topic Viewshed with elevation of Geographic Coordinate System in ArcGIS Spatial Analyst Questions

Viewshed with elevation of Geographic Coordinate System

TubaKumbara — Wed, 21 Apr 2010 19:56:25 GMT

Hi,
I have a problem . I am trying to perform viewshed analysis with GCS elevation data. My elevation data includes all world. The elevation values are all in meters. And I have observers shapefile with the same coordinate system of elevation layer. But I want to restrict the area in which the viewshed will be performed. In observers attribute table I added the fields RADIUS1 and RADIUS2 but I don't know exactly what value of RADIUS2 can be given. Because the coordinate system is in lat and lon . Whatever I give, always the result layer includes only 0s (NOT VISIBLE). But when I clip the elevation data and I omit the RADIUS fields, the result layer is OK.
Must linear unit take place in elevation data?If so this means that the elevation data must be in Projected Coordinate System when working with RADIUS values.
Could you please help me.Am I doing something wrong? Is there anything I must take into consideration?

Viewshed with elevation of Geographic Coordinate System

EricRice — Fri, 23 Apr 2010 18:35:52 GMT

Hi,

By default, the limiting distances RADIUS1 and RADIUS2 are interpreted as three-dimensional line-of-sight distances. To ensure that the slope distance is calculated correctly, both the ground units and surface ZUNITS must be in the same unit of measure. Your Z units differ from your ground units. Sometimes this is manageable with the z factor parameter, but your data is for the whole world so one z factor is not suitable at that scale. To obtain optimal results I recommend using an equidistant projection. Something like World Azimuthal Equidistant should do just fine. Project your points as well. The distance you specify in RADIUS 2 will have the units of the projection of the points.

Viewshed with elevation of Geographic Coordinate System

TubaKumbara — Mon, 26 Apr 2010 10:07:53 GMT

Hi,
I tried to project my all data to perform viewshed analysis. But I couldn't decide which projection exactly to use.There is no projection named World Azimuthal Equidistant. Instead there are North/South Pole Azimuthal Equidistant and World Equidistant Cylindrical/Conic. Which do you think gives best results? I tried Equidistant Cylindrical projection but the result elevation data doesn't show the elevation values in ArcGlobe. May the elevation data in the layer be lost?

Re: Viewshed with elevation of Geographic Coordinate System

EricRice — Mon, 03 May 2010 20:36:32 GMT

World Equidistant Cylindrical/Conic should be just fine. The message here is that you want an equidistant projection. Since your data is global you should avoid the polor projections.

Re: Viewshed with elevation of Geographic Coordinate System

RachaelHickling — Thu, 28 Apr 2011 11:55:48 GMT

I'm having exactly the same problem. When I leave out RADIUS2, the viewshed calculated with no problems. When I include it, I get a viewshed entirely comprised of zero values.

Was this problem ever resolved?

Rach.