Qgis.spatialthoughts.com-Tutorial Making Heatmaps Using QGIS and GRASS

7/21/2019 Qgis.spatialthoughts.com-Tutorial Making Heatmaps Using QGIS and GRASS

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Tutorial: Making Heatmaps using QGIS and GRASS

Please see Creating Heatmaps for an updated version of this tuto rial.

Heatmaps are one o f the best visualization t oo ls f or dense po int data. Heatmaps are used to easily

identif y f ind clusters where there is a high concentrat ion o f activity. In this tuto rial we will explore two

dif f erent method o f creating heatmaps in QGIS and work with crime related data to f ind crime hotspo ts with

cluster analysis.

QGIS 1.8 introduced a ‘Heatmap’ plugin which allows you to create heatmaps directly f rom QGIS. Prior to

that , one could use variety o f GRASS algorithms via the GRASS plugin in QGIS to create the heatmap. We

will use both these methods and obtain the heatmap. The data f or t his tuto rial comes f romLondon

datastore . There is a wealth of open data available and fo r this tutorial I downloaded the2011 Crime data

for Surrey.

Part 1: Heatmap Plugin

Note that the plugin method is experimental and can be slow/unstable at times. If you are working with a large

dataset or scientific analysis, GRASS method outlined in Part 2 is recommended. But the plugin method is

simpler and suited for quick visualiaztions.

To start, unzip the data to a f older. The data is in a CSV f ormat. Use the steps outlined in this

tutorial to import CSV data to QGIS. You have to specif y Northing’ as X and ‘East ing’ as Y f ields. The

CRS f or this data is OSGB 1936 British National Grid.

http://qgis.spatialthoughts.com/2012/01/importing-spreadsheets-or-csv-files-to.html

http://qgis.spatialthoughts.com/2012/01/importing-spreadsheets-or-csv-files-to.html

http://data.london.gov.uk/datafiles/crime-community-safety/police-uk-crime-data-surrey.zip

http://data.london.gov.uk/datastore/package/policeuk-crime-data

http://www.qgistutorials.com/docs/creating_heatmaps.html

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Save the layer as a shapef ile so it is easier to reuse without import ing again. Right click on the layer

and select ‘Save As’.



Name the shapef ile as crime.shp. Check the box next to ‘Add saved f ile to map’ and click OK.



The new vector ;ayer will be added to the table of content. Remove the older import ed layer by right

clicking and selecting ‘Remove’.



To use the heapmap plugin, we must enable it f irst . Click on Plugins → Manage Plugins. Scroll down

and make sure the box next t o ‘Heatmap’ is checked.



Open the Heatmap plugin by go ing to Raster → Heatmap → Heatmap

In the Heatmap plugin dialog the crime layer would be selected as the Input Point Vector. Name the

output raster as crime_heatmap. The Radius is the area around each point that will be used to

calculate the ‘heat’ a pixel received. The Decay ratio is how far a po int will inf luence o ther pixels.

Clicking on ‘Help’ will bring up a more detailed explanation o f the algorithm. For our analysis we would

like to have a pixel’s value determined by how many points are within 1 km (1000m) of the location,

so we select the radius at 1000 and Decay Ratio as 0. Also set the pixel resolution o f the output

raster to be 100. Click OK.



We have more than 100,000 points in our layer, so the heatmap process ing map take a while. ( In my

case about 30 minutes ). Once the heatmap raster is created, you will get a prompt to select t he CRS.

Select OSGB 1936 British National Grid and click OK.



You will see a gray rectangle covering your layer extents . This is the heatmap raster, but it doesn’t

have any styles applied to it.



Right click on the crime_heatmap layer and select Properties.



Under the ‘Style’ tab, select the Pseudocolor as the Color map and click OK.



Now you will see something resembling a heatmap.



Let’s do so me styling adjustments to view our heatmap better. Right click on t he crime_heatmap layer

and select ‘Properties’. In the ‘Transparency’ tab, move the slider f or Global transparency to 20%. We

also want to make the pixels with 0 values t ransperant, so click ‘Add Values Manually’ next to the

Transparent pixel list. Add the value 0 to the list. Click OK.



You will now see the underlying points along with the heatmap.



Now let’s identify clusters of ‘crime hotspo ts ’. Say you wanted to identif y areas with high crime

concentrat ion and deploy more patro l there. Or we may want to use these clusters in some further

analysis. We will use Raster Calculator to identify these clusters . Click on Raster → Raster calculato r.



You will have to decide on a threshold value. For our example, we will use 100 as a threshold to

identif y what is a ‘cluster’ and what is no t. In Raster calculato r dialog, name the output layer as

crime_heatmap_100 and check the bo x next to ‘Add result to project’. Input the expression

crime_heatmap@1 >= 100. What this means is that all pixels will values below 100 will be assigned a

value of 0 and all other pixels a value of 1. Click OK.



Now you will be able to see clusters in white pixels. We will convert t hese to vectors . Click on Raster

→ Conversion → Polygonize. ( Enable GdalToo ls plugin if you do not see this menu ).



Select crime_heatmap_100 as the input f ile. Name the output f ile as crime_clusters.shp. Check the

box next to ‘Load into canvas when f inished’ and click OK.

You will see the vector polygons f rom the crime_clusters layer. You will also not ice that t his layer

contains polygons f or 0 value areas also. We will need to remove these to get a layer with our

clusters. Right click on the layer and select ‘Open Attribute Table’.



Click ‘Advanced Search’ at the bot to m of the Attribute Table.



Enter the expression ‘DN = 0’ and click OK.



You will see t he polygons with 0 values will be selected. Click ‘Toggle Editing’ button at the bo tt om.

Once editing is enabled click ‘Delete Selected Features’ button. Click ‘Toggle Editing’ again and Save.



Go back to the main QGIS canvas and you will see the po lygon layer for your clusters. You can nowuse this po lygon layer in your f urther analysis.



Part 2: GRASS v.kernel module

The region set ting that we want change is the Resolution. This set ting will determine the size and

reso lution o f our heatmap. To match the set ting with the previous method, put 100 as the Cell width

and height. Click OK.



Now let us get t o the second method using GRASS. GRASS is another plugin that is built in and has

powerf ul analytical capability. I will quickly go through sett ing up a locat ion and mapset in GRASS. You

can refer to this tutorial f or more details on what they mean. Click on Plugins → GRASS → New

mapset.

http://qgis.spatialthoughts.com/2012/01/setting-up-working-grass-environment-in.html



Select a Database directory and click Next.



Select ‘Create new location’ and name it ‘uk’.



Def ine the projection as OSGB 1936 / British National Grid. Click Next.



For Def ault GRASS Region, click ‘Set current QGIS extent ’. Click Next.



Enter the mapset name as ‘crime’. Click Next.



Now that you set an active mapset, many more GRASS menu items will be enabled. Click Plugins →

GRASS → Edit Current Grass Region.



Now click on Plugins → GRASS → Open GRASS tools.



There are hundreds of analytical modules in GRASS. They are organized in diff erent sections . Click

on the ‘Modules Tree’ tab and f ind the v.in.ogr.qgis module.



Select the ‘crime’ points layer as input and name the GRASS vector as ’crime’. Click Run. Once the

import is f inished, click ‘View Output ’ to load the layer in QGIS canvas.



Open the GRASS tools again and f ind the module v.kernel.



Select the crime layer as input vector and enter 1000 as the Standard deviation. Name the output

raster map as crime_heatmap_grass. Click Run. ( A bug in QGIS 1.8 prevents t his module f rom running

correctly. This will be fixed in later versions, but if you encounter an error, f o llow the alternate

metho d below )



From the GRASS tools, select shell from the top.



This will open a windows commandline interf ace with relevant variables already set . Type v.kernel andpress Enter. This will launch the native GRASS interf ace for the module.

Enter the same values and click Run.



Once the process ing is f inished, click Plugins → GRASS → Add GRASS raster layer.



Select the crime_heatmap_grass layer and click OK.

You will see a grayscale heatmap. Right click and select

‘Properties’.



Select ‘Pseudoco lor ’ as t he Colormap and click OK. Now the raster will be st yled in a f amiliar heatmap

style. You can export this raster f rom GRASS to any format you like and use it f urther in your

analysis.

Qgis.spatialthoughts.com-Tutorial Making Heatmaps Using QGIS and GRASS

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