Tutorial 17, Part 2: Desert city, Aligning Data and Terrain

Don't Expect Perfection!

The moral of this story? Don't expect perfection. Not even close.

It would seem aligning graphs, objects and terrain to each other is not an exact science, as we discovered in our experiments at Waubonsie State Park in Iowa. We saw where the resolution of roadway data, determined by the number of nodes through which the geometry can be articulated, did not match up well with elevation data on smaller scales. So I banged my head against Tutorial 17 at least half a dozen times to make sure I understood what ESRI was trying to tell me about this subject.

There are three operations we can use to line up our data. Tutorial 17 instructs us to apply them in this order:

  • Graph Menu > Align Graph To Terrain: the alignment function applies to the nodes of a graph, in this case the roadway and power line geometry.
  • Shapes Menu > Align Shapes To Terrain: ESRI documentation says this tool “aligns shapes to arbitrary terrains (map layers with attribute ‘elevation’ defined)”. Remember that word “arbitrary”.
  • Layer Menu > Align Terrain To Shapes: aligns the vertices of the terrain surface to whatever shapes are selected.

Here are the results from the steps we followed:

Look over the offsets that were used each time we performed an alignment. Think about why streets, building footprints, trees, power lines and things like streetlight poles would be offset plus-or-minus from the terrain.  The Wall graph was projected twice; I was a bit confused and didn't do the second time.

Now, about that word “arbitrary”.

We've all seen visualizations where roadways, buildings and etc. appear on aerial photos draped over terrain surfaces. This tutorial tells me this is not the product we should expect after performing the alignments. ESRI documentation defines the terrain layer as "a special map layer that visualizes the elevation of the scene topography using image data. It also serves as a reference elevation (my emphasis) for align operations for scene objects such as shapes or graph nodes”.

Translation: the terrain data we’re using for these operations will not appear in your final visualization.

All of the alignments can be performed with the elevation layer turned off, and maybe that’s the way we should think of it. There may always be spots where there will be gross mismatches between data and terrain, no matter what you do. You can address and remedy problems on individual components by clicking them and using the Move tool (keyboard shortcut W).

We are looking for a best fit, not a perfect fit. Don't sweat it.

Leave a Reply

Your email address will not be published. Required fields are marked *