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GIS I – Digital Mapping
Activity 5 – Z-Surface Interpolation
In this activity we will work with data that was extracted from the EPA-TRI for the year of 2005 for Cook
County – this should provide useful context as you continue to work with your own TRI dataset. We’ll be
exploring some of the methods available to us for Z-Surface interpolation in ArcGIS Pro.
Interpolation is a process where we calculate a new raster layer that is an estimate of Z-values derived
from a discrete set of known data points. In mapping, the Z-dimension is the 3rd dimension – the most
common form of this is seen in terrain mapping, where X and Y represent horizontal positions, normally
using longitude and latitude, and Z represents the elevation of the land surface above mean sea level. It
has become common to see many different kinds of phenomena (things) represented in this way. For
example in the physical world air temperature, barometric pressure, and rainfall (all measured at nonrandom locations) are commonly mapped as Z-surfaces that are derived through interpolation. In the
social world many things are represented in the same way, for example population, crime, disease, and
pollution. These too are all measured or sampled at non-random locations.
In GIS this becomes a special problem that normally contains procedures for weighting the new data
based on the measured values of a particular attribute of the known points in reference to Tobler’s
Law. The interpolation process produces a raster Z-surface that represents an estimate over a particular
area. The following help sections are required reading from the ArcGIS Pro help files:
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Scenes
An overview of the 3D Analyst toolbox
How IDW works
IDW (3D Analyst)
Comparing interpolation methods
Mask (Environment setting)
You will also find very useful information on these techniques in the text, in particular Chapter
12. You’ll be working with a geodatabase from a zip compressed folder found in Content > Activities >
Activity 5 Data. The geodatabase inside this zip folder contains two layers:
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Cook_tri_2005 (Toxic Release Inventory point data for Cook County, Illinois – 2005)
tgr17031county (a detailed boundary polygon for Cook County)
Save the zip-folder to your desktop – then extract the content to a secure location and work from this
folder. You can either “extract” or just drag and drop the enclosed geodatabase (gdb) folder to your
desired location. We are at the point in the term where you should be able to solve minor problems
on your own.
Z-Surface Interpolation and Visualization
Start a new Map and open the two feature classes from the geodatabase. Interpolation tools can be found
by searching Tools for IDW. Interpolate a z-surface for the attribute TOTAL_ON_S (this is the total on-
site releases from Cook County facilities for the year 2005). In the Environments tab set the Mask and
the Processing Extent to tgr17031county.
Inverse Distance Weighted is a true value
interpolation, i.e. the height of the surface will be
exactly the reported value at the measured data
points – the TRI facilities for 2005. The estimated
values in the raster layer are based on the reported
TRI values, weighted by their distance from the
raster cell being calculated. The further from the
cell being estimated each measured value is the less
it will influence the estimated value. Since we are
using the closest 12 measured values to each cell we
are calculating, each calculation of a grid cell will
be a long and messy fraction. There are several
other interpolation methods including Natural
Neighbor, Spline, and Krige although not all of
these report or estimate values for the surface in the
range of the reported values for the TRI.
When you’ve completed and saved the new
interpolated raster layer make a powerful
visualization layout of Cook County that contains
all necessary marginal information and items
(formatted at 24″ x 36″ – 600 dpi – Geopdf) and place it in the Activity 5
Submissions folder in D2L to complete the activity. There will be no extensions to this
activity.
This is due Friday, November 10 at 6pm CDT.
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