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coverage vs shape files in a nutshell   am ka Nov 13, 2006
Re: coverage vs shape files in a nutshell   John Sobetzer Nov 14, 2006
Re: coverage vs shape files in a nutshell   Derek Law Nov 14, 2006
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Subject coverage vs shape files in a nutshell 
Author am ka 
Date Nov 13, 2006 
Message From what I gather, the main point of the matter is that

- shapefiles need to be in a geodatabase to analyse topology
- coverage files already have topology and don't need to be in a geodatabase.

I have read from conversations here how people have talked about doing away with coverages because "how would you store information about topology?"

My question back at that is, if you have all your shapefiles and put them into a geodatabase, doesnt that give you your topology back?

So is the main debate here of time management? Do you want to have to build a geodatabase, or do you want files in ready to analyze topolgy form?

I'm sure there are many other issues too (which I would love to know about), but is the above the main one?

thanks! 
   
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Subject Re: coverage vs shape files in a nutshell 
Author John Sobetzer 
Date Nov 14, 2006 
Message A coverage has perfect fabric topology, no gaps and no overlaps. The essense of editing coverages is editing arcs and labels, and when you are done you rebuild the polygons, abutting polygons sharing the same arc and hence same vertices making up that arc.

Shapefiles and feature classes have "independent" features, so that each polygon shape has its own shape field and is linked to an attribute record. Thus they can overlap or have gaps. When they are coincident each polygon shape has a line for their shared boundary. If you edit a polygon, say by splitting it, you've added vertices to its shape, but not those of the abutting shapes, and given the limits of precision, where the polygons previously had coincident lines they no longer do.

Shapefiles and feature classes can be set up in a way that sort of mimics coverages, by using lines and points. You edit the lines and points (including the attributes) and when you are done build polygons using both. The polygons have coincident lines where they abut because they come from the same source.

Shapefiles and feature classes can be edited in ArcMap using map topology tools as a way to reduce the likelihood of gaps or overlaps occurring, but I don't think that helps with issues raised by actions like the cutting of polygons, more with the moving of vertices or reshaping or adding polygons, and it doesn't deal with copying and pasting, merging or appending polygons, although using the construct features tool can be invoked.

The matter of topology rules, however, can go far beyond fabric topology. Thus in a geodatabase, not only can one have rules addressing overlaps or gaps, but there can be all sorts of rules covering other matters such as what kind of attributed lines can connect to other kinds of attributed lines. ESRI materials on topology go into these in depth. These are not available for shapefiles or coverages.

We predominately use coverages because the fabric topology guarantees accurate acreages and so far at least, only coverages can feasibly handle our complex overlays of large datasets. But I also use shapefiles in the lines and points mode to mimic coverages for smaller projects since ArcEdit has its own limitations and difficulties. 
   
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Subject Re: coverage vs shape files in a nutshell 
Author Derek Law 
Date Nov 14, 2006 
Message Hi MK,
For detailed information on the two data types, you can read the ArcGIS online help.

Coverages:
http://webhelp.esri.com/arcgisdesktop/9.1/index.cfm?id=165&pid=164&topicname=About%20coverages%20and%20INFO%20tables

Shapefiles:
http://webhelp.esri.com/arcgisdesktop/9.1/index.cfm?id=155&pid=154&topicname=About%20shapefiles,%20dBASE%20tables,%20and%20file%20types

Hope this helps,
Derek Law
ESRI