Geography 241
Lab Activity 4 - Z-Surface Interpolation, 3D Analyst, and ArcScene

In this lab we'll work with a shapefile that contains data describing the elementary schools in Chicago, circa 1997.   We'll be exploring some of the methods available to us for Z-Surface interpolation in ArcGIS.  Interpolation is a process in mathematics where we estimate or calculate new data points from a discrete set of known data points.  In GIS this becomes a special problem that normally contains procedures for weighting the new data based on the assumed strength of the known points in reference to Tobler's Law.  The following sections are required reading from the ArcGIS help files:

• 3D Analyst
     An Overview of 3D Analyst
     About 3D Surfaces
     vertical exaggeration for 3D documents
• Spatial Analyst
   Understanding interpolation analysis
  An Overview of Spatial Analyst
• Inverse Distance Weighted
      Implementing Inverse Distance Weighted (IDW)

You will also find very useful information on these techniques in the text, in particular Chapter 12.  You'll be working with the data from this compressed file.  The folder has two Shapefiles

• chi_mun_ll (a polygon of the City of Chicago, using lat-long)
• elem 1996 (a set of Chicago elementary schools in 1997 with several attributes)

I want you to always explore the software... try to push its limits.  Remember, when in doubt... take a chance and explore.  We are at the point in the term where you should be able to solve minor problems on your own.  If you're having a major problem, call in an expert - as always on the lab activities, it is ok to work with a partner..

Here's how to get started...

Open ArcMap and open the two shapefiles.

Use either the 3D Analyst or Spatial Analyst extension to interpolate a z-surface for one of the following attributes from the elem 1996 shapefile:

• LIMITED_EN (percent students with limited English language skills)
• LOW_INCOME (percent students low income)

You should use the Inverse Distance Weighted method (IDW) and try some different settings for the Power, Number of Points, and Cell Size.  This is a true value interpolation, i.e. the surface should be exactly the value at the point of interpolation.  There are several other interpolation methods, including Natural Neighbor, Spline, and Krige.  It might be fun to experiment with these and compare to IDW.  Here is some info about IDW interpolation:

IDW estimates cell values by averaging the values of sample data points in the vicinity of each cell. The closer a point is to the center of the cell being estimated, the more influence, or weight, it has in the averaging process. This method assumes that the variable being mapped decreases in influence with distance from its sampled location. For example, when interpolating a surface of consumer purchasing power for a retail site analysis, the purchasing power of a more distant location will have less influence because people are more likely to shop closer to home.

With IDW you can control the significance of known points on the interpolated values based on their distance from the output point. By defining a high power, more emphasis is placed on the nearest points, and the resulting surface will have more detail (be less smooth). Specifying a lower power will give more influence to the points that are farther away, resulting in a smoother surface. A power of 2 is most commonly used and is the default.

The characteristics of the interpolated surface can also be controlled by applying a search radius (fixed or variable), which limits the number of input points that can be used for calculating each interpolated cell.

A fixed search radius requires a distance and a minimum number of points. The distance dictates the radius of the circle of the neighborhood (in map units). The distance of the radius is constant so, for each interpolated cell, the radius of the circle used to find input points is the same. The minimum number of points indicates the minimum number of measured points to use within the neighborhood. All the measured points that fall within the radius will be used in the calculation of each interpolated cell. When there are fewer measured points in the neighborhood than the specified minimum, the search radius will increase until it can encompass the minimum number of points. The specified fixed search radius will be used for each interpolated cell (cell center) in the study area; thus, if your measured points are not spread out equally (which they rarely are), then there are likely to be a different number of measured points used in the different neighborhoods for the various predictions.

With a variable search radius, the number of points used in calculating the value of the interpolated cell is specified, which makes the radius distance vary for each interpolated cell, depending on how far it has to search around each interpolated cell to reach the specified number of input points. Thus, some neighborhoods can be small and others can be large, depending on the density of the measured points near the interpolated cell. You can also specify a maximum distance (in map units) that the search radius cannot exceed. If the radius for a particular neighborhood reaches the maximum distance before obtaining the specified number of points, the prediction for that location will be performed on the number of measured points within the maximum distance.

Here are some handy hints:

• Use a smaller cell size for a higher resolution surface..  It's cool..
• Use the 3D Analyst or Spatial Analyst - Options dialog to set the analysis mask to the chi_mun_ll shapefile.  This will "clip" your surface at the city boundary..
• Set the "extent" in the "Options" dialog to the chi_mun_ll shapefile.
• After creating your surface, you can right-click on the raster layer in the table of contents,  and "Make permanent.." to save it to a permanent location.  This creates a permanent ArcGrid file (raster data model) from the temporary file you just created.  Make sure you know where this is going (i.e. flashdrive or desktop).  Do this..

Now, here's the fun part.  After you've made it permanent, open an ArcScene window with the 3D Analyst toolbar .  This presents the 3D modeling interface for your pleasure.  Open the z-surface that you just made permanent above.  Use the Spatial Analyst tools in ArcScene to make a 3D model of your z-surface.  Play...  Here are some hints:

• When in ArcScene you have to set several parameters to make your model work.  These include settings in the layer properties under the "base heights" tab, the "rendering" tab (make it shaded), and the "symbology" tab (make it look cool with color..).
• You also must set the data frame properties under the "general" tab (vertical exaggeration? calculate from extent then play with it).  You can also set some illumination parameters here.
• Use the Navigate tool to rotate and manipulate your 3-D model in ArcScene.
• Fly the model in ArcScene with the Fly tool . Left-click will move you forward and speed up, right-click slows you down and "Esc" stops the bird..

Here are some hints that will help you create a stunning map at this point:

• You can add the schools layer as points and they can be symbolically manipulated as a thematic layer here.  Try it out.  You can set the "base height" of these symbols to either match that of the continuous Z-surface or to respond to a calculated expression from the available attributes and layers.  These two symbolic controls can be used separately or in tandem.  Experiment here.
• Try using a color scheme on your point layer that stands out from the color scheme of the surface.
• You can add text by creating a "new graphics layer" using the 3D Graphics toolbar.  Careful with this, it's a little squirrelly...
• You can export your "Scene" using any of a number of file formats.  Use the 2D selection on the file - export dialogue. A Tiff (biggest, most detail) works well or a JPEG is probably the most widely readable.  It can then be opened (actually inserted as a picture) in a layout window in ArcMap.  This allows you to use the layout tools of ArcMap on your Scene.
• You will likely want to create your final map in the ArcMap Layout interface using a composite of planimetric (2D) maps and orthographic (3D) views.

When you are completely stunned by what you've created, lay it out as a beautiful D-size map in ArcMap.  Write a short 1-2 page description that details exactly how you did this and turn that in with the map, that you print on the Designjet.  Turn in your map to complete the activity.  This activity is due Monday November 16 at the start of class.  Cheers.