topic Re: Determining correct format for OFFSETA in Viewshed creation in ArcGIS Spatial Analyst Questions

Determining correct format for OFFSETA in Viewshed creation

KrissyMoore — Thu, 12 Sep 2013 21:17:04 GMT

I want to create a viewshed for a 20m tall tower.

I have a DEM I created and projected into British National Grid using the project raster function

I have an observer point shapefile also projected in British National Grid. This shapefile has an OFFSETA field. I'm not sure what value to put into this field to represent 20 m. Is it just "20"? The linear measurements for the BNG projection are in metres but the vertical measurements are (apparently) in degrees. What does this mean for my OFFSETA value?

Thanks in advance for your time.

Re: Determining correct format for OFFSETA in Viewshed creation

DanPatterson_Retired — Fri, 13 Sep 2013 03:29:54 GMT

vertical measure in degrees? never seen a file like that, an offseta of 20m should work

Re: Determining correct format for OFFSETA in Viewshed creation

curtvprice — Fri, 13 Sep 2013 04:16:23 GMT

The linear measurements for the BNG projection are in metres but the vertical measurements are (apparently) in degrees. What does this mean for my OFFSETA value?

OFFSETA is a numeric field so it must simply be a number. It should be in the xy units of the coordinate system you are working in. If your vertical coordinates are different, (say, if your xy were meters and your z values were in feet) you's use a z-factor, and still use your xy units for offsets, etc.

I, like Dan, am confused as to your reference of vertical units in degrees in BNG.

Re: Determining correct format for OFFSETA in Viewshed creation

KrissyMoore — Fri, 13 Sep 2013 08:21:57 GMT

OK, thanks - so just "20" in OFFSETA will mean 20m.

In terms of the degree thing, I got it from Layer Properties but I wasn't sure if it was relevant or not (so, Angular unit =/= Vertical unit). Sorry, still new to the program!

[ATTACH=CONFIG]27434[/ATTACH]

Speaking of which - how do I then use this viewshed to create a fuzzy viewshed? I've tried to follow the explanations on this forum but I'm a bit stumped as to which rasters I need to use.

I have:

750prj = A DEM Raster of the terrain for my study area
10km = A circular polygon representing the area within which I want to investigate visibility
ChurchTower = A point 20 m high from which I want to estimate the viewshed of "750prj" within "10km"
ChurchView = A binary viewshed generated from "ChurchTower" + "750prj"

I know the next step is to create a Distance Decay Buffer using the Euclidean Distance Function in Spatial Analyst but I'm not sure what files to use at what stages. I've read through the explanations online but I can't really get my head around it. When I use 750prj or ChurchView as the source raster, the output raster is entirely black, and when I use the ChurchTower point as the input I get an error and it won't generate at all.

Is there some way I should turn the ChurchTower point into a raster?

The ultimate goal of this is to create a fuzzy viewshed which represents visibility fading with distance from the centre point.

Thanks again for all your help.

Re: Determining correct format for OFFSETA in Viewshed creation

curtvprice — Fri, 13 Sep 2013 13:49:01 GMT

When I use 750prj or ChurchView as the source raster, the output raster is entirely black, and when I use the ChurchTower point as the input I get an error and it won't generate at all.

Your source for the Euclidean Distance tool should be your Church Tower points. You should be able to provide a point input and it will project and rasterize on the fly. However, this will only work if you've set the environment output coordinate system, extent and cell size to match your viewshed raster.