Viewshed analysis in forested area

Question

I'm running viewshed analyses based on some countryside footpaths - I'm using a 5m DTM, but I want to incorporate more elements into this viewshed. I'll be converting shapefiles to rasters and assigning a z value so this can be added on top of the DTM.

However, the problem I'm envisaging is that when I add elements where viewshed points are present, this viewshed point will naturally be placed at the same level of the new DTM with added elements. I want to somehow have the viewshed points at the height of the original DTM. For example, if a footpath is going through a forest (which has been added as an element from the original shapefile) I don't want the viewshed point to be on top of this forest, but instead at the DTM "ground level". I understand that there will be no visibility for such instances but I would nonetheless like to keep such points.

I have the viewshed points as a point class and a line class. Perhaps by converting these into rasters then assigning this the DTM z value this can then replace other cells (i.e. where added forestry elements exist).

I feel there's probably a way of doing this in Raster Calculator, but I'm not sure exactly of the pathway to follow.

Any help would be greatly appreciated!

JeffVan_Etten · Answer

Jordan

For the points that would be on top of the forest, these would be null then wouldn't they as they cannot penetrate the forest canopy.  So you could just filter out these points and not run them and only run the points that are on the path.  These would then give you the areas where visibility would be achieved.

Maybe oversimplifying, but worth a thought.

Jeff

curtvprice · Answer

I'm using an OFFSETA of 1.7 as the viewer height, but I want this viewshed point to be "pinned" to the height of the original DTM height as opposed to the newly created DTM with forestry etc.

Seems like you could do this by adding the elevations of interest to the point table using Extract Values To Points and doing a field calculation to appropriately populate OFFSETA.

JordanWhite · Answer

I'm using an OFFSETA of 1.7 as the viewer height, but I want this viewshed point to be "pinned" to the height of the original DTM height as opposed to the newly created DTM with forestry etc.