[GRASS-user] GADM & Polygon files in general

[Thomas, Timothy (IFPRI)]

Thank you for your reply!  I find your answers and queries extremely helpful.

I tried to import GADM all at once, and it crashed badly.  Plus, in version 6.4, if it had run, it would have still taken a long time.  Australia alone took 30 hours of computer time.

I used the -o because even though it was in the same projection as my location, there was some complaining by GRASS, so I just chose to override the complaining.

I would not have chosen to use any snap or threshold settings, but GRASS 6.4 refused to import the shapes otherwise.

It may be that I was only able to get the ESRI Geodatabase version, and then I used Arc View to convert it to a shapefile.  Perhaps that step caused all of the snap issues.

   Tim

***************************************************************
Timothy S. Thomas, Research Fellow, IFPRI
t.s.thomas at cgiar.org       +1-202-862-4605     skype: timothy.s.thomas      Rm 5035



-----Original Message-----
From: Markus Metz [mailto:markus.metz.giswork at googlemail.com] 
Sent: Monday, August 09, 2010 3:10 PM
To: Thomas, Timothy (IFPRI)
Cc: grass-user at lists.osgeo.org; Nelson, Gerald (IFPRI)
Subject: Re: [GRASS-user] GADM & Polygon files in general

Thomas, Timothy (IFPRI) wrote:
>
> It leads naturally into a few questions.
>
>
> 2)  Since I have over 1,000 hours of computer time in converting using
> GRASS 6.4 on Linux with 8GB RAM, what in the world happened in GRASS 7
> that it can now do it in only 4 hours???
>
First, I imported the whole thing at once, not each country
separately. Second, some under-the-hood modifications in GRASS 7 of
the cleaning procedures result in reduced memory consumption and
faster import.

> 3)  Would someone be able to tell me whether the commands I have been
> using are the most efficient and correct, or whether that was the cause
> of the slow speed in the first place?
>
> For importing a shapefile, I used a command like:
>
>    v.in.ogr dsn=gadm1 layer=usa_gadm1 out=usa_gadm1 -o

Why the -o flag? GADM shapefiles come with a *.prj file, i.e.
projection information is available and should not be ignored.
>
> And if they do not have great boundaries, a command like
>
>    v.in.ogr dsn=gadm1 layer=usa_gadm1 out=usa_gadm1 snap=0.00002 -o
>
Looks ok, but gadm is AFAICT clean, snapping boundaries is not necessary.

> To combine 2 countries, I would use something like
>
>    v.patch -e in=usa_gadm1,can_gadm1 out=namer_gadm1
>
Watch out for category values when patching. I would prefer to import
the whole thing at once, or the county you are interested in.

> And to repair the boundaries, something like
>
>    v.clean namer_gadm1 out=namer_cln_gadm1 tool=snap,break,rmdupl
> thresh=0.00002

Looks ok, but for gadm this should not be necessary.

Markus M