I am trying to understand the viewshed and observer points tool in spatial analyst.
I want to show which areas of land can see a wind turbine, or put another way, from where can the wind turbine be seen!
In my simplified model I have just the DEM and a point shp which contains the following fields: OFFSETA and OFFSETB.
I have read the ESRI Documention and OFFSETA is the height of the observer over the DEM and OFFSETB is a height of an object over the DEM.
If I calculate the viewshed as follows: OFFSETA= 1.65 (Eye-level) and OFFSETB = 50 (the top of the windturbine) what am I actually calculating? Could it be all cells which the observer (at a height of 1.65m above the DEM) would observe if a 50m tall wind turbine would stand in every cell? in other words... these are the cells in which a 50m tall wind turbine could be seen from the observation point?
Because I want to know from where the turbine is visible, I thought I should put the observer´s eye-level on top of the wind turbine (50m). Now I can just calculate all cells that I can observe on the DEM + 1.65. If I can see then from the top of the turbine then I am assuming they can see me, right?
Now the practise: I created two viewsheds as above described.
Image 1: OFFSETA= 1.65 & OFFSETB=50
Image 2: OFFSETA50 & OFFSETB= 1.65
The two approaches gave two very different pictures When the OFFSETA = 50m I get these straight lines.
I am trying to understand why?