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

[Moritz Lennert]

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