[GRASS-user] v.to.rast gives empty raster layer

[Hanlie Pretorius]

Hi Moritz,

I'm not sure what you mean by 'Maris' first part of the solution'. I
saw only one part to Maris's suggestion (refining the region's
resolution) because I said in my initial mail that I had set extents
of the region to the contours_20m vector layer. Initially I did have a
resolution of 20m and got an empty raster layer as a result.

I share Micha's confusion about this. Although it doesn't make much
sense to have a 20m cell size (my bad) I would have thought that
v.to.rast would still have given me an output.

Regards
Hanlie

2011/5/3, Moritz Lennert <mlennert at club.worldonline.be>:
> On 03/05/11 15:47, Micha Silver wrote:
>> On 05/03/2011 04:34 PM, Moritz Lennert wrote:
>>
>>> On 03/05/11 10:20, Micha Silver wrote:
>>>> On 05/03/2011 09:11 AM, Hanlie Pretorius wrote:
>>>>
>>>>> Hi Maris,
>>>>>
>>>>> Thanks, your suggestion worked. With the region's resolution set to
>>>>> 5m, I got some output.
>>>>>
>>>>
>>>> Hi Maris:
>>>> Sorry for butting in on this, but I don't understand why the region
>>>> resolution should make any difference to v.to.rast??
>>>> Would you mind to explain?
>>>
>>> From the v.to.rast manual:
>>>
>>> "v.to.rast will only affect data in areas lying inside the boundaries
>>> of the current geographic region. Before running v.to.rast, the user
>>> should therefore ensure that the current geographic region is
>>> correctly set and that the region resolution is at the desired level."
>>>
>>
>>
>> Thanks, Moritz
>> What I'm trying to understand is the question about resolution. Would
>> you expect, as
>> Hanlie indicted, to get *no output* in the raster because of too course
>> resolution?
>> He claims that at 20m x 20m v.to.rast of a line feature created all NULL
>> cells, but at 5m x 5m the lines did appear in the raster.
>> ??
>
> Hanlie didn't tell us whether he applied Maris' first part of the
> solution as well (but I guess he did):
>
> g.region vect=contours_20m
>
> I agree with you that resolution does not make the difference. However,
> I guess that Maris meant that having a 20m resolution seems too coarse
> for 20m contour lines, but even that is not clear as all depends on the
> topography of the region in question. If you have 100m altitude change
> for 50m horizontal change, then 20m is a bad choice of resolution, but
> if you have 100m altitude change for 1km horizontal change, it should be
> more than enough.
>
> So, in conclusion, the region's resolution setting can make a huge
> difference in v.to.rast, but in this particular case does not explain
> why there was no output. This is certainly linked to the region's extension.
>
> Moritz
>
>