topic Re: Viewshed OFFSETA and OFFSETB in 3D Questions

Viewshed OFFSETA and OFFSETB

TanjaHumar — Tue, 17 Jul 2012 18:22:49 GMT

Hi, I have conducted a viewshed analysis using an OFFSETA (as the observer point) and OFFSETB as the target point. However, the results didn't seem right and I tried to verify the results with the line of sight tool of 3D analyst (OFFSETA = observer offset and OFFSETB = target offset). While the results didn't match at all, the line of sight tool-results made more sense. Once I swapped OFFSETA and OFFSETB (OFFSETA being the target pont...), it did match with the line of sight tool and my expectations...

However, in the ArcGIS HELP it is clearly written that OFFSETA should be the observer point and not the target point. What is now the correct approach??

Many thanks for your help!

Tanja

Re: Viewshed OFFSETA and OFFSETB

XuguangWang — Wed, 18 Jul 2012 17:43:48 GMT

Yes, OFFSETA represents observer offset. It has been that way since Workstation GRID.

Re: Viewshed OFFSETA and OFFSETB

TanjaHumar — Fri, 03 Aug 2012 05:33:48 GMT

Thanks for your answer, but the results just don't make sense this way and they don't match with the line of sight results!

Re: Viewshed OFFSETA and OFFSETB

ArtoVuorela — Wed, 02 Oct 2013 05:44:15 GMT

Hi Tanja. Sounds so familiar. I think I agree, swap OFFSETA and OFFSETB. I struggled with this a lot in 2009 when I did my first Viewsheds, I tried it back then and reported/complained to Esri Finland that the guidance is very, very confusing. Had to teach them what the things mean. But think it this way at least if you have some towers the visibility of which you wish to find out from whichever location: While OFFSETB is added to each pixel, it should be about 2 m, if a human being is observing from each cell, therefore OFFSETB should actually be said to be at the observation point in such cases at least. One observer cannot be in many towers (or pixels) the same time... OFFSETA "to be added to the z-value of the observation point" is to be added only to those few vector points in your feature class, not to each DEM cell. If they are the towers, they could be 100 m high or so.

Again yesterday out of office and without my old examples at hand, I run Viewshed by just looking at the on-line help, and did it the wrong way. The help is very counterintuitive for the case mentioned at least. Back then I also pointed out several confusing things about the image they use.