densitypointqgisvector-grid
I want to create a map as shown in the image, where every grid cell shows the number of points (from a point shape layer) that are within a grid cell. The number of points in a grid cell shall be coded as different shades of red.
How can I achieve that in QGIS?
This is a kludge but it does work - haven't found a way to go directly from points to raster yet (but am hoping someone gives a solution here!).
Starting with a point grid (random points in the Serengeti from the Vector|Research tools|Random points tool):
Create a polygonal grid of the same extent and cell size as the raster you'd like to have (this from Vector|Research tools|Vector grid):
Use Vector|Analysis tools|Points in polygon, using the polygon grid and the point layer:
This gives you a new grid (here colour coded by the number of points in each polygon cell):
Now use Raster|Conversion|Rasterize to convert this to a raster, using the PNTCNT attribute for the raster values and the same cell size as chosend for the grid:
... giving a raster with values as per the points in each cell:
If you have ArcInfo, 3D or Spatial analyst, you can use the create random point tool to generate your points.
First I would start with the intersection between your fishnet and a dissolve of your other feature class. This will give you one multipart polygon per grid cell including some of the yellow polygons.
Then you can place one point for each of the resulting feature classes using the optional parameters of "create random points"(take the result of the intersection as constraining_feature_class and set the number of points = 1).
As a remark, I would not use the 150 m constraint if I were you because you already have a lot of constraints, but you can use a "minimum_allowed_distance" parameter to avoid points falling at less than 150 m from each other.
If you don't have the appropriate licence, have a look at Geospatial Modelling Environment(previously Hawth's tools) and their geostratrandompnts tool
Best Answer
From this point it is not clear what you would like to have as map in the end. If you need a vector file, do the following in QGIS:
If you need a raster file, you can simply convert the vector file to raster (search "rasterize" in toolbox).
Keep in mind that "density" might be number of points per grid cell or number of points per actual grid cell area. In the second case, you would need to divide the attribute column of the number of points by the actual area of the grid cell using
"[name of field with number of points]"/$areain the field calculator.("field" equals "attribute column" in QGIS language )