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Best practices for simplifying coincident boundaries?

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#1
Dennis McClendon

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I'm working with congressional district boundaries that were probably compiled at 1:20,000 and I'm putting them on a page-sized map of California.  So I'd like to simplify and generalize them—but without having the coincident lines shift in different directions.  I'd be interested in hearing ideas about the best way to do this.  In ArcGIS, using IndieMapper, in Illustrator, using an Illustrator plug-in?

 


Dennis McClendon, Chicago CartoGraphics
chicagocarto.com

#2
David Medeiros

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ArcGIS has the Integrate tool (http://resources.arc...70000002s000000) which makes nearby lines coincident (you set the search tolerance).

 

I think Integrate maintains any overlapping locations in the two lines by adding a new vertex there and then sort of splits the difference everywhere else.


GIS Reference and Instruction Specialist, Stanford Geospatial Center.

 

www.mapbliss.com

 


#3
Dennis McClendon

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I don't seem to have the right license for that, but going through some old paper files yesterday I was reminded of MapShaper.

 

 

The results look promising so far, though I'm not sure what the difference is between polygons and polylines.


Dennis McClendon, Chicago CartoGraphics
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#4
David Medeiros

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I don't seem to have the right license for that, but going through some old paper files yesterday I was reminded of MapShaper.

 

 

The results look promising so far, though I'm not sure what the difference is between polygons and polylines.

Polygons are polygons of course, closed with an area and perimiter. Polyline are just lines, if the yhappen to close they may look like a polygon but will not have a fill or area attribute, only length.


GIS Reference and Instruction Specialist, Stanford Geospatial Center.

 

www.mapbliss.com

 


#5
Charles Syrett

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I've used MapShaper many times, and it seems to me to be one of the most excellent tools to come out of the GIS world. It has a way of allowing you to generalize lines quickly, easily, and intuitively - a true cartographic tool. However, for Dennis' problem I'm afraid it won't do the job, since it appears to work with each polygon separately. In that sense, it has no advantage over, say, the Simplify tool in FreeHand. Great for polyines (open paths), lousy for polygons (closed paths).


Charles Syrett
Map Graphics
http://www.mapgraphics.com



#6
David Medeiros

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I've used MapShaper many times, and it seems to me to be one of the most excellent tools to come out of the GIS world. It has a way of allowing you to generalize lines quickly, easily, and intuitively - a true cartographic tool. However, for Dennis' problem I'm afraid it won't do the job, since it appears to work with each polygon separately. In that sense, it has no advantage over, say, the Simplify tool in FreeHand. Great for polyines (open paths), lousy for polygons (closed paths).


Charles Syrett
Map Graphics
http://www.mapgraphics.com

 

Map Publisher incidentally, will allow you to simplify both polygons and lines topoligically (I think this is what you're talking about). It has it's own simplify tool that weeds points but maintans co incedent lines. I use it in combination with the Illustrator simplify to get nearly exact line contiguity, and smooth lines.


GIS Reference and Instruction Specialist, Stanford Geospatial Center.

 

www.mapbliss.com

 


#7
P.Raposo

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Tricky problem. In Arc, I suggest converting the polys to just lines, using the Polygon to Lines tool and the "IDENTIFY_NEIGHBORS" option, which, if I understand correctly, will mean that the resultant network of lines will not include coincident, overlapping lines at the polygon borders. Then, simplify that network of lines. Then, re-poligonize, if necessary.

 

If you need the attribute data from the origonal polys, then calculate centroids for the new polys, and use a spatial join to find the original polys underneath - this should workout as long as you haven't simplified the geometry drastically. Spatial join again from the now attribute-enriched points to the new polys, and your new polys will have all the attributes from the input ones.



#8
mfarmer

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It's probably too late for this, but I'd think about getting new congressional districts from the census bureau, apply topology, and simplify the lines with XTools.



#9
Dennis McClendon

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No, I'm back on this problem today.   Polygons to Lines is not available at my license level.  

 

Why get "new congressional districts from the census bureau?"  What is "apply topology?" 


Dennis McClendon, Chicago CartoGraphics
chicagocarto.com

#10
mfarmer

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Congressional districts, like counties, municipalities, etc., from the Census Bureau's TIGER download site are already polygons made up of coincident lines. Bottom line is you won't have the adjacency issues you were showing in your OP. They're digitized at 1:100,000 scale, which is more than adequate for your needs. Dealing with topology is overkill, ignore that part (but to answer your question, it applies rules to geodatabase feature classes, so that you can enforce rules that eliminate gaps, overlaps, etc.).

 

http://www.census.go...tiger-line.html, choose "113th CD" for the 113th Congressional Districts.

 

XTools (http://www.xtoolspro.com/) can convert polygons to polylines within ArcGIS. It also claims to smooth polylines using either beziers or b-splines; I haven't tried that tool yet, but let me say I like the XTools that I've tried. You probably wouldn't want to smooth the lines that are also state boundaries, so you may need to do some work breaking lines so that you can unselect them for smoothing.



#11
Dennis McClendon

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Thanks.  Census Bureau is where I originally downloaded the files from, but because each district is a closed polygon, any of the tools I have simplifies one polygon one way and the adjacent one a different way, producing the mismatch I showed.

 

XTools looks interesting.  Will it work with the lowest-level Arc license, whatever that's called this week?


Dennis McClendon, Chicago CartoGraphics
chicagocarto.com

#12
mfarmer

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XToolsPro should work with all levels of ArcGIS, as far as I know. It is shareware, and you'll have to register to use it. After a month or so, some of the more high level functions will go away if you don't pay for them, but some basic things like area calculations, polygons to polylines, smoothing, remain. I'd recommend trying it. Note that when you do the conversion from polygon to polyline,  you have the choice to make one line or two between adjacent features. You'll want just one since you'll be simplifying the lines, so be sure to look at every option in the window of that operation.

 

Both "Make Polylines from Polygons" and "Smooth Polylines" are under the "XToolsPro"->"Feature Conversions" menu.

 

I have no connections with the developer behind XToolsPro, I'm just a satisfied and grateful user. It comes in handy.



#13
loximuthal

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I don't know when you originally downloaded the CD boundary files or which version you need, but we've got several vintages of Congressional Districts available at http://www.census.go...f/cbf_cds.html, and the latest posted (111th Congress) has several levels of generalization already applied.  The state files are generalized to 500k and there are two versions of the US files, one at 5m and one at 20m.  If you need an earlier set of district boundaries, I'm afraid we only have the detailed set.  Some folks in my office are working on the 112th, but I don't know when they'll be available.


Andy McIntire
US Census Bureau

#14
Dennis McClendon

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Well, I would need the 113th (also the link doesn't work).  But I'm also doing California state senate and assembly districts, so the need remains.


Dennis McClendon, Chicago CartoGraphics
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#15
loximuthal

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Oops.  Sorry about the broken link.  Not sure what happened there.  Here's another link, this one to the 113th (don't know why I forgot what Congress we're up to by now):

http://www.census.go...tiger-line.html

 

Looks like it does not have generalized boundaries, so it still won't help much.  I just checked and it looks like generalized 113th CDs will be available in a few weeks.  Maybe too late for you, but at least the'll be out.


Andy McIntire
US Census Bureau




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