[GRASS-dev] [bug #4488] (grass) v.to.rast: wrong cells rasterised often

[Maciek Sieczka]

On Fri, 26 May 2006 20:57:49 +1200
Hamish <hamish_nospam at yahoo.com> wrote:

> Way back when I spent a bit of time trying to figure out how v.to.rast
> worked. While I never saw anything like this at that time*, I can
> offer some possible insights:
> 
> 
> Lines and area borders are treated differently. IIRC, lines tended to
> "activate" every cell they crossed, while area boundaries (without
> category numbers) activated the cell if more than half the cell was
> covered by the "in"-side of the area.
> 
> I'd have to dig into the mailing list archives to find the
> screenshots, but figuring it out was enough trouble that I bothered
> update the man page: ("Otherwise:" section)
> 
> http://freegis.org/cgi-bin/viewcvs.cgi/*checkout*/grass6/vector/v.to.rast/description.html?rev=HEAD
> 
> 
> [*] I did take a fair amount of time zooming way in on lines and
> areas, so I think I would have seen your problem if it was a common
> error - do you get the same results using GRASS 5.4?

So I did the checking. And yes, v.to.rast results in 55 and 61 are
*exactly* the same, as to which cells it picks. It seems v.to.rast
didn't change at all in this regard. Same wrong cells are output by
v.to.rast in both 55 and 61.

> Also, check that your raster region is not like:
> res=50 w=1075 e=1125
> 
> and after zoom in the region is like:
> res=50 w=100 e=150
> 
> as that can have the appearance of shifting raster cells 25m while the
> lines stay in the same place. (that one was another few days of head-
> scratching for me)

I'm aware of these issues, they used to give much trouble too. I took
care not let such shifting take place in this case.

Step by step how I proceeded in 61:

	$ g.region vect=streams res=10 -ap
	projection: 1 (UTM)
	zone:       33
	datum:      wgs84
	ellipsoid:  wgs84
	north:      5684380
	south:      5675370
	west:       596040
	east:       602820
	nsres:      10
	ewres:      10
	rows:       901
	cols:       678

	$ v.to.rast input=streams output=streams_rast use=val value=1
	$ d.rast streams_rast
	$ d.zoom
	$ d.rast.num -f streams_rast grid_color=grey
	$ d.vect streams

	$ g.region -p
	projection: 1 (UTM)
	zone:       33
	datum:      wgs84
	ellipsoid:  wgs84
	north:      5678900
	south:      5678690
	west:       599560
	east:       599780
	nsres:      10
	ewres:      10
	rows:       21
	cols:       22

Step by step how I proceeded in 55:

	$ v.in.shape input=streams/streams.shp output=streams5
	$ v.support streams5
	$ g.region vect=streams5 res=10 -ap
	projection: 1 (UTM)
	zone:       33
	datum:      wgs84
	ellipsoid:  wgs84
	north:      5684380
	south:      5675370
	west:       596040
	east:       602820
	nsres:      10
	ewres:      10
	rows:       901
	cols:       678

	$ v.to.rast input=streams5 output=streams_rast5
	$ d.rast streams_rast5
	$ d.zoom
	$ d.rast.num -f streams_rast5 grid_color=grey
	  (note -f is ignored, a bug in 55's d.rast.num?)
	$ d.vect streams5 col=red

	$  g.region -p
	projection: 1 (UTM)
	zone:       33
	datum:      wgs84
	ellipsoid:  wgs84
	north:      5678900
	south:      5678700
	west:       599560
	east:       599820
	nsres:      10
	ewres:      10
	rows:       20
	cols:       26

See the attached PNGs for the displayed result.

Thanks for your interest.

Maciek

P.S.
In case of any doubts the testing dataset is still here:
http://www.biol.uni.wroc.pl/sieczka/udostepnione/grass/vtorast/test.tar.bz2


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