[GRASS-user] Unable to get an horizon from a DEM using r.horizon

GRASS GRASS user
1

Hi all,

My goal is to extract the visible horizon given a point using a DEM I have successfully imported into GRASS. Using the r.horizon command it returns me some good result for some part of the horizon. However at some other angle in returns me the 0 value which means that no horizon has been found. I cannot understand why we cannot found a horizon. There always be an horizon, even if a point lies on lower altitude than you. I have tried to modify the r.horizon command to return horizon values which are also below you but it doesn't work well and returns me some weird results.

Anyone having some clues about that?

Thank you a lot in advance.

Best,

Mathieu Monney

2

Mathieu Monney wrote:

My goal is to extract the visible horizon given a point
using a DEM I have successfully imported into GRASS. Using
the r.horizon command it returns me some good result for
some part of the horizon. However at some other angle in
returns me the 0 value which means that no horizon has been
found. I cannot understand why we cannot found a horizon.
There always be an horizon, even if a point lies on lower
altitude than you. I have tried to modify the r.horizon
command to return horizon values which are also below you
but it doesn't work well and returns me some weird results.

Anyone having some clues about that?

not directly, but you might try the r.los module, or r.viewshed from
wiki addons.

Hamish

3

r.cva from wiki addons too.

4

Thanks for the answer.

However the command you proposed are really time consuming as there computing the entire viewshed. I only need the last visible points from a given position. Do you know any other option to do that?

Thanks.

Mathieu

Le 28 août 2009 à 03:57, Hamish a écrit :

Mathieu Monney wrote:

My goal is to extract the visible horizon given a point
using a DEM I have successfully imported into GRASS. Using
the r.horizon command it returns me some good result for
some part of the horizon. However at some other angle in
returns me the 0 value which means that no horizon has been
found. I cannot understand why we cannot found a horizon.
There always be an horizon, even if a point lies on lower
altitude than you. I have tried to modify the r.horizon
command to return horizon values which are also below you
but it doesn't work well and returns me some weird results.

Anyone having some clues about that?

not directly, but you might try the r.los module, or r.viewshed from
wiki addons.

Hamish