I have performed a bare-earth viewshed analysis of a roadway corridor using a DEM, but upon further inspection on Google Streetview, realize that part of the road has dense trees along it. I need to figure out a way to incorporate this treeline into the analysis. It seems that the OFFSETB field will accomplish what I need: "The OFFSETB item indicates a vertical distance in surface units to be added to the z-value of each cell as it is considered for visibility." The steps I would follow would be:

1) Segment the observer line feature for where trees are located

2) Add an OFFSETB field and add a value of 10m for the treeline segment

Am I understanding the OFFSETB function correctly?

  • If you have access to a DSM instead of a DTM you might be a better result. Commented May 30, 2015 at 14:38

1 Answer 1


It depends what you want to do.

Offsets are used to place the observer (OFFSETA) and the observed object (OFFSET B) above ground. For example, offset B of 10 meter tells you if you would see the top of a 10 m high tree from the observers position. OFFSET B is applied to any possible location.

If you want to use the trees as obstacle, I recommend that you add the tree height to your DEM (e.g. with raster calculator)

  • Can you provide me with some steps as to how I would only add tree height to parts of the DEM where the treeline is located with raster calculator?
    – Angela
    Commented May 28, 2015 at 13:22
  • 1
    feature to raster with the height of the trees as attribute field. then Con(Isnull("your_raster_with_trees"), "DEM", "DEM" + "your_raster_with_trees")
    – radouxju
    Commented May 28, 2015 at 14:16
  • Thanks! I followed your formula (though I don't quite understand it), and I can tell that it is calculating correctly; however, it resulted in holes where my raster with trees was, though not exact. It seems like this is an issue with consistent cell sizes between the DEM and my raster with trees. Do you know how to resolve this?
    – Angela
    Commented May 28, 2015 at 18:57

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