I am facing a problem with viewshed analysis, when I am using > 1 point till 15, not all tthe points are represented at the viewshed (i am using points as nodes not as points in a polyline) which is logic, but the most important is that I am a bit confused with, which is the non visible area.Is it always the first row with the value 0?
or are you confused about the tabular outputs? from here
Observer Points stores the binary-encoded information about which observation points can see each raster cell. This information is stored in the VALUE item.
To display all the regions of the raster that can be seen only by observer 3, open the output raster attribute table and select the row where observer 3 (OBS3) equals 1 and all other observers equal 0. The regions of the raster that can be seen only by observer 3 will be highlighted on the map.
You remember well Mr Patterson, no it's not that with the tabular points. I am confused with the fact that I have built an application to run several viewsheds for 1, 2, 3..15 points, and of course in the viewshed I get, I don't have always 2,3,4,....16 rows as the number of points + the invisible area are, but less, and moreover I am asking to esnure that, the invisible area is always in the first row of the viewshed?
yes it reads very confusingly, but from the help example
"select the row in the table where OBS3 has a 1 AND all other rows have a zero"
yup got it ... sort of. I think you are going to have to do it interactively and get the table figured out, or... as suggested, run each point, one at a time
From the tests I runned until now for 5 points:
1. Always the first row of the viewshed tool is the non visible area.
2. If you run the viewshed tool for each of the 5 points separately you will have different visible and non visible areas than if you run the shp with the 5 points at once in the viewshed, however the non visible area remain always the same, the first row with value "0".
3, If you run the process from the observer points you will get the pixels that are visible from each point "the raster that can be seen only by the specific observer" which fits with the viewshed if you run it separately for one point (1.) each time, but it has very big differences with the viewshed that comes up if you run the viewshed tool with input the shp where all points are together in one file (2.)
! So the non visible area is always the first row even if you run the viewshed for 1 point or for 15.
! Still remains unknown why the viewshed raster has less rows than the number of points you used.
Which makes me want to test 5 points in sequence on one side or a barrier which we know is high enough that nothing can be seen on the other side..., I would suspect that you might end up with 2 rows, but ... something to test on the list
When you run a viewshed analysis with multiple points it is a collaborative process. So for each pixel in the output it will have the value of how many of the points in the analysis can see it. So if none of the points can see that pixel then the value will be zero, if five points can see that pixel then the value will be five.
If what you require is to have each point show individually what it can see then you will need to run the process on each point by selecting them individually before you run the process each time. This method will give you an output raster for each point. You may be able to find a tool somewhere that will iterate through each point rather than have to select and run each time.
Hope this info helps.
I understand what you mean but can't I choose as an input also a shp with maximum 15 points to run the viewshed? I think that I can do it, from 1-15 points right? My problem is that the viewshed for the 15 points for example it will appear a viewshed raster with only 7 rows in which I won't know which correspond to which point and that's ok, but also I don't know what row is that that represents the invisible area. Is it the first row with value 0 always?
If you select each row individually in the table the corresponding cells with that row will be highlighted, then you will know the cells that correspond with the invisible area, is a new attribute field added for each iteration?