[GRASS-user] fixing overlapping areas with snap and min_area

[Tyler Smith]

Hello,

I am trying to import a shapefile that contains polygons. Using v.in.ogr with default settings works, but generates overlapping areas:

```
v.in.ogr input=~/path/to/data/ output=soils_omafra 

...

-----------------------------------------------------
139375 input polygons
Total area: 1.87116E+11 (713211 areas)
Overlapping area: 465.913 (142064 areas)
Area without category: 1.0631E+09 (137984 areas)
-----------------------------------------------------
Copying features...

Building topology for vector map <soils_omafra at PERMANENT>...
Registering primitives...
Building areas...
WARNING: Area of size = 0.0 ignored
WARNING: Area of size = 0.0 ignored
WARNING: Area of size = 0.0 ignored
```

After several hours testing different values of snap and min_area, I have come up with the following:

```
v.in.ogr input=~/path/to/data/ output=soils_omafra snap=4.52e-07 min_area=10.0

...

-----------------------------------------------------
138109 input polygons
Total area: 1.87116E+11 (138565 areas)
Overlapping area: 34.3593 (4 areas)
Area without category: 1.0631E+09 (421 areas)
-----------------------------------------------------

[no warnings reported]
```

If I increase snap any more I start creating additional areas, so I don't think I can automatically get rid of the remaining 4 areas.

However, when I plot those areas and look at them, they are below the min_area threshold I've set:

```
d.what.vect map=soils_omafra

...
soils_omafra at PERMANENT:
Type: Area
Sq_Meters: 6.307
Hectares: 0.001
Acres: 0.002
Sq_Miles: 0.0
Layer: 2
Category: 2
...
```

My map is in a lat-long projection, and the map unit is meters. I thought setting min_area=10.0 would drop all areas smaller than 10 sq meters, but my problem areas here are only 6.3 sq meters?

Have I misunderstood what min_area does, or am I setting it incorrectly? I notice that the default is 0.0001, which seems really small if the units is actually meters.

More generally, I'm not sure how critical these overlapping areas are, or when I can safely ignore them. For my current project, I'm going to use the imported polygons to create rasters for analysis in R.

Thank you for your time,

Tyler

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