Adding points from coordinates

The easiest way is to use QuickWKT

Count the number of features in area

  1. select the right layer
  2. on the top panel, third row, select the tool Select Features by area or single click
  3. select features
  4. Open the attribute table for layer. In the heder, there should be the number of selected features

Layers

Adding layers

To add a layer, use Layer -> Add Layer. Each layer type has its own option in this menu.

Temporary layer

Temporary layers are useful for experimenting. Whether we want to add a point, line, or area, we need a layer for that even if we do not intend to save the result. The right layer for this is the temporary scratch layer we can add it by:

Layer -> Create layer -> New Temporary Scratch Layer...

Invalid layer

The layer can be marked as invalid because of a missing or invalid index column. Each Postgis layer needs an id column consisting of unique integer values.

Postgis layer

Postgis layers are layers that are loaded from a Postgis database. A postgis layer can be created from:

  • a table,
  • a view,
  • or a query.

In all cases, the layer data has to fulfill the following requirements:

  • the layer has to have a column with unique integer values (id) that is used as a primary key,
  • the layer has to have a column with geometry data, and
  • the layer definition must not modify the data in the database (due to read-only transaction mode).
    • nextval() is therefore not allowed
    • creating temporary tables is not allowed

Debugging

As an open-source project, QGIS has many bugs.

Frequently, the program does not behave as expected, but no problem is reported. In such cases, the best way to find out what is going on is to open the log window: View -> Panels -> Log Messages.