I am very happy that Vision Vancouver Councillor Reimer tabled a notice of motion yesterday called "Protecting Vancouver’s Public Green Space" which included the following statement:"The Greenest City Action Plan adopted by Council in 2011 sets a target that every person in Vancouver lives within a five-minute walk (400 metres) of a park, greenway or other public green space by 2020"
My first thought when reading this was simple: how will this indicator be measured? Specifically, I recalled a recent diagram in the Oakridge rezoning application slides (a project I fully support, just for the record) that shows 400 metre buffers from surrounding parks:
The trouble with this diagram is the 400 metre buffers appear to originate from the park centroids. Alternatively, I beleive 'accessibility' has actually been measured based on a buffer from the outer edge of parks. In other words, if I leave my house and walk 400 metres and arrive at the edge of a park, I would consider it to be accessible. I decided to explore these two interpretations; the following map shows the area within a 400 metre buffer of park 'centroids' in medium green, and the area within a 400m buffer from park 'areas' in lighter green:
In addition, I also clipped each 2011 Census Dissemination Block to the 400 metre buffer from the edge of parks, and calculated the fraction of each block that was within the threshhold. The fraction of the area was then used to calculate a proportion of the population per block that has no park accessibility. For example if a block had 100 people and it was 75% within the buffer zone, 25 people were considered to have no access to a park. As a result of this analysis, I determined that 79,407 people or 13.2% of the population of the City of Vancouver live greater than 400 metres from the edge of a public park. These people are shown using the yellow proportional symbols in the map below:
I also reviewed the Greenest City progress report and noted the indicator the City has selected is significantly simpler.Their indicator is "Percent of city's land base within a five-minute walk to a green space". In other words their target is based on area instead of population. Which is fully valid, but I still wonder how if they are measuring their covered area based on buffers from the park centroids (as shown on the Oarkridge map) or from outer edge of the parks (which would yield a larger total covered area and I am fairly confident this has been done). At the end of the day, I do not mean to criticize the outstanding work the City is doing, I am simply trying to highlight how an oversimplified diagram can lead to ambiguous results.
I am excited to learn more about this motion as it is developed because I have been working on many projects related to this theme at Golder Associates Sustainable Communities Group over the past few months. For example, I have conducted extensive GIS analysis and visualization to support the City of Vancouver's ongoing Integrated Stormwater Management Plan (ISMP)and the John Hendry (Trout Lake) Park Master Plan.
I understand this Notice of Motion is referencing much more area than just parks, and exactly what should be considered 'greenspace' will likely be a topic for a future post. For example, community gardens and greenways are referenced in the Greenest City action plan as features that satisfy the 'greenspace accessibility' target. Alternatively, given my recent focus on stormwater management I would also suggest it may be equally important for the City to implement a 'no net loss of pervious surfaces' target to maximize onsite infiltration of rainwater.
I hope this post shows that GIS tools are not always as objective as they appear and it is important for policy makers to consider how various calculation methods can impact progress towards targets.
