Viewshed analysis: Am I using the correct methodology?

[RobertBuckley]

Hi,

I have a project which entails doing a visibility impact analysis of planned and existing wind turbines and windparks. I will be doing this with arcgis 10 and the spatial analysis extention.

Looking into the forums I see that this is sometimes rather confusing due to the amount of variables which need to be considered. Mr Hubner has answered most the questions in a very professional manner but there are still a couple of questions which I have.

Data
-Point SHP with a single turbine position.(height of turbine = 100m)
-DEM (12,5m resolution)
-Polyon SHP with residential areas, woodland and industrial sites.

Senario 1.
Create viewshed showing raster cells which can see the wind turbine.

Viewshed settings:
OFFSETA = 100 (height of turbine)
OFFSETB = 1,65 (average height of human)
RADIUS = 10000 (10km Radius)
Z-Factor = 1 (default) (is this some sort of conversion setting from ft to meters?)
####I still do not understand what the "SPOT" attribute should do??#####

Is this a correct way of completing a reliable visibility analysis?

Senario 2.
Create viewshed showing raster cells which can see the wind turbine- also taking into account the landscape features in the polygon SHP (woodland, Indiustrial areas)

- I created a raster in the same resolution as the DEM (12,5) based on the polygon SHP. The value field contains the assumed heights of the various landscape categories (e.g resedential = 6m, etc)

- I mosaiced the landscape raster to the DEM adding the heights of the landscape raster to the base height of the DEM. The DEM now contains heights of the earth plus assumed heights of features (woodland etc)

Viewshed settings:
OFFSETA = 100 (height of turbine)
OFFSETB = 1,65 (average height of human)
RADIUS = 10000 (10km Radius)
Z-Factor = 1 (default)


#########
my questions are...

1. the documentation mostly uses the example of someone standing on the point and looking out and the taks would be to find out which raster cells can this person see from his position based on his height (e.g 1,65cm tall). However....if i turn the question around and ask which cells (positioned 1,65cm above the DEM) can see an imaginary point positioned 100m above the ground( e.g a wind turbine), would I simply change the OFFSETA to 100m and the OFFSETB to 1,65? In this case the DEM is raised by 1,65m to assume that a person has his eye level in every raster cell 1,65cm above the base DEM.
Do I understand this correctly? Is it safe to assume that I can swap OFFSETA and OFFSETB as a I like, to analyse either 1. those cells that can see a point or 2. whether a point that can see certain cells? It is this difference that is confusing me slightly because I´m trying to establish whether If I set OFFSETA to be 100m and OFFSETB to 1,65 whether it is safe to assume that from the resulting viewshed the cells with value 1 can see the wind turbine in it´s entirety form ground level to 100m?

I hope that I have explained this well enough to provoke an answer.


yours,

Robert Buckley