Viewshed results clarification in ArcMap 10.1

Question

I'm running a viewshed analysis is ArcMap 10.1, and I'm concerned about the results that I'm getting which if leading me to believe that my current understanding of viewshed variables may not be correct. Specifically, I'm concerned that I'm getting no viewshed values (GRIDCODE = 0) is areas were I'm fairly certain there should be a viewshed (based on personal knowledge).

The purpose of this viewshed analysis id to evaluate oceanfront views. I have a set of observation points that are centroid points to a home, and a lidar raster as my elevation input. The building footprint which the observation points rests in has been set to an elevation value of 0. Within the observation point file I have attribute fields for SPOT, AZIMUTH1, AZIMUTH2, RADIUS1, RADIUS2. I am currently not incorporating offset values.

I am understanding the SPOT value to be the height of the observer point measured from a height of 0. Is this correct? So, if I have the SPOT value listed as 13 and my map units are in feet, the observer height is being taken at 13 feet. Am I correct on this? If I am correct (which I'm fairly certain I am), is there something I may be missing that could be causing the off results I believe I'm getting?

Answer 1

From http://resources.arcgis.com/en/help/main/10.1/index.html#//00q90000008n000000:

SPOT

The SPOT item is used to define the surface elevations for the observation points.

Offset

The offset is the vertical distance (in surface units) to be added to the z-value of a location on the surface. There are two offset items, one defining the elevation to be added to the observer location and the other defining what will be added to each cell to be considered for visibility. [...]

Your spot understanding is right. However, my suggestion is to use offsets instead of spots, because you have to be sure that the observer points are above the DEM surface and avoid to understimate the height of the observers and, consequently, the visibility of targets. In short, the offset grants you to have a better control of the position of the observer, because it's always a well known distance.