Strange results when I execute the intervisibility analysis

[lluisvicens]

Hi, I'm performing an intervisibility analysis and the results I get, from my perspective, are quite surprising.

I have two point layers (windmills and points of interest), and a DEM. With these three layers, I execute (twice) the «Create viewpoints» tool, one for the windmills and one for the points of interest. For the windmills, I define an observer height of 60 meters and a target height of 2 meters. For the points of interest, I define an observer height value of 2 meters and a target height value of 60 meters. In both cases, I'm using the same DEM.

Then, the point is that when I execute the «Intervisibility network» (also saving negative links), I obtain different results depending on which layers are acting as observer points or as target points. Performing the intervisibility analysis from points of interest to the windmills, I only get 2 positive links (and 286 are negative) while, when I execute the same analysis but on the other way around (from windmills to points of interest) I get up to 93 positive links (195 negative links).

¿Should don't I get the same positive links in both cases? As far as I know (perhaps I'm wrong on the way the algorithm operates) if I can see a point A from a B point, then I should be able to see the point B from the point A. ¿Am I wrong?

Thanks for your help. Lluís

[zoran-cuckovic]

Hello, This is surely a bit tricky to grasp ... The parameters should be understood as observer and as target. Your windmills as observers have 60 meters but only 2 as targets.

So, analysis 1 : windmills = 60 m to observers = 60 m (as target!); analysis 2 : observers = 2 m to windmills = 2 m (as target!).

Does that make sense ? Best, Zoran

Le ven. 15 mars 2019 à 14:42, Lluís notifications@github.com a écrit :

Hi, I'm performing an intervisibility analysis and the results I get, from my perspective, are quite surprising.

I have two point layers (windmills and points of interest), and a DEM. With these three layers, I execute (twice) the «Create viewpoints» tool, one for the windmills and one for the points of interest. For the windmills, I define an observer height of 60 meters and a target height of 2 meters. For the points of interest, I define an observer height value of 2 meters and a target height value of 60 meters. In both cases, I'm using the same DEM.

Then, the point is that when I execute the «Intervisibility network» (also saving negative links), I obtain different results depending on which layers are acting as observer points or as target points. Performing the intervisibility analysis from points of interest to the windmills, I only get 2 positive links (and 286 are negative) while, when I execute the same analysis but on the other way around (from windmills to points of interest) I get up to 93 positive links (195 negative links).

¿Should don't I get the same positive links in both cases? As far as I know (perhaps I'm wrong on the way the algorithm operates) if I can see a point A from a B point, then I should be able to see the point B from the point A. ¿Am I wrong?

Thanks for your help. Lluís

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[lluisvicens]

Thank you so much for your kind reply. Probably (at least for me, sorry) it is a little bit unclear behavior. And perhaps won't also be easy to explain it to my students in a 'rational' way when windmills area 60 meter height and POI 2 meter height. In any case, I will follow your instructions carefully.

Thanks a lot for your time!

[zoran-cuckovic]

I've written a tutorial to make things more clear.