topic Re: Viewshed analysis in forested area in ArcGIS Spatial Analyst Questions

Viewshed analysis in forested area

JordanWhite — Tue, 02 Jul 2013 19:15:13 GMT

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!

Re: Viewshed analysis in forested area

DanPatterson_Retired — Tue, 02 Jul 2013 19:20:09 GMT

read up on the use of OFFSETA which must be incorporated into the shapefiles table to represent a viewers height (ie 2 m or whatever your want), your observer point will be set above the ground by this distance. Similarly, you can add polygon's and their heights to the DEM giving the allusion of not seeing through the forest.

Re: Viewshed analysis in forested area

JordanWhite — Tue, 02 Jul 2013 19:28:27 GMT

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.

Re: Viewshed analysis in forested area

curtvprice — Tue, 02 Jul 2013 22:51:48 GMT

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.

Re: Viewshed analysis in forested area

JeffVan_Etten — Mon, 02 Sep 2013 07:53:26 GMT

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