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I downloaded GADM administrative level 0 worldwide and it seems very simple file: 255 rows of polygons representing countries. But the size of the file is very large: >500 MB. Why it's happening? Usually table with 255 rows should be roughly few MB's.

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    Can you describe (attach a print screen of) the variables (fields) types? Most of a shp file size is allocated to the dbf file. This includes all tabular information, including geometry. I guess that many fields in conjunction with complex geometries cause inflate the file size. – dof1985 Jul 17 '16 at 14:08
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    Little correction: The geometry is stored in the .shp file. I am sure OP's data is huge because of the .dbf. – bugmenot123 Jul 17 '16 at 14:28
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    Actually, a "huge" .DBF is impossible with only 255 features -- Each row is limited to 4000 bytes by the dBase-III spec, and 1Mb hasn't qualified as "huge" since the mid-1980s. – Vince Jul 17 '16 at 19:23
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    Technically, a perfectly accurate map of country boundaries would require an unlimited amount of space. – JonathanReez Jul 18 '16 at 9:24
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The dataset is so large, because the borders are very detailed.

If you are looking for a worldwide overview, you better take the data from Natural Earth. They provide datasets suitable for different scales. http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/cultural/ne_10m_admin_0_countries.zip is the smallest one.

5

These 255 rows represent 255 countries and each row is not a single polygon rather it is a multipolygon. Multiplogon for each country is due to the presence of large number of disjoint coastal areas or in other words islands. If you check the number of points involved in creating China alone, it goes beyond 7,00,000 reaching 11MB.

You can check the size of each shapefile in diva-gis site.

You can reduce the size of these shapefiles in QGIS by using vector-> geometry tools ->simplify geometries.

  • How is dissolving helps reducing the file size? The amount of vertices will be the same. – Matte Jul 17 '16 at 15:25
  • @Matte I beg your pardon, we need to use simplify geometries option in geometry Tools. – GeoSpatialEarth.in Jul 17 '16 at 15:29

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