INASAFE seems like an interesting QGIS plugin but as I wade through the docs, I really need some sample data to play with in order for me to understand. There is a tutorial which uses Indonesia data, but I can't find that dataset either.
If anyone is using this and has any example datasets, it would be very appreciated.
OK, after working thru the INASAFE tutorial it was unclear how to use Census data, so I waded thru the python scripts. It became clear that INASAFE couldn't deal with my data. I'm not familiar enough with either QGIS or python to add my capability to INASAFE so I decided to try my luck doing a similar analysis using QGIS (newbie here, but a bold one).
INASAFE appears to work well with OSM building data and Indonesia has done a terrific job mapping what appears to be thousands of their buildings in OSM, but my county has very few and census data has much more detailed data.
So I took my 2010 census block data, which had fields of POP10 for # of people in block and HOUSING10 for # of housing units in block and added a calculated AREA field. Since I had everything in Google Mercator, this area is in some nonsense units, but I don't care about that. As long as cutting the polygon in half gives an area half as big, then it will work for my purposes.
Next I did an intersect with some circa 2002 FEMA 100-yr floodplanes (remember when FEMA floodplanes were free!). On the intersect layer I added a calculated AREA field called FloodAREA.
I calculate the flooded Population as POP10*FloodAREA/AREA (ratio of area of flooded polygon to original polygon). As expected, some times FloodAREA=AREA (completely flooded block), and sometimes was less (partially flooded block), but never more (sanity check).
I ran basic statistics on the intersect layer and got the surprising result that 45,000 people (10% of pop) in my county live in 100 year flood plane.
I was wondering about thresholding - if polygon has like 0.1 people flooded, probably shouldn't count it - people and houses are not uniformly distributed in block. Especially in flood plane, I suspect lowest spots are less likely to have houses built on them. So, if a block has 250 people, for example, and it shows 25 people flooded - this really is saying that lowest 10% of block is flooded. Housing density rarely seems to approach 50%, so it's very possible (likely?) that when 10% of a block is flooded, there are not 10% of the houses in the flooded area.
So 2 questions:
- Are there any ideas or standards out there for discarding partial people from the sum or requiring minimum % of block flooded before counting?
- How do I get a sum of a field in QGIS with a thresholding formula applied (like sum(if Flooded_Pop>1 and FloodAREA/AREA>0.05)?