[GIS] QGIS 2.10, csv points to vector grid

csvgrids-graticulesrastervector

I have a series of point data from a .csv file that I would like to display as a heatmap raster, the .csv data have various attributes with them and I would like to adjust the color ramp based on which of those attributes I select.
My first thought was to apply a color ramp to a vector grid based on how many points are in the grid cells.

However, I'm a beginner at both GIS and this is my first time using QGIS. I'm happy to thrash around on my own. I've tried to create a vector grid, but pretty sure I've done it wrong because although a layer was added to the TOC I saw nothing on the canvas.

Best Answer

For a quick 'heat map' view of your data:

Use the Layer > Add Layer > Add Delimited Text layer menu option from QGIS

Browse to your CSV file, and ensure your X and Y fields are being used properly:

Click OK and you'll be asked what coordinate system to use. Assuming you're using LAT/LON in WGS84, choose EPSG:4326, then click OK and your points should be added to the map:

Now double-click the layer you just created, choose the Style option, then from the pull-down in the top-left, choose Heatmap, and then Apply, and you should see your points rendered as a heat map:

Play around with those settings to get something you like!

In order to use the attributes you've mentioned, simply adjust the layer settings, query defining the features you're viewing, etc.

Otherwise, I can never get the 'heatmap' tool to work...

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QGIS – Generating Centroids for Vector Polygons Based on Raster and Land-Use Data

When performing analyses of raster data polygon-by-polygon (with non-overlapping polygons), think first of zonal operations. By definition, these are statistical summaries for groups of cells identified by polygons, with an automatic iteration over the polygons.

OK, but exactly what summary? Look to the output for a clue: a centroid will be designated by an (x,y) coordinate pair. Therefore, you want some kind of weighted averages of coordinates.

Evidently, "development density" should serve as the weights. Coupling this with the definition of a weighted mean gives us a prototype algorithm:

Create a grid of x-coordinates and a grid of y-coordinates. Call these [X] and [Y] respectively.
Let [W] represent the weighting grid (development density). Compute [X.weighted] = [W] * [X] and [Y.weighted] = [W] * [Y]. (This is a local operation: it is performed cell by cell.)
Obtain zonal sums of [X.weighted], [Y.weighted], and [W] itself. Instead of zonal sums, you may use zonal averages: the final results will not differ. (To do this, you may first have to rasterize the polygon layer.)
Bearing in mind that the zonal sums will have one value for each polygon, join them on common polygon identifiers and (using a field calculation) divide the zonal sum of [X.weighted] by the zonal sum of [W] to obtain the weighted mean of [X]. Repeat, mutatis mutandis, for Y.

Now, QGIS uses GRASS GIS for raster operations. The GRASS zonal mean is performed by r.average. I suspect (but have not confirmed) that row() and col() in r.mapcalc will compute [X] and [Y] for you. Finally, r.mapcalc will also perform the local multiplications needed in Step (2).

[GIS] How to load multiple vector layers into QGIS’ map canvas using python scripting

I figured it out! It was something very small. Basically, the layers you would like to see displayed on the canvas is provided through canvas.setLayerSet(list_of_layers).

# total list of layers actually displayed on map canvas
canvas_layers = []

# load vector layers
for files in os.listdir(source_dir):

    # load only the shapefiles
    if files.endswith(".shp"):

        # create vector layer object
        vlayer = QgsVectorLayer(source_dir + "/" + files, files, "ogr")
        print(files)

        # add the layer to the registry
        QgsMapLayerRegistry.instance().addMapLayer(vlayer)

        # combine extent of the current vector layer with the extent of the created "extent" rectangle object
        extent.combineExtentWith(vlayer.extent())
        canvas_layers.append(QgsMapCanvasLayer(vlayer))


# set extent to the extent of a larger rectangle so we can see all geometries       
canvas.setExtent(extent)

# provide set of layers for display on the map canvas
canvas.setLayerSet(canvas_layers)

Best Answer

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QGIS – Generating Centroids for Vector Polygons Based on Raster and Land-Use Data

[GIS] How to load multiple vector layers into QGIS’ map canvas using python scripting

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