Importing shapefiles with shp2pgsql

If you need to import a shapefile in PostGIS, you have at least a couple of options such as the ogr2ogr GDAL command, as you have seen previously, or the shp2pgsql PostGIS command.

In this recipe, you will load a shapefile in the database using the shp2pgsql command, analyze it with the ogrinfo command, and display it in the QGIS Desktop software.

How to do it...

The steps you need to follow to complete this recipe are as follows:

1. Create a shapefile from the virtual driver created in the previous recipe using the ogr2ogr command (note that in this case, you do not need to specify the -f option, as the shapefile is the default output format for the ogr2ogr command):

   $ ogr2ogr global_24h.shp global_24h.vrt
   

2. Generate the SQL dump file for the shapefile using the shp2pgsql command. You are going to use the -G option to generate a PostGIS spatial table using the geography type, and the -I option to generate the spatial index on the geometric column:

   $ shp2pgsql -G -I global_24h.shp chp01.global_24h_geographic > global_24h.sql
   

3. Analyze the global_24h.sql file (in Windows, use a text editor such as Notepad):

   $ head -n 20 global_24h.sql
   SET CLIENT_ENCODING TO UTF8;
   SET STANDARD_CONFORMING_STRINGS TO ON;
   BEGIN;
   CREATE TABLE "chp01"."global_24h_geographic" (gid serial PRIMARY KEY,
   "latitude" varchar(80),
   "longitude" varchar(80),
   "brightness" varchar(80),
   ...
   "frp" varchar(80),
   "geog" geography(POINT,4326));
   INSERT INTO "chp01"."global_24h_geographic" ("latitude","longitude","brightness","scan","track","acq_date","acq_time","satellite","confidence","version","bright_t31","frp",geog) VALUES ('-23.386','-46.197','307.5','1.1','1','2012-08-20','0140','T','54','5.0','285.7','16.5','0101000000F0A7C64B371947C0894160E5D06237C0');
   ...
   

4. Run the global_24h.sql file in PostgreSQL:

   $ psql -U me -d postgis_cookbook -f global_24h.sql
   

   Tip

   If you are on Linux, you may concatenate the commands from the last two steps in a single line in the following manner:

   $ shp2pgsql -G -I global_24h.shp chp01.global_24h_geographic | psql -U me -d postgis_cookbook
   

5. Check if the metadata record is visible in the geography_columns view (and not in the geometry_columns view as with the -G option of the shp2pgsql command, we have opted for a geography type):

   postgis_cookbook=# SELECT f_geography_column,   coord_dimension, srid, type FROM geography_columns   WHERE f_table_name = 'global_24h_geographic';
   

    f_geography_column | coord_dimension | srid  | type  
   --------------------+-----------------+-------+-------
    geog               |            2    | 4326  | Point
   

6. Analyze the new PostGIS table with ogrinfo (use the -fid option just to display one record from the table):

   $ ogrinfo PG:"dbname='postgis_cookbook' user='me' password='mypassword'" chp01.global_24h_geographic -fid 1
   INFO: Open of `PG:dbname='postgis_cookbook' user='me' password='mypassword''
   using driver `PostgreSQL' successful.
   Layer name: chp01.global_24h_geographic
   Geometry: Point
   Feature Count: 30326
   Extent: (-155.284000, -40.751000) - (177.457000, 70.404000)
   Layer SRS WKT:
   (unknown)
   FID Column = gid
   Geometry Column = the_geom
   latitude: String (80.0)
   longitude: String (80.0)
   brightness: String (80.0)
   ...frp: String (80.0)
   OGRFeature(chp01.global_24h_geographic):1
     latitude (String) = -23.386
     longitude (String) = -46.197
     brightness (String) = 307.5
     ...frp (String) = 16.5
     POINT (-46.197 -23.386)
   

7. Now open QGIS and try to add the new layer to the map. Navigate to Layer | Add PostGIS layers and provide the connection information, and then add the layer to the map as shown in the following screenshot:

   

How it works...

The PostGIS command, shp2pgsql, allows the user to import a shapefile in the PostGIS database. Basically, it generates a PostgreSQL dump file that can be used to load data by running it from within PostgreSQL.

The SQL file will be generally composed of the following sections:

• The CREATE TABLE section (if the -a option is not selected, in which case, the table should already exist in the database)

• The INSERT INTO section (one INSERT statement for each feature to be imported from the shapefile)

• The CREATE INDEX section (if the -I option is selected)

Note

Unlike ogr2ogr, there is no way to make spatial or attribute selections (-spat, -where ogr2ogr options) for features in the shapefile to import.

On the other hand, with the shp2pgsql command, it is possible to import the m coordinate of the features too (ogr2ogr only supports x, y, and z at the time of writing).

To have a complete list of the shp2pgsql command options and their meaning, just type the command name in the shell (or in the command windows, if you are on Windows) and check the output.

There's more...

If you do not prefer using the command-line utilities, you can still export your shapefiles, even multiple ones all at once, by using shp2pgsql-gui, which is a GUI software that can also be used as a plugin in pgAdmin. From its interface, you can select the shapefiles to import in PostGIS and select all the parameters that the shp2pgsql command allows the user to specify as shown in the following screenshot:

PostGIS 2.0 onward, shp2pgsql-gui is also a GUI for the pgsql2shp command (there will be a recipe about it later). It allows the user to select one or more PostGIS tables and export them to shapefiles. The GUI lets the user specify all the options that can be used in the pgsql2shp command.

There are other GUI tools to manage data in and out of PostGIS, generally integrated in the GIS Desktop software. In the last chapter of this book, we will take a look at the most popular ones.