Adding elevation data from contour lines to point layer using QGIS?

Question

I want to calculate the number of vacant lots within 300m from a given building. My source file contains point layers of buildings and vacant lots.

Here's a short description of my workflow. I've successfully completed steps 1 and 2. How do I complete step 3 ?

1. Create heatmap of vacant lots @300m radius.
2. Create a layer of contour lines based on the heatmap.
3. On "buildings" layer, add attribute indicating which contour line each building is located in.

Answer 1

Why convert the vacant lot's to an heatmap? It's easier than that:

• Create a buffer of 300m around each buildings (don't use the dissolve option);
• Use a Spatial Join to determine what lot points are within each building buffer;
• Use group stats plugin or dissolve, to count, sum, ... the lot point attributes by building.

Answer 2

I suggest different algorithm:

1. Surround each building with 300m radius circle polygon. If your building is point - it's easy. Else - extract each building to a point at his centroid.

2. Calculate how much vacant lots exist in each building 300m surrounding polygon. Again if the lot is polygon - Extract it to point at his centroid.

[Usually GIS tools like ArcGis, GM... have build in tool for all this together. Not familiar with QGIS. Usually they plot it as heat map].

In Global Mapper it's Density Grid tool. In QGIS I suggest if possible to work directly with .shp files, store them in PostGis DB (All open source and free) and then just execute PostGis query's (This is what the build in tools in GIS SW doing in the background anyway...). For example (maybe the last select has sql bug, the point is demonstrate the intention...):

WITH
    build AS (  --buildings polygons to points
        SELECT ST_Centroid(buildings.geom) AS centroid_geom, buildings.id AS id
        FROM buildings
    ),
    lots AS (  --vacant lots polygons to points
        SELECT ST_Centroid(vacant_lots.geom) AS centroid_geom, vacant_lots.id AS id
        FROM vacant_lots
    ),
    build_surrounding AS (  --build radius arroung each building
        SELECT id AS id, ST_Buffer(build.centroid_geom, 300)  --Assuming layers in metric units
        FROM build
    )
SELECT sum(ST_Within(lots.centroid_geom, build_surrounding)) AS counter, build_surrounding.id
FROM build_surrounding, lots;