Skip to content

Latest commit

 

History

History
111 lines (73 loc) · 3.46 KB

qgis.md

File metadata and controls

111 lines (73 loc) · 3.46 KB
Raw

Quick introduction to QGIS

QGIS is an open source GIS Desktop software that allow to create, visualize, analyze and publish geospatial information. It can be downloaded for free from their webpage.

Im this section of the documentation of QGIS you can find detailed information about the different options of the QGIS UI.

In this workshop of Geoinquietos you can find an Introductory workshop to QGIS.

Connect QGIS to a PostgreSQL/PostGIS database

In order to connect QGIS to a database we will use the option Layer -> Add PostGIS layer

Then, a new menu will appear and we will need to set the paramaters to enable to connection between the database and QGIS

Adding layers from database to QGIS

If we don't want to load all the data of the layer we can add a filter to our data by clicking on the option Set Filter, this option will open a new menu where we can select a condition to filter our data.

We can use different plugins of QGIS to apply spatial queries to the tables of our database and display the result on QGIS:

  • Spatial queries
  • DBManager

Installing a plugin to QGIS by selecting using the PLugins manager:

Importing data from QGIS to database

Download world_borders dataset from here.

Open the DBManager and select the option "import layer" to import the layer to the database:

Spatial queries with QGIS using DBManager

  • Centroids: SELECT ST_Centroid(geom) as geom FROM world_borders2

  • Buffers filtered: ST_Buffer(ST_Centroid(geom)::geography, 500000)::geometry as geom, admin FROM world_borders2 + SET filter to admin= 'Spain'

  • Gridding a geometry using PostGIS

We can apply the next SQL query to create a grid using PostGIS:

WITH usext AS (
    SELECT
        ST_SetSRID(CAST(ST_EXTENT(geom) AS geometry), 4326) as geom_ext, 
        60 as x_gridcnt,
        40 as y_gridcnt
    FROM world_borders2
    WHERE admin = 'United States of America'
),

grid_dim AS (
    SELECT (
            ST_XMax(geom_ext) - ST_XMin(geom_ext)
        ) / x_gridcnt AS g_width,
        ST_XMin(geom_ext) as xmin,
        ST_XMax(geom_ext) as xmax,
        (
            ST_YMax(geom_ext) - ST_YMin(geom_ext)
        ) / y_gridcnt as g_height,
        ST_YMin(geom_ext) as ymin,
        ST_YMax(geom_ext) as ymax
    FROM
        usext
),
grid AS (
    SELECT  
        x, y,
        ST_MakeEnvelope(
            xmin + (x - 1) * g_width,
            ymin + (y - 1) * g_height,
            xmin + x * g_width,
            ymin + y * g_height,
            4326
        ) AS geom
    FROM
        (SELECT generate_series(1, x_gridcnt) FROM usext) as x(x)
        CROSS JOIN
        (SELECT generate_series(1, y_gridcnt) FROM usext) as y(y)
        CROSS JOIN 
        grid_dim
)

SELECT ST_Intersection(grid.geom, world_borders2.geom) as geom from grid, world_borders2
WHERE ST_Intersects(grid.geom, world_borders2.geom) AND world_borders2.admin = 'United States of America'