- QGIS 2 Cookbook
- Alex Mandel Víctor Olaya Ferrero Anita Graser Alexander Bruy
- 562字
- 2021-07-16 11:12:35
Importing data from text files
Data can be imported from text files, providing some additional about how the geometry information is stored in the text. This recipe shows you how to create a new points layer, based on a text file.
How to do it…
- Select the Add delimited text layer menu entry from the Layer menu. You will see a dialog like the following one:
- In the upper field, enter the path to the
elev_lid792_randpts.csvfile in the sample dataset. That file contains a points layer as text. - Once you enter the file path or select it in the file browser that can be opened by clicking on the Browse button, the fields in the lower part of the dialog will be filled, as shown in the following screenshot:
The X field and Y field drop-down lists will be populated with the fields that are available, which are described in the first line of the text file. Select X for X field and Y for Y field. Now, QGIS knows how to create the geometries and has enough information to create a new layer from the text file.
- Enter a name for the layer in the Layer name field and click on OK. The layer will be added to the QGIS project, as shown in the following screenshot:
- No information about the CRS is contained in the text file or entered in the parameters dialog, so it must be added manually. In this case, the CRS used is
EPSG:3358. To set this as the CRS of the layer, right-click on the layer name and select Set layer CRS:
- In the CRS selection dialog, select the
EPSG:3358CRS and click on OK. The layer now has the correct CRS.
How it works…
Data is read from the text file and processed to create geometries. All the fields in the table (all data in a row in the text file) are also added, including the ones used to create the geometries, as you will see by right-clicking on the layer and selecting Open attribute table, as shown in the following screenshot:
Along with the CSV file, this file may contains a CSVT file, which describes the types of the fields. This is used by QGIS to set the appropriate type for the attributes table of the layer. If the CSVT file is missing, as in our example's case, QGIS will try to figure out the type based on the values for each field.
There's more…
Layers created from text files are not restricted to point files. Any geometry can be created from the text data. However, if it is not a point, instead of selecting two columns, you must place all the geometry information in a single one and enter a text representation of the geometry. QGIS uses the Well-Known Text (WKT) format, which is a text markup language for vector geometries, to describe geometries as strings. Here is an example of a very simple CSV file with line features and two attributes:
geom,id,elevation LINESTRING(0 1, 0 2, 1 3),1,50 LINESTRING(0 -1, 0 -2, 1 -3),2,60 LINESTRING(0 1, 0 3, 5 4),3,70
See also
- To know more about the WKT format, you can go to http://en.wikipedia.org/wiki/Well-known_text
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