I am new to PostGIS. I am looking to have a simple bounded (-200 < x, y, z < 200) data set of 1,000,000 points on a plain XYZ graph. The only query I need is a fast K nearest neighbors and all neighbors such that the distance is less than < N. It seems that PostGIS has a LOT of extra features that I do not need.
What do SRID do I need? One that does not concern with feet or meters.
Am I right that I need to use the function
ST_3DDistance to query for the K nearest neighbors with LIMIT K? or with a maximum distance of N.
To add a column, I need to use SELECT AddGeometryColumn ('my_schema','my_spatial_table','geom_c',4326,'POINT',3, false);. Is that correct?
What is the difference between a 3D point and a PointZ?
Will AddGeometryColumn ensure that my distance query is fast?
Is PostGIS the right choice for my use case? The rest of my DB is already integrated with PostgreSQL
Thanks!
What do SRID do I need? One that does not concern with feet or meters.
You don't "need" a srid. If your data is a in a coordinate system, find the right srid, otherwise, use 0.
Am I right that I need to use the function ST_3DDistance to query for the K nearest neighbors with LIMIT K? or with a maximum distance of N.
Yes, you're right.
To add a column, I need to use SELECT AddGeometryColumn ('my_schema','my_spatial_table','geom_c',4326,'POINT',3, false);. Is that correct?
Yes, but I'd use 0 for srid, instead of 4326 (that is for degrees).
What is the difference between a 3D point and a PointZ?
PointZ is a 3d Point.
Will AddGeometryColumn ensure that my distance query is fast?
AddGeometryColumn will just add some constraints to the table, ensuring that the geometries you insert are coherent with the column definition.
I don't think you need it, but you could try adding an index to your geometry column using CREATE INDEX index_name ON schema.table USING gist (geom_col);
Is PostGIS the right choice for my use case? The rest of my DB is already integrated with PostgreSQL
I think it is the easiest way, not necessarly the "right" one.
You could also implement a distance function without postgis, storing the three coordinates in three numeric fields.
Related
I'm using Postgres v13.
I couldn't find a clear example of how to achieve this basic calculation. I'm totally confused about how to handle geometry and geography points.
I have a table that stores points in the format geography(Point, 4326) alongside their timestamp.
I need to obtain the total distance in meters between timestamps A and B, and I need it to be super specific using spherical calculations. To be clear, there may be N points.
So far I've been using this query, but the distance is way off for long distances and I don't understand if there is any difference in creating a line using geometry points or geography points:
SELECT ST_Length(ST_MakeLine(lh.position::geometry order by report_time), TRUE)
FROM location_history AS lh
WHERE lh.device_id = 1
AND lh.report_time BETWEEN '2022-10-10T13:25:00.000Z' AND '2022-10-11T13:25:00.000Z'
GROUP BY lh.device_id;
Does this query make sense? ST_MakeLine only accepts geometry points and confuses me. Is there another way of creating a line with geography points?
ST_Distance is used in every example I could find, but it just compares 2 points!
Thanks!
I have a table called h2combines in Postgres that has two points-geometry fields: geom1 and geom2, and some other fields. I want to select all records that geom1 and gemo2 are with 400 miles. I tried this:
SELECT * from h2combines WHERE ST_DWithin(geom1, geom2, 643738);
However, it returned all rows. Seems I misunderstand something here.
Thanks!
Welcome to SO.
To get the distance in meters/miles you have to cast your geometries to geography, e.g.
SELECT * FROM h2combines
WHERE ST_DWithin(geom1::geography, geom2::geography, 643737.6);
Keep in mind that calculations using GEOMETRY and GEOGRAPHY are made differently, and so are their results. GEOGRAPHY calculates the coordinates over an spherical surface (which can be much slower than GEOMETRY) and uses meters as unit of measurement, while GEOMETRY uses a planar projection and uses the SRS unit.
I have to store logical 3d coordiantes of object in postgres database. Each object typicaly has from 50-1000 points and probably never exceed 10000.
My intension is to use column of type real [][] in postgres.
I looked also postGis extension and wonder if it is suitable solution, but could not answer myself of several questions:
Which spatial reference should i use - only need logical coordinates x,y,z could i specify left or right coordinate system - this is the part that mostly confuses me?
2.How should orgnaize data - line geometry seems natural way to me?
Would be posible to find distance between two points in the array (line geometry)?
It would be natural to use the PostGIS geometry(pointz)[] as data type, an array of three-dimensional points.
Here is an example that shows a constant of that type and calculates the distance between the points:
WITH x(p) AS (
SELECT '{POINT Z (1 2 3):POINT Z (3 0 2)}'::geometry(pointz)[]
)
SELECT st_3ddistance(p[1], p[2]) FROM x;
st_3ddistance
---------------
3
(1 row)
I know that it might be dumb question, but I'm searching for some time and can't find proper answer.
I have PostgreSQL database with PostGIS installed. In one table I have entries with lon lat (let's assume that columns are place, lon, lat).
What should I add to this table or/and what procedure I can use, to be able to count distance between those places in meters.
I've read that it is necessary to know SRID of a place to be able to count distance. Is it possible to not know/use it and still be able to count distance in meters basing only on lon lat?
Short answer:
Just convert your x,y values on the fly using ST_MakePoint (mind the overhead!) and calculate the distance from a given point, the default SRS will be WGS84:
SELECT ST_Distance(ST_MakePoint(lon,lat)::GEOGRAPHY,
ST_MakePoint(23.73,37.99)::GEOGRAPHY) FROM places;
Using GEOGRAPHY you will get the result in meters, while using GEOMETRY will give it in degrees. Of course, knowing the SRS of coordinate pairs is imperative for calculating distances, but if you have control of the data quality and the coordinates are consistent (in this case, omitting the SRS), there is not much to worry about. It will start getting tricky if you're planing to perform operations using external data, from which you're also unaware of the SRS and it might differ from yours.
Long answer:
Well, if you're using PostGIS you shouldn't be using x,y in separated columns in the first place. You can easily add a geometry / geography column doing something like this.
This is your table ...
CREATE TABLE places (place TEXT, lon NUMERIC, lat NUMERIC);
Containing the following data ..
INSERT INTO places VALUES ('Budva',18.84,42.92),
('Ohrid',20.80,41.14);
Here is how you add a geography type column:
ALTER TABLE places ADD COLUMN geo GEOGRAPHY;
Once your column is added, this is how you convert your coordinates to geography / geometry and update your table:
UPDATE places SET geo = ST_MakePoint(lon,lat);
To compute the distance you just need to use the function ST_Distance, as follows (distance in meters):
SELECT ST_Distance(geo,ST_MakePoint(23.73,37.99)) FROM places;
st_distance
-----------------
686560.16822422
430876.07368955
(2 Zeilen)
If you have your location parameter in WKT, you can also use:
SELECT ST_Distance(geo,'POINT(23.73 37.99)') FROM places;
st_distance
-----------------
686560.16822422
430876.07368955
(2 Zeilen)
postgresql 9.5 + postgis 2.2 on windows.
I firstly create a table:
CREATE TABLE points (
id SERIAL,
ad CHAR(40),
name VARCHAR(200)
);
then, add a geometry field 'geom':
select addgeometrycolumn('points', 'geom', 4326, 'POINT', 2);
and create gist index on it:
CREATE INDEX points_index_geom ON points USING GIST (geom);
then, I insert about 1,000,000 points into the table.
I want to query all points that within given distance from given point.
this is my sql code:
SELECT st_astext(geom) as location FROM points
WHERE st_distance_sphere(
st_geomfromtext('POINT(121.33 31.55)', 4326),
geom) < 6000;
the result is what I want, but it is too slow.
when I explain analyze verbose on this code, I found it dose not use points_index_geom (explain shows seq scan and no index).
So i want to know why it dose not use index, and how should i improve?
You cannot expect ST_Distance_Sphere() to use an index on this query. You are doing a calculation on the contents of the geom field and then you are doing a comparison on the calculation result. Databases may not use an index in such a scenario unless you have a function index that does pretty much the same calculation as in your query.
The correct way to find locations with in a given distance from some point is to use ST_DWithin
ST_DWithin — Returns true if the geometries are within the specified
distance of one another. For geometry units are in those of spatial
reference and For geography units are in meters and measurement is
defaulted to use_spheroid=true (measure around spheroid), for faster
check, use_spheroid=false to measure along sphere.
and
This function call will automatically include a bounding box
comparison that will make use of any indexes that are available on the
geometries.