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. Refer to Section 8.8, “PostGIS
Function Support Matrix” to see what is currently supported and what
is not.
Great.Thank You. It works fine in database. I have following code from PHP..it returs like, Query Failed:
$locationresult=pg_query($con,"SELECT id,name FROM gps.locationnames WHERE ST_DWithin(location,ST_GeographyFromText('POINT(lon lat)'),500)") or die ('Query Failed:'.pg_last_error($con));
What is the problem here..
I'm using PostgreSQL with PostGIS. All my data has already decimal lat/long attached to it (i.e. -87.34554 33.12321) but to use PostGIS I need to convert it to a certain type of SRID.
The majority of my queries are looking for data inside a certain radius.
What SRID should I use? I created already a geometry column with SRID 4269.
In this example:
link text the author is converting SRID 4269 to SRID 32661. I'm very confused about how and when to use these SRIDs. Any lite on the subject would be truly appreciated.
As long as you never intend to reproject/transform the data to another coordinate system, it doesn't technically matter what srid you use. However assuming you don't want to throw away that important metadata, and you do want to transform it, you will want to ensure your assigned srid matches the data, so postgis knows what to do when the time comes.
So why would you want to reproject from epsg:4269? The answer is because certain types of queries (such as distance) make no sense in this 'unprojected' world. Your units are in decimal degrees, and a straight measurement of x decimal degrees is a different real distance depending where in the planet you are.
In your example above, someone is using epsg:32661 as they believe it will give them better accuracy for the are they're working in. If your data is in a specific area of the globe, you can select a projection that's accurate for that area. If it spans the entire globe, you have to choose a projection that does 'ok' for your needs.
Now fortunately PostGIS has a few ways of making all this easier. For approx distances you can just use the st_distance_sphere function which, as you might guess, assumes the earth is a sphere. Or the more accurate st_distance_spheroid. Using these, you don't need to reproject and you will probably be fine for your distance queries except in edge cases. Newer versions of PostGIS also let you use geography columns
tl;dr - use st_distance_spheroid for your distance queries, store your data in geography columns, or transform it to a local projection (when storing, or on the fly, depending on your needs).
Take a look at this question: How do you know what SRID to use for a shp file?
The SRID is just a way of storing the WKT inside the database (you may have noticed that, altough you store lat/long points, the preferred storing is a long string with number and capital letters).
The SRID or EPSG can be different for the country/state/... altough there are some very common ones especially the 2 mentioned by you. If you need specific info what area uses what SRID, there is a database for handling that.
Inside your database, you have a table spatial_ref_sys that has the information on what SRID PostGIS knows about.
What is the best way to calculate AND add a field to a data file that shows the crow-fly distance (in miles) between two zip codes for each record (250K+) in a file? THANKS
Use this page for the raw distance information. Then use this website for the formula from Adam Bellaire's response to calculate the distance.
There is no reason to use an external service as zip codes really don't change all that often.
Enjoy!
Get a Google Earth API Key and use the API to calculate distances between two zip codes in your language of choice.
UPDATE:
If a web service isn't for you, you can check my OLD Visual Basic Posting on planet source code. It has a database of zip codes, their lat/long positions, and some VB code to calculate the distances between two zip codes. The Zipcode DB will probably require some updating, and it's in a MS Access format (but you can move that data anywhere).
You could use the Yahoo Geocoding Service to first get the latitude and longitude corrodinates for each zip code, then simply use the haversine formula to get the distance between any two sets of latitude/longitude data.
Here is a c# implementation of the haversine formula.
Enjoy!
I have a situation that's similar to what goes on in a job search engine where you type in the zipcode where you're searching for a job and the app returns jobs in that zipcode as well as in zipcodes that are 5, 10, 15, 20 or 25 miles from that zipcode, depending on preferences set by the user.
How would you calculate the neighboring locations for a zipcode?
You need to get a list of zip codes with associated longitude / latitude coordinates. Google it - there are plenty of providers.
Then take a look at this question for an algorithm of how to calculate the distance
I don't know if you can count on geonames.org to be around for the life of your app but you could use a web service like theirs to avoid reinventing the wheel.
http://www.geonames.org/export/web-services.html
I wouldn't calculate it, I would stored it as a fixed table in the database (only to change when the allocation of ZIP codes changes in a country). Make a relationship "is_neighbor_zip", which has pairs (smaller, larger). To determine whether two codes are neighboring, check in the table for specific pair. If you want all neighboring zips, it might be better to make the table symmetric.
You need to use a GIS database and ask it for ZIP codes that are nearby your current location.
You cannot simply take the ZIP code number and apply some mathematical calculations to find other nearby ZIP codes. ZIP codes are not as geographically scattered as area codes in the US, but they are not a coordinate system.
The only exception is that the ZIP+4 codes are sub-sections of the larger ZIP code. You can assume that any ZIP+4 codes that have the same ZIP code are close to each other.
I used to work on rationalizing the ZIP code handling at a company, here are some practical notes I made:
Testing ZIP codes
Hopefully has other useful info.
Whenever you create a zipcode, geocode it (e.g. google geocoder api, saving the latitude and logitude) then google the haversine formular, this will calculate the distance (as the crow flies) from a reference point, which could also be geocoded if it is a town or zipcode.
To clarify some more:
When you are retrieving records based on their location, you need to compare each longitude and latitude DECIMAL with a reference point (your users geo-coded postcode or town name)
You can query:
SELECT * FROM photos p WHERE p.long < 60 AND p.long > 50 AND p.lat > -10 AND p.lat > 10
To find all UK photos etc because the uk is between 50 and 60 degrees longitude and +-10 latitude (i might have switched long with lat, i'm fuzzy on this)
If you want to find the distance then you will need to google the haversine formula and plug in your reference values.
Hope this clears things up a little bit more, leave a comment if you need details