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Creating a boundary from a grid

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#1
anniec

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I have a square mile grid which covers the entire county. Within each square of the grid is a population figure. I have identified urban and suburban areas based on the population figures and exported them to separate shapefiles. So basically I have several groups of squares which represent both suburban and urban areas (if that makes sense). The remaining portion of the grid (where populations did not meet the criteria for urban and suburban areas) are considered rural areas.

I have been asked to create boundaries from this grid. Besides digitizing the boundaries myself, is there any way to use the software to create them? My boss was talking about doing a raster, but I'm not sure how that would work with my existing data.

If anyone has any ideas about how to create a boundary, I would appreciate it!

#2
pfyfield

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When you say "grid," is this vector data? Grid usually implies raster, but exporting shapefiles makes me think the data are vector. You could merge then dissolve the shapefiles. Merge (or union) would create a single shapefile for all the urban, suburban or rural areas, and dissolve would eliminate the interior linework.
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#3
anniec

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Yes it's a polygon file I created using Hawth's Tools. Each square in the grid is one square mile.

I think my supervisor is looking for a way to create more fluid boundaries without them being so square. I know it sounds strange and I don't fully understand it either. He was thinking that I could create a raster and somehow that would create more fluid boundaries but I don't see how. The individual data sets are also located in the square mile grid, I just exported them out into separate files.

#4
Laura Miles

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Hmm, I don't know if I would alter the boundaries of the data, as the underlying population data sounds to be based on these boundaries. To represent the same population data with altered boundaries would be compromising the integrity of the data. But I suppose this also depends on the scale the data will be displayed at.
You could convert to a raster with cell size = 1 mile and then back to polygon with the option to simplify polygons checked, which should smooth out your blocky appearance.
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#5
frax

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Just dissolve the grid polygons, that you have classified. Look up the usage for the dissolve tool in the toolbox, and dissolve on your class (urban/suburban/rural) attribute). No need to look at raster processing.
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#6
anniec

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I didn't think a raster was necessary either. I'll give your suggestions a try and hopefully I'll come up with something satisfactory!

Thanks for all your help!

#7
Laura Miles

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The raster processing I suggested was intended only for smoothing the appearance of the boundaries, not creating them- sorry if I wasn't clear. Dissolve is the appropriate tool for the boundary creation, but it won't make the boundaries less chunky looking- if this step is necessary to do I think this raster processing is one way to achieve that and perhaps what the boss had in mind when he mentioned rasters.
Now that I've given it more thought you could use the "Smooth" tool on the Advanced Editing toolbar in an ArcMap edit session after dissolving your boundaries. This is probably simpler than raster processing.
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#8
Dennis McClendon

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I think this is going to be rather clunky unless it's an exceptionally large county.

It seems like your boss's idea is to blur all the urban squares together to make a sort of generalized outline of the urban area. Now I could see doing that with quarter-sections, but square miles? Even in a 40 x 40 mile county, there will only be 1600 squares. My avatar is 64 x 64.

I'd suggest doing it with census tracts instead. You already have the geometry, and the boundaries are more likely to be things such as rivers, freeways, or swamps that actually have something to do with where boundaries of urban or rural areas actually occur. Suppose you have a mile-square that perfectly straddles a river. On one side it's urban and on the other side it's rural. What purpose is served by saying that—on average—it's suburban?
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#9
anniec

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I tried giving the Dissolve tool a try and had a hell of a time with it. Luckily there are only a few suburban and urban areas so doing it manually isn't a problem.

Yes our county is big, and there is one subdivision in particular where one square mile is urban and then right next to it is suburban.

We are in the beginning stages of this project so we are trying to figure out a way to make it work. Initially my boss wanted to take each street and count the number of dwellings each mile, but I tried to explain to him that those are linear miles when in fact we should be using square miles (according to the criteria given to us by the requester).

I might try to play around with Kernel density today. It's my understanding that you can use that to find population per square mile. I've never tried it but I'm going to give it a shot today.

#10
Dennis McClendon

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But why would you do it by square mile? Frederick County is only 415 sq mi, a 25 x 25 grid. Why not do it by block group or even census block?
Dennis McClendon, Chicago CartoGraphics
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#11
Laura Miles

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I have a square mile grid which covers the entire county. Within each square of the grid is a population figure...


Don't you already have population per square mile then? Kernel Density will output a raster as well. I'm not sure exactly what your inputs are or the desired output but from what you've indicated I wouldn't think you'd need to go there...

#12
anniec

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I can ask the fire chief about doing it based on Census data. He gave us a section of a code they use which specifies staffing and response time in relation to urban, suburban, and rural areas. For each of those areas, there are demographics listed (i.e. Urban >1000 people per square mile, Suburban 500-1000 people per square mile, etc.).

The goal of the project is to use the population figures listed in the code to determine urban, suburban, and rural boundaries of the county. Those boundaries will then be used to determine under-served areas, which may need a new fire station or substation. Hope that makes things more clear.

Thanks again for all your suggestions.

#13
Dennis McClendon

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May I just say—as respectfully as I can—that what we have here is a profound misunderstanding of the problem (if not the assignment). Because the national standards are given in terms of population per square mile does not mean that's how you should analyze the county.

If you have a single countywide fire department, it's probably most appropriate to do a countywide analysis of population (or dwellings) per fire station. After all, the ones in the suburban areas respond to calls from nearby rural areas. But if you want a finer grained analysis of where fire stations are needed, you should be doing a nearest-neighbor analysis, or one based on routing, of distance from nearest fire station to dwellings. If you don't yet have the data showing where all dwellings are, do it to census block centroid, and weight it by number of units per block.

GIS is capable of doing this type of sophisticated analysis with relative ease. Instead, you're asking how can you muddy the picture until it becomes unrecognizable, merely to distinguish between different shades of mud.
Dennis McClendon, Chicago CartoGraphics
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#14
anniec

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Hi Dennis,

Thanks for your reply. I just wanted to clarify that the goal is to identify population boundaries as specified in the code provided. The chief is aware that we can use GIS to find the best location for a fire station, but at this point he is not interested in that. Right now he just wants to see population boundaries on a map, not taking into account existing first-due areas or fire stations. I have not had an opportunity to discuss the Census data with him, but I will the next time I speak with him.

Thanks again for your input.

#15
Dennis McClendon

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The misunderstanding is thinking that a square-mile standard requires you to classify square miles. Each piece of Census geography, whether blocks, block groups, or tracts, has a known area and a known population. So you can easily calculate the "density per square mile" for areas as small as a single block. Based on that, you can then dissolve and generalize the blocks into larger areas with urban, suburban, and rural densities.
Dennis McClendon, Chicago CartoGraphics
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