[GRASSLIST:3269] Re: Rastering

[J.C.M. van der Kwast]

Hi Erin,

Your mail cleared some things up for me. The patching of the narrow raster
to the whole map is a thing that works, but it's very labour intensive. I
also noticed that when zooming in and overlaying the vector map and grid
that there's actually no gap. It seems the maps get rastered oke, but
actually is only oke at a small scale. I've tried blowing up the map so the
number of cellls rastered would be greater. This doesn't help.
Just one remark, I don't see how finer vectors can help with this problem.

Greetings,

Sjors

BTW import to arvcview8.1 is done oke. All small areas are visible in
monitor, though (but I don't mind) exaggerated a little.




>
> Sjors,
> I think you are right that it has to do with scale and resolution.  One
> thing that helps visualize this is to zoom in on a small spot of your
> raster and then superimpose the vector and a grid (with d.grid if your
> region boundaries start on whole units equal to your resolution) to see
> where the gaps are and why.   If the digitized polygon takes up less than
> 1/2 of a grid square it does not show up.  If the feature is less than the
> resolution, you can not deal with it.
>
> I have had similar needs and problems dealing narrow polygons, in my case
> with roads and narrow spots along meadows.  I digitized them as areas
> because I wanted a raster with representative road widths and I want my
map
> to reflect the connectivity.  I tried finer resolutions, eventually it
> works, but that is a pain, I don't really want to work with a 2m x 2m
> raster.  One compromise is to make the features labeled lines in v.digit,
> then rasterize those vectors and r.patch the narrow raster on the other
> map.  Often the only connectivity is at the corners, so it's not really
one
> continuous polygon, but it's better than nothing.  Also, of course, your
> feature is exaggerated, but depending on your question, that may not be a
> big problem.  I believe the only alternative is to work with a finer
> resolution, or vectors.  (I used the latter, when trying to calculate
> landscape metrics.)
> Good luck.
>
> Erin O'Doherty
> Laramie, WY, USA
>
>
>
>
>                     "J.c.m. van der
>                     Kwast"                  To:     grasslist at baylor.edu
>                     <jkwast at home.nl>        cc:
>                     Sent by:                Subject:     [GRASSLIST:3247]
Re: Rastering
>                     owner-GRASSLIST@
>                     baylor.edu
>
>
>                     02/27/02 09:20
>                     AM
>
>
>
>
>
>
> Hi Karine,
>
> Yes, I've checked region settings, there's no problem at all. I ddin't
> mention it before, but I've also tried to buffer the cells (r.grow (1
> cell) and r.buffer, and with mapcalc tried to define them as areas again
> ( in mapcalc use some like newfile = file + buffered_file). This does'nt
> seem to work as well, unless the buffer is like 5 m which would
> exaggerate the features enormously. Also the neighborhood module, has
> been tried. Same story, you must exaggerate enormously (e.g. buffer 9
> cells up to 13 cells). I've been trying afterwards to thin the map with
> r.thin leaving me the result I've started out with. I think the problem
> has more to do with scale and resolution. Well I'll keep on trying this
> and that, thanks anyway,
>
> Greetings,
>
> Sjors
>
>
>
>
> Karine ZUERCHER wrote:
>
> >Sjors,
> >
> >Have you checked the region settings (g.region -p) just before doing the
> >v.to.rast?
> >You should have the nsres & ewres equal to 1 &  rows & cols 5000.
> >
> >Hope that may help
> >In Kindness
> >
> >Karine
> >
> >On Wednesday 27 February 2002 15:43, J.c.m. van der Kwast wrote:
> >
> >>Hi everybody,
> >>
> >>For some time now I'm wrestling with the rasterization of thin
> >>linear/area objects. The case is as follows, xy projection, area 5000 x
> >>5000 m (or units), grid 1 x1. I've got thin areas of 3 m wide and trying
> >>to raster them. I would expect to see them as thin linear objects in a
> >>raster map of the whole area. But the areas are broken. When zooming in
> >>extremely then the areas seem to form lines or areas again, but are they
> >>continuous? Maybe it has something to do with the scale?
> >>When given a z value and patched with a dem, the dem is correctly
> >>displayed but the thin areas are broken into pieces (as they are in the
> >>whole raster map). Does it mean that I should divide the map up into
> >>small portions with a desirable resolution, raster them and patch them
> >>back together in the desired resolution? Personnally I don't think it
> >>will help as the areas formed do not seem to be continuous.
> >>Some time ago I wrote a mail with the title landscape. The problem is
> >>much the same, only with rastering linear features. As my background is
> >>not GIS (but agriculture) it could be I'm trying to do something which
> >>isn't possible at all. But looking at it logically, a feature of 1 cell
> >>in a grid of 1 x1 should at least take the value I've given it. If it
> >>doesn't show up at the screen I can live with it. Problem is, it does
> >>sometimes will and sometimes won't show up.
> >>I've been playing around with a lot of functions and manuals but still
> >>this puzzles me greatly. If anyone has any answers, I'd be most
grateful,
> >>
> >>Warm regards,
> >>
> >>Sjors
> >>
> >
>
>
>
>
>