By now you should have a pretty good understanding of how InaSAFE works and its operations.
You know how to add layers, and how to define keywords so that InaSAFE can recognise layers appropriately. Now that we can use InaSAFE effectively to run scenario analyses, we will look at other QGIS functionality that will help us in preparing contingency plans.
In this module, we will learn how to perform a GIS analysis in order to determine an appropriate evacuation route in the event of a disaster.
The term fastest route indicates the route a person can take between point A and point B that will allow them to cover the distance in the least amount of time.
Similarly, shortest path indicates the route that will allow a person to get from point A to point B with the least distance travelled. In theory, this would be a straight line between point A and point B, but in reality this is impractical, because travelling in a straight line means climbing hills and going around buildings and fences. Of course this is why we use roads, and why we calculate fastest and shortest routes using roads.
We will use the Road Graph plugin in this module, which does just that. If we provide two points, the plugin is able to calculate either the fastest route or the shortest path between them.
We will be continuing with the previous example, which you should have saved. We won’t be using the InaSAFE plugin in this module, so you may close the panel if you like.
The Road Graph plugin calculates either the shortest or fastest route between two points, so we need to provide a start point and an end point for an evacuation route.
Of course an evacuation route should be for all people in an area, but we can experiment with different start points and see if evacuation routes will be different in different areas.
Your starting point will be marked as a green point and the coordinates of the point will be recorded in the Start input box.
Now we need to assign the destination of our evacuation route. Where will people be evacuated to? Because this is an example, we don’t have a great idea of where an appropriate place would be. We might use GIS to determine appropriate locations, which would most likely be high ground in the event of a flood. For this example, we will choose a destination at the south-east corner of the village.
In this example, the length of the fastest route between our two points is about 1.97 kilometres and the travel time is 0.0788 hours, which is about 5 minutes. The time in our example is determined by the distance and our default speed of 25 km/hr. The speed can be changed, and can even be set to different amounts for each segment of road.
The route appears on our map:
In this module we’ve learned how to calculate the shortest distance between two points using the Road Graph plugin. Using this you can easily determine evacuation routes from various areas. Evacuation routes are important for contingency plans, and those living in threatened areas can be educated with the quickest and safest routes to take in the event of an emergency.