Epi Info Map Question: merging areas that are separated in the shape file

My shape files are a bit old and don't correspond to the current geographic categorization system of the data I'm mapping - eg. regions "1" and "2" in the shapefile are combined as region "3" in the data. One solution would be to reclassify the data (rates) for region "3" by reclassifying as regions "1" and "2" and assigning both the rate of region "3" (and deleting mention of region "3"), so that on a chloropleth map the same rate is assigned over the area covered by regions "1" and "2", aka region "3". Then I'd just have to using drawing software to erase the line between regions "1" and "2".  I'm certain there's a better way to do this, though, and if anyone can tell me how, I'd be very happy.

Thanks,

L

Tags: Epi Map

Views: 919

Replies to This Discussion

Unfortunately, there is no easy way to merge regions. (Dividing a region into two is somewhat straightforward, though.) The way you'll have to do it is to go into Epi Map's shapefile editor, which is accessible by going to Shape (Boundary) > Create/Edit from the Epi Map navigation menu.

From here, you can simply drag the vertices from one polygon over the top of another to create the appearance of a single region. You can also delete regions and divide regions. This editor is very crude, however, and given that there is no undo option, you will want to be very careful less you have to start all over from scratch.

You may also be able to find updated shapefiles for the state/country/province you're working with on the Internet. One example is the the US Census Bureau which has many shapefiles of the states and counties available to the general public for download.

Let me know if this helps.
Thank you very much for your response! I'll be getting the data in a few hours and will be trying out a combination of techniques, including what you suggested.
Hi Laura. Here's how to edit the shapefile and merge the two regions into one.

The free GIS software MapWindow gets the job done easily.

Download the previous version 47RC3 (the latest version 47SRa has a bug with merge):
http://www.mapwindow.org/download.php?show_details=45

Be sure to make a backup copy of your original shapefiles because MapWindow will alter the files in the merge process. Launch MapWindow application and follow these steps:

1. Use File-->Open and select your shapefile to edit

2. Click Plug-Ins-->ShapeFile Editor

3. Use the "Merge Two Shapes" tool (click icon on tool bar) to combine the two adjacent polygon areas. The result polygon should include both areas

4. Use the "Attribute Table Editor" tool (click icon on tool bar) to edit identifying information of the combined area. You should find the first area in a table row (second area is gone and there is no longer a row for it). Change the attribute values in this row for the new combined area

5. Use the "Identifier" tool (icon on tool bar) to view the attributes of the area to confirm that changes are made

Note: Changes are automatically made to the shapefile in the merge process -- there is no option to keep the original and save the result as new shapefile.

Now just be sure your data uses the identifier of this new region in the shapefile.

Let us know if you have more questions or prefer to have the shapefiles edited for you.

Have a nice day!

Roy Ing
EpiSuccess.com
Thank you very much for your response! I have never used MapWindow before and am glad to have the opportunity to try it.
Hi Roy,
This was really helpful. I'm trying to use the same technique now to merge two specific regions into one region, but the one region that I want to create has another "island" region within it. So, basically, I am trying to create a square with a smaller square inside of it and the larger square is currently cut in half. Everytime I merge the two halves of the big square, the small island square disappears. It's still in the attribute file, but it isn't visible...both in mapwindow and in epi info map.
How can I resolve this issue?
Thanks
Laura
Hi Laura. To create an island or "hole" in a "doughnut" shaped polygon, you can use the free GIS software QGIS (Quantum GIS).

List of software versions (including Windows, Linux and Mac OSX):
http://www.qgis.org/en/download/current-software.html

Download and install the software (e.g. Windows version):
http://linfiniti.com/downloads/QGIS-1.4.0-1-No-GrassSetup.exe

Here's a quick start guide:

1. Launch the software and open your shapefile by selecting (menu) Layer --> Add Vector Layer (select shape file)

2. To locate the area to edit, use the "Pan", "Zoom In" and "Zoom Out" tools to navigate

3. To edit features (points/lines/polygons), click "Toggle Editing" (pencil icon on top left)

4. To cut a hole inside a polygon, click "Add Ring" tool (icon), then (left) click on area to add points for the inner polygon to form the island or hole (right click when done). To undo, select (menu) Edit --> Undo

5. Use the other tools to edit points (nodes), lines and polygons

6. To save the result to a new shapefile, click (menu) Layer --> Save as Shapefile

Here is a recent blog on "Polygon Shapefile Editing with QGIS":
http://geospaced.blogspot.com/2009/12/polygon-shapefile-editing-wit...

QGIS Manual:
http://download.osgeo.org/qgis/doc/manual/qgis-1.0.0_user_guide_en.pdf

For questions and help on QGIS, visit the forum at:
http://forum.qgis.org/

Have a nice day!

Roy Ing
http://www.EpiSuccess.com

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