How to use the intersects function in QGIS?

How to use the intersects function in QGIS?

I would like to have an example on how to use the Intersects function in the Field Calculator of QGIS. Basically, I want to know which feature from a polylines layer intersects a polygons layer and store the result in an attribute. (0 = not intersecting, 1 = Intersecting).

Which is an example of an expression in QGIS?

Based on layer data and prebuilt or user defined functions, Expressions offer a powerful way to manipulate attribute value, geometry and variables in order to dynamically change the geometry style, the content or position of the label, the value for diagram, the height of a composer item, select some features, create virtual field

How to intersect a layer of a map?

Since the attributes of layers are not exposed I could not use in built expressions. Where main_map is the Item ID of a map. This will return a QGIS Geometry which can then be used to intersect the map layer. In order to achieve what I needed, I had to write a custom QGIS function to do the map intersects with the layer features.

How are topographic maps printed in QGIS server?

Since we are using QGIS Server to print the map all the logic is set up in print composer. All maps generated are dynamic (users can select a specific area within South Africa to print) and QGIS Server will use the map template to generate the map. On each topographic map is printed map sheets that correspond to the printed area as depicted below

Where do I find the expression tab in QGIS?

The Expression tab provides the main interface to write expressions using functions, layer’s fields and values. It contains widgets to: type expressions using functions and/or fields. At the bottom of the dialog, is displayed the result of the expression evaluated on the first feature of the layer.

How to group and Count attribute data in QGIS?

I can classify the data and enable feature count in the layer list but I cannot get counts in the attribute table. With the new virtual layer you can now use SQL on any layer in QGIS, for example the airport Shapefile from the QGIS sample data: To count the number of airports by USE type and compute the average elevation use:

What are the types of expressions in QGIS?

This group contains operators (e.g., +, -, *). This group contains functions to handle conditional checks in expressions. This group contains math functions (e.g., square root, sin and cos). This group contains functions to convert one data type to another (e.g., string to integer, integer to string).

How to create a new field in QGIS?

Load the shapefile railroads.shp in QGIS and press Open Attribute Table. Click on Toggle editing mode and open the Field Calculator dialog. Select the Create a new field checkbox to save the calculations into a new field. Add length as Output field name and real as Output field type, and define Output field width to be 10 and Precision, 3.

How to create sum column in QGIS field calculator?

So what I would do is create a new integer or real column (eg. “Sum”) and add the value you obtain from any of the methods into the Expression window. Then create another column (eg. “Percentile”) and edit the following expression to suit your column names:

How big is the QGIS field calculator test data?

The test data is only about 20 lines and points – it can be downloaded from https://drive.google.com/file/d/12rUV_pAaevs9vATbIsgc9QJvvziRygGN/view?usp=sharing for labeling the line layer.

How to count the number of lines in a polygon?

The output file example.shp contains the attribute count, which gives you the number of lines within each polygon. As already mentioned in the QGIS wiki link you provided you can find the tool in the ftools sextante toolbox. The Name is “Sum Line length” and it needs a polygon and a line shape.

Which is feature from polylines layer intersects polygons layer?

Basically, I want to know which feature from a polylines layer intersects a polygons layer and store the result in an attribute. (0 = not intersecting, 1 = Intersecting). I’m aware of the geoprocessing tools “Clip” or “Intersect”, but is it possible in the Field calculator ?

How to calculate the elevation of a polyline?

The polyline file contains a numeric attribute (elevation in meters). I would like to select all polylines that are contained within (and intersect) each polygon grid cell. I would then like to count the number of lines inside each cell, sum their elevations (from the attribute table) and report the average elevation of polylines in each cell.

How to set intersection attribute in spatial query?

Instead, use the Spatial query plugin to first select all features that intersect the polygon layer. Then use field calculator and only update selected features to set the intersection attribute to 1.

What happens when I use the field calculator?

When you click on the field calculator icon the dialog opens (see figure_attributes_3 ). If the layer is not in edit mode, a warning is displayed and using the field calculator will cause the layer to be put in edit mode before the calculation is made.

How to calculate the intersection attribute in Excel?

What you can do is calculate intersections with specific geometries, e.g. Instead, use the Spatial query plugin to first select all features that intersect the polygon layer. Then use field calculator and only update selected features to set the intersection attribute to 1.

How to select features from a layer in QGIS?

Select features from a layer using expressions. Deselect features from a layer using the Attributes toolbar. Using Query Builder to show a subset of features from a layer. Natural Earth has a nice Populated Places dataset. Download the simple (less columns) dataset For convenience, you may directly download a copy of datasets from the link below:

How to clip and projecting vector data with QGIS?

Clipping and projecting data with QGIS Extract the dataset to a simple path such as C:dataPhiladelphiaBaseLayers. Open the folder and explore it a bit. You’ll see a bunch of shapefiles that you can add to QGIS and examine. These datasets use a geographic coordinate system and cover the large region of greater Philadelphia.

How to get rid of attributes in QGIS?

Right-click the ne_10m_populated_places_simple layer and select Properties. In the General tab, scroll down to the Feature subset section. Click Query Builder. Enter the same expression you had entered earlier and click OK. Back in the main QGIS window, you will see rest of the points disappear.

How to select only selected features in polygon?

If you want to actually export the points which intersect your selected polygon to a new shapefile then ” Vector->Geopressing tools->Intersect ” and again tick the “Use only selected features” checkbox. The input layer will be your points layer, and the select layer will be your polygon layer. I can suggest using a QGIS Plugin namely Select Within.

How to select polygons in a PostGIS database?

I’m using QGIS 1.8 with a PostGIS database and I have three PostGIS-layers in QGIS a lines layer, a points layer and a polygon layer. What I’m trying to do is to select a polygon in the polygon layer and mark all points within this polygon.

Where do I find the function list in QGIS?

The Function List contains functions as well as fields and values. View the help function in the Selected Function Help. In Expression you see the calculation expressions you create with the Function List. For the most commonly used operators, see Operators.

Where to find field calculator in QGIS 2.8?

Visit http://docs.qgis.org/2.8 for QGIS 2.8 docs and translations. The Field Calculator button in the attribute table allows you to perform calculations on the basis of existing attribute values or defined functions, for instance, to calculate length or area of geometry features.

What can you do with a field calculator?

The Field Calculator button in the attribute table allows you to perform calculations on the basis of existing attribute values or defined functions, for instance, to calculate length or area of geometry features. The results can be written to a new attribute field, a virtual field, or they can be used to update values in an existing field.