Where did my Skyline go?

11-24-2010 04:20 AM
by Anonymous User
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The Skyline tool is a new tool for 3D Analysis in ArcGIS 10. Many of you may have seen this tool demo’d at the User Conference in July; don’t worry if you missed it though, the demo is still available. Watch the “ArcGIS is 3D Capable” (section 14) video available at the following link: http://video.esri.com/watch/43/arcgis-is-3d-capable. (The portion of the video that is specific to the Skyline tool and model begins at the 6:13 mark.)

The Skyline tool will create a feature that displays a line or multipatch feature class containing the results from a skyline silhouette analysis. The output can be combined with the Skyline Barrier Tool to perform shadow analysis, flight path analysis or can be used to evaluate how the skyline view changes with prospective building construction.

I want to take a moment to point out that there is a line in the Web help documentation that many miss that needs to be emphasized. The line appears in the first paragraph and states, “the analysis is conducted from observer points above a functional or virtual surface.” For the tool to work effectively, the observer points must be above the surface used in the optional inputs or not contained by the multipatch features in the ‘input features’ dialog box. When the point is below or contained, the output is not created successfully, therefore resulting in an empty feature, and no one likes an empty skyline feature.

So, as a workaround, if you are trying to create points that will model a specific point, you need to adjust the elevation of the observer points to be above the surface, especially if you are interpolating the location of the observer. Once you have determined the elevation of the observer point, be sure that it is not the same elevation as the surface.

If you are unfamiliar with how to increase the elevation of the 3D points you have created and do not have the elevation in the attribute table, consider the following workflow.

  1. Use the Add XY Coordinates tool to generate the X, Y and Z coordinates for each point. (Yes, I know the name of the tool says XY, but if the point is 3D it also includes the Z coordinate.)
  2. Add a field (double or float to preserve the decimal).
  3. Right-click the field, and use the Field Calculator to add to the elevation value (ultimately, it does not matter what value you provide, it just has to be above the surface).
  4. Now use the Copy Features tool and under Environments > Z Values set ‘Output has Z Values’ to ‘disabled’ prior to running the tool to make the feature 2D.
  5. Use the Feature to 3D by Attribute tool and select the new field for the elevation to convert it back to 3D at the new elevation.

Now the new observer point should be able to create your skyline normally. So to recap, be sure your observer point is above your surface or not contained by your multipatch, or else your Skyline analysis will be empty and you will still be asking, “Where did my Skyline go?”

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Jeff Swain, Esri Support Analyst - Raster group, Esri Support Services