Creating dummy variables between layers based on their geometries in QGIS [closed]

Question

I have the following data:

• administrative boundaries for the United States:
  https://biogeo.ucdavis.edu/data/gadm3.6/shp/gadm36_USA_shp.zip
• vector layer containing coastlines
  https://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/physical/ne_10m_coastline.zip
• river layer
  https://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/physical/ne_10m_rivers_lake_centerlines_scale_rank.zip

Question I. For a boundary layer

How to create a dummy variable being 1 if a district lies on the coast and 0 otherwise. Alternatively, I also want to know the distance from the district to the closest coast.

Question II. For a river layer

How to create a dummy variable where the dummy is 1 if a county is crossed by a river and 0 otherwise. The thing now is, that each river is assigned a scale rank variable. I only want the dummy to be 1 if rivers with scale ranks between 1 and 5 cross the county .

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I still did not manage doing the task using the field calculator.

Answer 1

This solution might be not the best one in terms of time consumption and performance because of the data size (calculating for the whole world). However, if you want to achieve a "rapid" result (here you do not need to set up anything) IMHO this is what you need. Nevertheless, handling large data sets will be much more efficient by means of PostgreSQL with PostGIS extension. Perhaps writing a PyQGIS script is even better or as was suggested by @csk via using the Field Calculator.

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I can suggest using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer...

Let's assume we have the following layers 'boundaries', 'coastlines', and 'rivers', see image below. Numbers over the lines represent the 'scalerank'. I did not use the data provided in the question, because of the large volume.

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Question I. For a boundary layer

With the following query, it is possible to create a dummy variable being 1 if a district lies on the coast and 0 if not, including a distance from the district to the closest coast (this will only give a value when a district does not intersect the coastline).

SELECT "boundaries".*,
(CASE
    WHEN "boundaries".id IN
        (SELECT "boundaries".id
         FROM "boundaries", "coastlines"
         WHERE st_intersects("boundaries".geometry, "coastlines".geometry))
    THEN '1'
    ELSE '0'
    END) AS dummy_var,
    ROUND(ST_Length(ST_ShortestLine("boundaries".geometry, "coastlines".geometry)),2) AS distance
FROM "boundaries", "coastlines"
GROUP BY "boundaries".id
ORDER BY MIN(ST_Length(ST_ShortestLine("boundaries".geometry, "coastlines".geometry)))

The output Virtual Layer with its Attribute table will look as following

In this case, be careful with geometric predicates since 'boundaries' and 'coastlines' cannot only intersect, as well as touch etc. Hence, you may extend the query with the OR operator, i.e.

WHERE st_intersects("boundaries".geometry, "coastlines".geometry) OR st_touches("boundaries".geometry, "coastlines".geometry)

Moreover, mind the CRS. Work with one that provides a distance in miles, meters etc.

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Question II. For a river layer

With the following query, it is possible to create a dummy variable 1 if a river crosses a district and 0 otherwise and for rivers with scale ranks between 1 and 5.

WITH "rivers_1_5" AS (
    SELECT *
    FROM "rivers"
    WHERE "scalerank" >= 1 AND "scalerank" <= 5
)
 
SELECT "boundaries".*,
(CASE
    WHEN "boundaries".id IN
        (SELECT "boundaries".id
         FROM "boundaries", "rivers_1_5"
         WHERE st_intersects("boundaries".geometry, "rivers_1_5" .geometry))
    THEN '1'
    ELSE '0'
    END) AS dummy_var
FROM "boundaries"
GROUP BY "boundaries".id

The output Virtual Layer with its Attribute table will look as follows.

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Answering the question in comments: "Where/How can I enter the code?"

In QGIS 3 paste the code and then run Add

In QGIS 2 paste the code and then run Ok

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Answer 2

How to find out how far a county is from the nearest coastline:

1. Install the NNJoin plugin.
2. Run the NNJoin plugin, using the county layer as the first input, and the coastline as the second input. It will create a duplicate of the county layer, with an additional 'distance' field that tells how far the nearest coastline is.

How to create a field that tells whether a district is on the coast

1. Install the RefFunctions plugin (this adds custom functions to the Field Calculator, including the intersecting_geom_count function)
2. Use the Field Calculator to create a field using an expression like: intersecting_geom_count('coastline')>0. The field value will be 0 (false) when the county doesn't intersect a coastline, and 1 (true) when it does.

Note: If the coastline layer doesn't perfectly match the county boundaries layer there may be coastal counties that don't get counted because their mapped boundaries don't actually intersect the coastline layer. To solve this issue, create a small buffer of the coastline layer. Use the coastline buffer instead of the coastline layer in step 2 above.

How to create a field that tells whether a district has a river of rank 1 through 5:

1. Filter the river layer by the rank field, using this filter expression: "rank" <= 5 AND "rank" > 0
2. Follow steps 1 and 2 above, using the river layer instead of the coastline layer.