Spatial Analysis of Different Types of Traffic Accidents with QGIS

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Purpose

The purpose of this tutorial is to give users an understanding of what free and open source software (FOSS) is and demonstrate how to use Quantum GIS (QGIS) by using spatial data. This tutorial will be analyzing the spatial data of different types of traffic accidents that occur in Barrhaven, Ottawa using QGIS.

Introduction

The city of Ottawa has many programs regarding the safety of transportation and safer roads. A Safer Roads Ottawa Program is committed to preventing or eliminating road deaths and serious injuries. The program is conducted by Ottawa Fire Services, Ottawa Paramedic Service, Ottawa Police Service, Ottawa Public Health, and the Public Works Department. They are conducting the program through culture change, community engagement, and developing a sustainable safe transportation environment. Another program called the Safety Improvement Program includes all three E's of road safety: education, enforcement, and engineering. Locations chosen to be studied are usually higher-than-average traffic collision rate locations. The Safety Improvement Program is carried out on an annual basis because that is when year-end traffic collision and volume data is available.

Data

The data used in this tutorial is from Carleton University's Maps, Data, Government Information Centre (MADGIC) and the City of Ottawa's Open Data Website. The data used in this tutorial are a polygon shapefile of the wards of Ottawa, a point shapefile of traffic accidents, a polyline shapefile of Ottawa roads, a polyline shapefile of Ottawa railways, and a polyline shapefile of the pedestrian network. You may use any other data for another city or location as long as there is a polygon shapefile of your chosen city or neighbourhood, a polyline shapefile for roads, and a point shapefile for accidents, as well as any other information you would like to analyze. To get ready for this tutorial, you should download all of these shapefiles and keep them in a working folder on your PC to make your work easier to locate.

Quantum GIS (QGIS)

Quantum GIS, also known as QGIS, is a great free and open source software (FOSS GIS tool). The purpose of QGIS is to allow users to create maps with many capabilities provided by core functions and plugins. QGIS is available for Windows, Mac OS X, and Linux. Download the long term release repository (most stable version) of QGIS (v.2.14) to begin this tutorial.

Tutorial

Now that you have downloaded the software and shapefiles, it is time to begin.

Setting Up the Environment

Go to the Project menu, select Projection Properties. The Projection Properties windows will open. Select CRS from the left hand menu and then toggle on Enable 'on the fly' CRS transformation. The CRS that should be selected is NAD83(CSRS) / MTM zone 9 if in the Ottawa region. Click Apply and OK.

Setting Projection to NAD83(CSRS) / MTM zone 9

Adding Shapefiles into QGIS

Once QGIS is launched, go to the Layer menu, Add Layer and select Add Vector Layer. In the pop-up window, set your source type as File and click the browse button under the Source header. Browse to where your shapefile is an double-click it or highlight it and select Open. Insert each of the shapefiles you would like to analyze. Modify each shapefile to look the way you would like it to by double-cliking the layer from the Layers Panel on the left hand side, then selecting Style from the left hand side of the popup window. Here you can change the symbol type (such as gradient, single symbol, categorized etc), the colour, the size and more.

Display of all of the imported layers

Editing Attribute Tables: Selecting by Expression

Right click on the accidents layer and select Open Attribute Table. An attribute table of the accident layer will appear and it will have the different types of impact and the two street names where the accident occurred. Once you have the attribute table open, select “Select Features Using an Expression”. A Select By Expression window will appear, and in the function list, expand the Fields and Values and select Impact Type. Select the "all unique" button in the bottom right corner beside “Load values” and a list should appear of the unique values from this attribute. The field values will have the list of different impact types that were seen in the attribute table.

In the expression box of the select by expression window. When you double click on the Impact Type, "ImpactType" will appear. Once that appears, you select the equal (=) sign where the available operators are listed. The next step is to select a field value. In this example, the first field value selected is "Angle". The expression should be "ImpactType" = 'Angle'. Once this expression has been selected, the impact types that are 'angle' are now highlighted on your map.

Selection Expression

Saving Selected Points (Shapefile)

The selected points that are now highlight can be saved as their own shapefile. To do this, right click on the accidents layer and select Save As and a window will appear. In this window, the format that the shapefile should be saved under is "ESRI Shapefile", click the browse button to choose where the file will go and give it a name. Toggle on “Save only selected features” and "Add saved file to map" and then click OK.

Angle impact accident type point shapefile after it has been added to map

Once a new shapefile has been made and added to the map. Repeat "Editing Attribute Tables" and "Saving Selected Points (Shapefile)" steps until all the impact types have been saved as their own individual shapefiles.

Map of Ottawa with all the different impact types as their own individual shapefiles (Wards shown in different colour scheme to better highlight different accident types)

Selecting a Specific Area

To select a specific area, open the attribute table for the wards-2010-2 shapefile layer. Click on the "Select features using an expression" button.

The Select By Expression window will appear, and in the function list, expand the Fields and Values and select the specific area you want to study. In this case, it is Barrhaven. Because the wards in this file is listed by number and not name, we must consult a reference list.

Different Wards in Ottawa and their respective wards numbers

The expression should be "WARD_NUM" = '3' because we want to select Barrhaven. If we were wanting to select more than one ward at a time, our expression would be for example "WARD_NUM" = '3' OR "WARD_NUM" = '22' which would select both Barrhaven and Gloucester-South Nepean. To save the selected shapefile, right click on the wards-2010-2 layer and select Save Selection As and a window will appear. In this window, the format that the shapefile should be saved under is "ESRI Shapefile", give the shapefile a name and toggle on "Add saved file to map".

Highlighted Barrhaven

Clipping Files

When the newly merged shapefile has been made, you can simply clip all of the other information (accidents, roads, railways, pedestrian paths) onto the study area as well. To do this, go to the vector menu, select Geoprocessing Tools and then select Clip. A clip window will appear and the input vector layer will be what shapefile you want to clip onto the output vector layer. For example, to clip the roads, the input vector layer should be Road Segments and the clip layer should be the Barrhaven shapefile we made from our previous selection. For output shapefile, you can browse to the folder that contains all of your other shapefiles and choose a name for your clipped file.

Clipping window

When clipping the other shapefiles, the clip layer remains the same, but the input vector layer changes. Format the style of your clipped shapefiles to your liking.

The output of all layers being clipped to the AOI

Analyzing the Data

To see how many of each accident type occur, we can look at the attribute table. For example, in our recorded data for Single Vehicle Unattended Accidents in the Barrhaven ward, 10 incidents occurred.

Circle highlights the total number of features within the selected layer for which the attribute table was opened

To see how many accidents occur within 100 m of a railway track, we can first create a buffer around our clipped railways shapefile. Select Vector -> Geoprocessing Tools -> Buffer. In this example, a buffer distance of 100m was used and the segments to approximate value was 25 (this mean that our finished polygon will have 100 sides (the higher the segments to approximate value, the smoother the buffer). We save this in our desired folder with a file name of our choice.

Buffer window with explained values

Because we want to know how many accidents of any type occurred within 100 m of a railway, we can clip our original accidents shapefile to our Barrhaven polygon.

Clipping our original accident shapefile to our AOI

Here’s the outcome:

Outcome of buffer and clip

To find out how many accidents occurred within our buffered region, we can make a clip of accidents in Barrhaven from our railway buffer.

Clipping accidents that occur within railway buffer

From the output layer and it’s attribute table we can see that only one accident occurred within 100 m of a railway in Barrhaven.

Accident that occurred within buffered region

Using these methods, you can perform a multitude of operations to interpret things such as how many accidents occur within a distance from pedestrian pathways, how many accidents occur within a ward, how many accidents occur within a distance from railways, how many of a certain accident type occur within a distance from other accident types etc.

Making a map

Once we have produced the data that we want to showcase, we can use QGIS to make an aesthetically pleasing map. First, turn on all of the layers that you would like to be showcased and turn off all others by selecting and unselecting the checkbox next to each layer name. I want to showcase different types of accidents that occur within Barrhaven so I have selected these layers:

Selected layers

Select Project -> New Printer Composer and then type a name for your map in the text box and select OK (it does not have to be the same as the title you place on your final map). In the pop-up window, select Layout -> Add Map and then draw a rectangle on your canvas where you would like your map. To reposition your map display, select Layout -> Move Content. Now you can drag your map into place and zoom in and out. Under the Item Properties tab on the right hand side, you can also change the scale to get the perfect sizing.

Changing the scale of the map

To add a title, select Layout-> Add Label and draw a box for your title. Under Item Properties -> Label you can type your title in the textbox and change the font type, size, colour etc. To add a north arrow, select Layout -> Add Image and draw a box on your canvas. Under Item Properties -> Search directories you can view different North Arrows and choose which one you would like to use.

Choosing a north arrow

Similarly, you can add a scale bar by Selecting Layout -> Add Scalebar.

To add a legend select Layout -> Add Legend. To show only the legend items that are present on your current map, select the icon that resembles a filter cone under the Item Properties -> Legend items menu. To rename legend items, double click a legend item and enter the text you want displayed. You can also rearrange legend items by dragging them up and down. Once you are happy with your map, you can export it as an image, SVG or PDF by selecting Composer from the top left hand menu and choosing either Export as Image, Export as SVG or Export as PDF.

Final Map

Conclusion

QGIS is an intuitive and user-friendly software that in this instance has allowed the production of a map that showcases traffic accidents in the Barr haven ward of Ottawa as well as allowed the user to analyze different scenarios. This is just an example of the many ways that a few shapefiles can be combined to produce a multitude of useful and easily understood data

References

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