In ArcMap, turn on the Spatial Analyst and 3D analyst extensions (View/Toolbars). You will see a toolbar appear but if you click on the pull-down menu you will see nothing is highlighted. You must enable the extensions (Tools/Extensions).

Last week we worked with a raster elevation format, the DEM. Rasters aren't the only way to store elevation data. The triangulated irregular network (TIN) is a way to store elevation data (or other surface data) as a set of triangles derived from irregularly spaced sample points. Sound odd? Let's take a look.

In ArcMap, add the DEM of part of Hopewell Township (\\ad-rsc\data\teach\intgeo\rowan\arc8class\hopewell\penndem_sp). To create a TIN from a raster data set containing elevation, activate the 3d Analyst/Convert/Raster to Tin command. Be sure you're saving the TIN to your directory. Once the TIN is created, zoom in to the hilly area in the northwest part. Notice how the surface is composed of triangles. The TIN display defaults to hillshaded in order to give you an idea of the 3D-ness, but it's not really 3D, is it? Don't you feel kind of cheated? Let's fix that.

To get to something approaching real 3D, we need to launch ArcScene, ArcGIS's 3D scene viewer. To do this, click on the ArcScene icon to the right of the 3D Analyst toolbar. The ArcScene viewer looks a lot like the ArcMap interface, so you should feel relatively comfortable with the controls. Copy the TIN in ArcMap and paste the layer into ArcScene. Now that looks a bit more like it, huh? If New Jersey wasn't so flat it would look even better. Luckily we can exaggerate the vertical relief (View/Scene Properties/General). Try a vertical exaggeration of 10.

Let's see what we can do with the controls. The pan and zoom functions work similar to those in ArcMap. Since we're in 3D, we can also rotate the scene using the navigate control. Go slow, you might get dizzy. One of the super nifty neato things we can do is fly-through the scene using the fly-through control. Once this control is actviated the position of the mouse determines the direction the camera moves, left mouse button increases the fly-through speed and the right mouse button decreases the speed.

Now that you've had enough pretending you're a bird, let's try displaying something more exciting than just the TIN. Add an aerial photo (\\ad-rsc\data\teach\intgeo\rowan\arc8class\hopewell\771.jpg) to the scene. Hmm, that's not what we wanted. We need to tell ArcScene how to drape the photo over the TIN. Under Layer Properties/Base Heights, check the box that tells ArcScene to use the TIN as the data layer to obtain the base heights from. Turn off the TIN and explore the draped aerial photo a bit.

We can also drape thematic data over the TIN. Under Scene Properties/Extent, set the extent of the scene to be equal to the extent of the aerial photo. This means that only data overlapping the aerial photo will displayed (theoretically). Now add \\ad-rsc\data\teach\intgeo\rowan\arc8class\hopewell\pennlulc to the scene. Set the symbology to use the field "level1", the transparency to 60%, and tell ArcScene what layer to use for the base heights. As you can see, ArcScene isn't perfect, but given the right circumstance you may find it useful.

Some other applications of ArcScene you should be aware of but we won't demo:

• ArcScene can display data polygonal data such as building footprints in 3D if they have appropriate height data.
• ArcScene can save fly-throughs in movie-formats for viewing outside of ArcGIS.

Viewshed Analysis -

Ok, so flying through the landscape is cool and all, but what about real 3-D analysis? Let's go back to ArcMap. In 3-D Analyst, we can create topographical profiles of lines, draw lines of sight from points, and find the steepest path downhill from a specific point. One of the more widely used 3-D analyses is the creation of viewsheds. A viewshed is simply the land area visible from any given point. You can perform viewshed analysis in either 3-D Analyst or Spatial Analyst. We need two data layers in order perform a viewshed analysis, a TIN or raster representing elevation and a point or line layer representing the vantage points we want to determine the viewshed for. We'll use the TIN we created earlier as our elevation data. The vantage points are represented in \\ad-rsc\data\teach\intgeo\rowan\arc8class\hopewell\viewpoints. They are prospective overlooks on a trail along the ridge in the north of the DEM. Create the viewshed (Surface Analysis/Viewshed under the 3-D or Spatial Analyst menu), accepting the default cell size. Save the viewshed to your directory. The output is a raster data layer, automatically symbolized to show what is visible and not visible from the points. Open the attribute table for the viewshed and you'll see it's a little more complicated than that. Cells with a value of 0 are not visible from either point, cells with a value of 1 are visible from one point and those with a value of 2 are visible from both.

Assignment -

Make a map using the viewshed results and the landcover found at \\ad-rsc\data\teach\intgeo\rowan\arc8class\pennlulc.shp. Your map should emphasize the Level 1 land use categories visible from either point. Although you can't tell precisely which areas are viewable from which point, you can make a general guess. Explain on your map which point should be the overlook point if the goal is to emphasize the rural nature of the area.

Assignment due Monday, April 7th.