Splitting polygons

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09-06-2011 07:48 AM
by Anonymous User
Not applicable
Original User: dougstanley

Returning to use of ArcMap after 5-year absence and I've forgotten a few things... I am using ArcView 10 and, unfortunately, under a tight deadline. Please help.

Layers: Layer 1 contains polygons of sales territories. Layer 2 contains polygons with population projections by MSA.

Problem: Sales territory boundaries sometimes cross (or divide) MSAs. Using the Geoprocessing tool, I can clip the MSA to adhere to the territory boundary, but the value in the population field remains unchanged.

I want the software to apportion the population projection for the split MSAs to the appropriate Sales Territory. Many moons and versions of ArcView ago I knew how to do this. Any help appreciated.
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10 Replies
by Anonymous User
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Original User: dkwiens

I'm not aware of any tool that will do this automatically for you, but it seems like you should be able to calculate the final population by multiplying the initial population by the proportion of the initial area in the new area. For example, if your final area is 60% of the initial area, then the final population should be 60% of the initial population.

1.) Calculate the initial area in a new column so it won't change after your overlay (ie. not in the automatically updated GDB Shape_Area column)
2.) Overlay your polygons
3.) Calculate the new area (or if you're using a GDB there will be an updated area column)
4.) Calculate the proportion of the initial area in the new area (new area/initial area)
5.) Calculate the new population (initial population * area proportion)
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by Anonymous User
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Original User: dougstanley

Thanks for your suggestion. I'm working to implement it. First, a bit more background and a question:

The MSA layer started life as an ESRI-provided package. It included a SQMI field that contains the number of square miles for each MSA. The layer is GCS.

Problem: The Calculate Geometry tool does not accept GCS. It requires that one use a Projected Coordinate System. When I choose a a PCS that the software tells me is compatible with my GCS data, and I use the Calculate Area Tool, I get a different number than the SQMI number that came with the layer. Is that to be expected and is it acceptable for my purposes?

Thanks again,

Doug
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DarrenWiens2
MVP Honored Contributor
I suggest you project your original data to a new file using the Project tool, first (to an equal area projection if you're picky about high accuracy). When you calculate the area, make sure it's reporting the number in the desired units - depending on how you do it, you may be able to choose the units or it may default to the projection units (e.g. meters).
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by Anonymous User
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Original User: dougstanley

With your help, I am gaining on this and am now very close to accomplishing the objective. Thank you. Your suggestions for the next step would be much appreciated.

I have 19 MSAs that I need to split and assign to multiple territories because territory boundaries cross them. I had originally thought using the Clip tool would divide the MSA polys where they are crossed by territory boundaries but this isn't working as expected. What would be the best approach?
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DarrenWiens2
MVP Honored Contributor
Intersect will give you the areas split by both territory and MSA. Union will give you the entire original area split into polygons showing territory, MSA, or both, whichever is the case. You should also check out Identity if you're interested in keeping all of the territory but only overlapping parts of MSA, or vice versa.
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by Anonymous User
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Original User: dougstanley

Intersect was EXACTLY what I needed. Thank you so much for your help.
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NobbirAhmed
Esri Regular Contributor
Just for future reference (I guess your problem is solved already):

First, run Make Feature Layer with MSA as input. Make sure, in the Field Info parameter, you check mark the 'Use Ratio Policy' column for population field.

Next, run Intersect between the output of Make Feature Layer and your other polygon features (territory).

Your resulting output will have a POPULATION field with population as per ratio of the intersecting area.

This is simpler and you don't need to perform any field calculation.

See the attached image for Field Info parameter. You'll get detail info on this parameter here:

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Make_Feature_Layer/00170000006p000000/

Go down to the last but one usage.
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by Anonymous User
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Original User: dsstanley

Just for future reference (I guess your problem is solved already):

I came back to this months later and tried your suggestion. Much simpler. Thanks!
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guruprasadmv
New Contributor
Hi,

We are using Microstrategy BI tool and are trying to implement US MSA map (metropolitan statistical area) using ESRI.

This is first time we are using ESRI and not sure how to start.

Following are few information on our project.

        ESRI has been configured in microstrategy and ready for use
        our requirement is to show sales numbers for each of the MSA in a USA map
        we have a microstartegy attribute which represents the MSA elements

We need to know the following.

       as MSA is not available as a standard region in ESRI, how do we geocode
       do we need to use shape file or longitude/latitude
       how do we create shape file
       or how do we get longitude/latitude information

       how do we use shape file or longitude/latitude for geo coding
       how do we configure US MSA map and use the same in MSTR document

basically, we are trying to implement this for the first time and do not know how to start and what should be available to start.

Please share any information/document on the same.

Thank you for your help.
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