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I have a census tract shapefile with the population of a minority group and total population.

I am trying to analyze this group by high and low risk flood zones.

The problem is that not all of the census tracts are completely covered by the flood shapefiles so the estimate wouldn't be accurate.

Is there a way to get an accurate estimate of the people in the flood zones?

Most of these tracts are urbanized.

  • As per the Tour, which I note you have not yet taken, there should be only one question asked per question. – PolyGeo Mar 16 '16 at 2:01
  • For your first question, I'd say you are on the right track. If you have the time and access to better datasets/imagery, you will get a more accurate population assessment if you can ascertain where people live. If not, you can approximate this by assuming an evenly-distributed population, and calculating the population in the flood zone based on the proportional overlap – Stephen Lead Mar 16 '16 at 2:17
  • Thank you for your help, which tools could I use for the latter? Also poly I shortened the question, thanks! – XSC Mar 16 '16 at 4:06
  • @XSC see MaryBeth's answer below, which suggests starting with the Tabulate Intersection tool. Let us know if that doesn't help – Stephen Lead Mar 16 '16 at 5:19
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Population is an estimate, as it stands, so I'm assuming you are looking for a 'best-guess' type of workflow that will give the best results for the littlest amount of time spent.

ArcGIS's Tabulate Intersection tool may be of interest to you. You can input two polygons (flood zone and population block groups, for example) and tabulate what percentage of that group falls within a flood zone.

You may also want to try a select by location on the amount of buildings that fall within the flood zones, if you have access to that data. If you have access to your county/locality's population and housing data or land use, you may be able to make even more accurate assumptions using the buildings layer or by tabulating intersections--industrial land use, for example, will not likely have any population associated with it, where residential will--You could clip out residential land use from the block groups and see where that falls within the flood zones.

  • Actually, the tool is fantastic but I am getting some weird results in the percentage. Should input zones be the tract file, zone fields be the pop estimate and input class field be the flood zones? (The flood zones are clipped to the same extent as the tracts so only the flood zones inside the tracts are included). Thanks!! – XSC Mar 16 '16 at 15:45
  • You may have to play around with it a bit? From how I understand your question, I believe that you will want to check the flood zones against the input class, so you get the percentage of the population zone against the size of the flood zone (as long as your flood zones are clipped to that area. – MaryBeth Mar 16 '16 at 17:31
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    I got it to work, it's amazing! The difference between the hand counted and the data from the tabular is less than 500, it's amazing! Thank you!! – XSC Mar 16 '16 at 23:30

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