[GRASS-user] v.isochrones : error message

[Moritz Lennert]

On 05/04/15 12:53, image93 wrote:
> Hello,
>
> Below, the  computational region setting :
> --------------------------------------------------------------------
> projection: 99 (Lambert Conformal Conic)
> zone:       0
> datum:      towgs84=0,0,0,0,0,0,0
> ellipsoid:  grs80
> north:      7037330.05419347
> south:      -0.94580653
> west:       0
> east:       798655
> nsres:      1
> ewres:      1
> rows:       46916
> cols:       5324
> cells:      249780784
> --------------------------------------------------------------------
>
> When i modify the resolution nres/ewres from 1 to "150", i have no longer
> error message. But when i try a process with a resolution with a value <
> 150, the process is endless...
>
> According to you, the results generated at this resolution (150) would be
> relevant? valid?
>
> I loaded the output vector into qgis. The results are seem incoherent. The
> curves show that we can go in 15 minutes on the other side of Paris! In
> reality, From my starting point we would go much further toward toward north
> than south west...
> I use OSM data. I edited and populated the max speed field thanks to the
> value of another filed (type of road).
> How to explain this incoherence? Due to v.isochrones? Due to resolution? Due
> to input maxspeed values ?

Difficult to say without the data. At this stage, v.isochrones should 
probably be considered beta, notably because of a significant rewrite, 
so it might be v.isochrone's fault. Then again, as I mentioned in my 
response to your other mail, I'm not sure that you are calling 
v.isochrones with the correct input data.

Could you make your data available off-list, so that I can experiment 
with it myself ?

>
> I am annoyed because it seems impossible to generate the results for a
> better resolution than 150.

That is not normal, As I told you offlist, I've been able to run 
v.isochrones in a 152,198,630 pixel region on the entire OSM network of 
Belgium within 1-2 minutes depending on settings. That's double the 
number of pixels of what you would get with a 2m resolution.

Moritz