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I am working on a project doing spatial analysis at a statewide level. I am looking at a form of transportation coverage in a state, and part of my analysis involves buffers and drive time polygons. I am using a geodatabase (therefore ArcGIS) for all my work, so new features will be exported as feature classes in feature datasets.

To visualize: I have said state border, and some dissolved buffer rings around some points, and they cover parts of the state. I want to create a feature class for the parts of the state these buffer rings don't cover, AKA gap areas. Is there a tool I can use or create to do this?

I know I could draw them in manually in editor, but I am unsure if that's the most precise way to generate these feature classes. To me, I am thinking this operation would be like an inverted "Clip" in that I take the two layers, and instead of pulling out the clipped area, it would take everything not in the clipped area.

Thoughts?

*Edit: Sorry I never received notifications in my inbox. Anyway, yeah it's a Desktop Basic license, my bad.

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Can you edit your Question to include the license level(s) of ArcGIS for Desktop that you have access to, please? This I easy with Advanced but needs a slight workaround for Basic. –  PolyGeo Jun 30 at 21:32
    
Your use of the word 'gap' is a little confusing. Generally when I read that word it means there is no data in an area, not the area of one layer that isn't overlapped by another. If you take a look at the ArcGIS How Union Works page you'll see a good example of this. –  Chris W Jun 30 at 22:05
    
Related and almost a duplicate but not quite because of additional details: gis.stackexchange.com/questions/102105/… –  Chris W Jun 30 at 22:49
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Rather than using a text description to help us visualize what you are asking, would you be able to include a picture, please? –  PolyGeo Jun 30 at 23:18
    
I edited my post to reflect the license I have. Thanks for all the good suggestions everyone. –  rachel.passer Jul 1 at 13:35

4 Answers 4

up vote 1 down vote accepted

There's another option that isn't as elegant as Erase or Union and involves a few more steps, but it can be done at Basic and without third party tools (which can be an issue if you don't have administrative rights on the machine).

  1. Make a copy of your buffer layer, bufferdummy. If you have more than one layer, you can select them all and export/copy them to a new feature class.
  2. In bufferdummy, select everything and use the Merge tool on the Editor dropdown menu. It doesn't matter which feature's attributes you choose to preserve.
  3. Now make a copy of your state border, notcovered This will become the feature class you're trying to create.
  4. With only bufferdummy and notcovered editable (turn other layers off - be careful because this command will affect all editable layers), select the bufferdummy polygon and on the Editor dropdown choose Clip. Don't set a buffer and leave it on 'discard area of intersect'. The other option would do what the Clip GP tool does - trim away outside the shape instead of inside it.

You can then delete bufferdummy and you should be left with notcovered as a single poly (possibly multipart) that is all of your state border not covered by a buffer. You might want to explode it to get rid of multiparts if there are any, but I have a feeling you'll end up with one big shape and holes cut out of it rather than separate pieces. You can leave this as its own feature class or add it to your buffer layer (name attribute of greater than max buffer or some such).

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I like it. It's a bit more interactive and is a good solution for more than two feature classes... also as it's in editor you've got an UNDO opportunity. –  Michael Miles-Stimson Jun 30 at 22:47
    
Yeah at first I thought this was a bit of a lengthy process but I think while it might have a few more steps, it is more...adjustable? Thank you! –  rachel.passer Jul 1 at 13:23
    
Also you're right, I don't have admin rights on my computer here and if I need to get a higher license or install a third party extension, I have to justify it yada yada yada. I think I'll try this way first. –  rachel.passer Jul 1 at 13:44
    
@rachel.passer It sounds lengthy to describe it, and it does have more steps than a single tool, but it's really not that bad once you get the technique down. Back at a digitizing job I had we used this all the time because most licenses were just View/Basic and we had a 100% coverage requirement. It's a fast way to create shapes from other shapes when you don't have some of the more advanced tools. I still use it (or the Clip tool at least) to create coincident boundaries rather than using Trace on the boundary I've already created. –  Chris W Jul 1 at 18:55
    
I know it's a little late, but I started getting to this part of my project today and I tried your suggestion above. It worked! But FYI, #4 is worded in a confusing way compared to what actually happened. When those two layers are editable, you said select the "bufferdummy" polygon and clip that on the editor. When I tried this, I actually clipped the "notcovered" polygon, not the "bufferdummy" one for it to work properly. Then just save the edits and voila, perfect gap in coverage areas. Thanks! –  rachel.passer Jul 9 at 14:33

Erase is the tool for this in ArcGIS.

http://resources.arcgis.com/en/help/main/10.1/index.html#//00080000000m000000

Kind of like an "opposite clip"

You need a higher level license to do this.

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What you have is not really a gap though but I get the gist of it and I pretty certain this is your tool. Always bothers me it is not in the .view license. –  user two seven two three nine Jul 1 at 1:28
    
So, if I do not have a higher license available to me, what would I use? I have Desktop Basic. –  rachel.passer Jul 1 at 13:20
    
I actually think a simple Union will do what you want but you will need a few more steps. See Michael's answer below. Install Xtools and use their erase is another option. Or install Qgis and use difference ---gis.stackexchange.com/questions/62949/… –  user two seven two three nine Jul 1 at 13:26
    
Thanks for the QGIS Erase link. It would probably be not worth the trouble to convert my feature classes into shapefiles to use QGIS, but glad to know I have the option there. –  rachel.passer Jul 1 at 14:01

The ArcGIS Erase tool will work, but as user two seven three nine says, you have to have the required license level to use it. The Xtools Pro extension suite has an Erase tool built in that will do the same thing and is free. Some of the features of Xtools require that you buy a license after the 14-day evaluation period is over, but the Erase tool (and a ton of other very useful ones) is free to use forever.

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Thank you, I will check out Xtools Pro. I appreciate the suggestion! –  rachel.passer Jul 1 at 13:21

If you don't have an info level of license you can do it using union, which is limited to two feature classes in basic and standard, but that's enough.

Union your two layers, to keep it simple join the attributes with FID only, and then delete the ones that have a FID value from the second feature class. There are a few ways to delete but if you wish to do this in a model make a feature layer with an appropriate where clause and use delete features to remove the unwanted areas.

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Beat me to it again! :P I was also going to add this option would allow you to address true gaps between the layers - see my question comment. –  Chris W Jun 30 at 22:01
    
It wouldn't have been by much @ChrisW. Erase is much easier if you have the license, putting this into a model makes a defacto erase. –  Michael Miles-Stimson Jun 30 at 22:11
    
Looks like I have a lot of options! :) –  rachel.passer Jul 1 at 13:24

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