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*edited file links below

I'm working on several NGO projects at the moment, all of which are reliant on being able to use a district level map with some 490+ features, and I am running into some trouble with my geometries.

I spent most of the weekend trying to figure out how to snap the features together in this file (https://dl.dropbox.com/u/49434182/492%20Kabupaten%20Google%20Mercator.zip), to get rid of overlaps and slivers. Running the fTools validation tool, it tells me that I have more than 200 problems with this file. So, being a sensible person, I wanted GRASS to do the work for me. However, no matter how I tweaked the parameters of v.in.ogr, I couldn't eliminate the gaps or slivers when I went to export it again. GRASS would generate one layer of (basically) the geometries I wanted, and two layers of slivers.

I was able to get my hands on an official shapefile from the national statistics agency (found here: https://dl.dropbox.com/u/49434182/indokabkot_497_2010.zip), which although way more detailed, also has invalid/overlapping geometry according to fTools. I'm unsure of the right projection with this file, so that's also a bit of an issue.

I'm using QGIS 1.8, and am at a bit of a loss as to where I've gone wrong. Can anyone provide pointers before my data is due for a presentation next week?

Thanks!

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  • Just providing the .shp file without the other files that are used by the shapefile format doesn't allow anyone else to look at the data. You can check for possible coordinate systems at the EPSG registry, epsg-registry.org. Ah, Indonesia. The NEIEZ ones might be possible, but most CRS are narrow TM zones.
    – mkennedy
    Mar 13 '13 at 18:59
  • better stuff your hole files into a .zip and then publish a link
    – Bernd V.
    Mar 13 '13 at 22:04
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    Check also the v.clean module which you can use after import of the data. Note that GRASS GIS 7 (which you can install in parallel to GRASS 6), offers improved topological features at higher processing speed.
    – markusN
    Mar 14 '13 at 8:37
  • Thanks for the input everyone. I've updated the links to the files, so you can play around with the files. Sorry for the mistake there. I've also started downloading GRASS 7, and will see if that offers any help. I presumed that the v.in.ogr function already performed all the functions of the v.clean toolbox, but is that not the case?
    – Justin
    Mar 14 '13 at 9:58
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The snap option in v.in.ogr and v.clean in GRASS are basically the right way to go. But looking at the file you published, I'm afraid that you can't fix all problems without manual editing.

My tip: Get familiar with the GRASS digitizer and invest a day or two to build a topological correct data layer.

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  • 1
    Has the GRASS digitizer advantages over the QGIS digitizing tools?
    – AndreJ
    Dec 9 '13 at 8:21
  • Yes, it is a topological digitizer. Extra hint: GRASS GIS 7 is even better at topology, see grass.osgeo.org/grass7
    – markusN
    Feb 13 '14 at 23:49

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