Introductory Geoprocessing With the Model Builder
E x e r c is e 9 C b an d 9 b: S lo pe an d Er o s io n P o t e n t ia l
Copyright L. Herrington & A. Badruddin, Syracuse 2011
In this exercise you will use the DEM to find tax parcels that are possible sources of
sedimentation of the town’s rivers. This exercise is a combination of raster and vector
processing intended to show you how you can have both raster and vector data work to solve a
problem. It is also an exercise for using a number of the raster tools like Filter, the Slope, Raster
to Polygon conversion, Reclassify, etc.
Setup
1) Open ArcMap.
2) In C:\Workspace create a folder called Sedi and in that folder create a file GeoDatabase
called SediScratch
3) In the Geoprocessing menu (main toolbar) establish the Current GeoDatabase as Marti and
the Scratch GeoDatabase as SediScratch.
4) Load the following Feature Class and Raster layers from Marti into Map:
a) Feature Classes River and town Outline
b) dem_lewismart
c) Open dem_lewismart’s properties dialog.
d) Select the Display Tab and check the top three check boxes. Look at the rest of the stuff
in the dialog.
Q11 – What does “Nearest Neighbor” mean?
Q2 - Is the DEM “discrete data”?
Q3 – Are the elevations in feet or meters._______ How do you know that?
In Explorer search for Podunk’s Profile for Martinsburg, NY. You will find the
elevation of Martinsburg there. You can figure out whether the dem is in feet or meters.
Where is Martinsburg? 462,034, 4,842,348 approximately.
e) Select the symbology Tab.
 The show window is probably set to Stretched, if not select Stretched.
 Select various Color Ramps and determine which you like best. Left clicking in the
ramp window allows you to toggle the Graphics View (ramp’s colors or name). With
graphics view on you can select one of ESRI’s elevation ramps.
f) Change the Show window to Classified.
g) Set the number of classes to 25 and pick a ramp you like. What you want is to be able to
see clearly what the structure of the DEM is. Sometimes a Qualities ramp works better
than a Quantitave ramp. Classifying a raster to 25 classes make for a large Table of
Contents!
h) Click the Classify button to see what the histogram of the data looks like.
 The Classification Method is probably set to Natural Breaks. This is not too good for
what you are doing since you are not interested in Natural Breaks!
 Check that the number of classes is still set to 25.
 Set Method to Equal Interval and note how the divisions change.
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Questions are to be answered in the Results page at the end of this document
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Exercise 9b
 Click OK to get out of the classify dialog.
5) Click Apply so you can see how the DEM looks. If you don’t like what you see try different
settings.
6) Note that the statistics for the DEM are available in the Classification dialog
7) When done click OK
Data Preparation
Usually there will be several things you have to do to a project’s data to get ready for
Processing. Since you are going to be processing raster data for slope etc. you will generally
have to smooth the DEM to remove errors generated in the creation of the data. Smoothing is
done by a low pass filter using the Filter tool (Spatial Analyst.
Smoothing
8) Quite often when we are going to do operations (like calculating slope) DEMs will have to
be smoothed to remove banding and increase the autocorrelation between rows of data.
But remember that smoothing (low pass filtering) changes the elevation data so you will
want to be sure about what has been done to the DEM. Smoothing with a 3x3 kernel that is
passed over the DEM effectively changes the resolution of the data as was discussed earlier
in the course.
9) What is the effect of smoothing the DEM on computed slope %?
a) To smooth the DEM use the Filter tool as a low pass filter.
b) Build a model called Smoothing1
 Drag or copy the tools Filter and Slope into your Toolbox2
 Drag Filter into your model.
(i) The input raster is dem_lewismart .
(ii) Filter type should be "low" for a low-pass smoothing filter.
(iii)
Set the output raster name to something like Filter_Dem1 (raster name
MUST be <=13 characters)
 Run the model and look at the resulting raster..
(i) Compare its stats with those of the original.
Q4: Are they significantly different? What is a significant difference?
Add
the Slope tool and run the model again. Be sure to set Output measurement to

PERCENT_RISE!
 Add the result to the Table of Contents and
take a look at it. Set a number of classes
and pallet that makes sense to you.
 Add a second Slope tool and use the
original DEM as its input (see figure 7b.1. )
 Run the model
10)Now symbolize the slope rasters the same way
and compare them.
Q5 Are they different
Figure 7b.1. Model Smoothing1.
2
By now you should be able to find these tools!
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Exercise 9b
11)Too see exactly what the difference is use the Minus tool to subtract the smoothed DEM
from the original.
12)The Result looks more or less like a random surface! Until you zoom in then things look a
little weird (Figure 7b.2).
13)Go to the symbolization dialog for the Result and click on
Classified.
a) Click on the Classified button (upper right) and look at the
histogram. The distribution looks almost normal (in a
statistical sense). The Classifications say that the range
in values is from ~-44 to ~+53. The mean is 0.05 and the
SD is ~1.7
14)The statistics are more or less those of a random surface but
visually the layer shows some strange banding.
15)Usually more than one Filter pass is needed to smooth things
Figure 7b.2. Display of the
out. So modify the Model so that the DEM is filtered three
raster difference between the
times (Figure 7b.3).
two slope rasters.
Figure 7b.3. Model with three filter passes
16)Turn on the results for the 3rd filter layer (DEMfilt3 in
our model). Most of the weird banding is gone and it
looks like the steep slopes are in the drainage gulleys in
the escarpment (See Figure 7b.4.)
END OF EXERCISE EXC9B
Turn in answer sheet,
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Figure 7b.4. DEMfilt3 with
Rivers. Red is >= 15%.
Exercise 9b
The Problem (EX9b due at next class)
These instructions do not hold your hand as much as the first part did. You have to think a bit :-(
You are to build a model or models to do this analysis. NOTE: there have been significant
changes to this part since the paper version was distributed!
SEE PAGE
17)You are charged with finding Agricultural and Residential tax parcels that contain steep
slopes (>= 15%) that could be causing erosion and thus siltation of the town's streams. The
requirements are as follows:
a) The areas of Slope >= 15% must be of a minimum size of 1/2 acre.
 They must be within 50 meters of rivers.
 Since Forested areas are usually not sources of sediment the areas isolated for a)
must have areas of forest cover removed.
b) Only Agricultural and Residential parcels (CLSs between and including 100 and 200)
are of interest.
18) You have been charged by the Environmental Management Council of the town to
prepare two maps.
a) A Sedimentation Area map showing the areas of possible sedimentation sources over a
hill shade (look it up) of elevation parcels. Parcels, Rivers and Streets should also be
visible and the parcels should be symbolized as the landuse (CLS). This map is for the
general public and the town officials.
b) A Sedimentation Investigation map showing parcels that intersect, in the Select by
Location and Select Layer by Location Tool sense, the 50 meter buffer around rivers.
The illustrated sedimentation sources should be a minimum size of ½ acre only.
c) There is a bonus if you actually compute the area per parcel of sedimentation sources.
Note:
In the ESRI world the process of Intersection has two meanings. The word in the main menu
Select by Location tool means to just select items that intersect. In the Toolbox world the
Intersect Tool actually does a selection and a Clip as is shown in the tool’s help panel.
19)Thus you need to integrate the two needs
a) Simply identify areas of 1/2 acre or more that contain slopes greater than or equal to
15% that are within 50 meters of a river. The survey people could use maps or GPS
units to find these areas.
b) Identify parcels that contain area(s) of slope >= 15%. That way a list of addresses and
homeowners can be used to guide the inspectors.
Method a) would be easier for you but b) would be easier for the inspectors. Since
method a) would be a precursor to method b) you are tasked to report both methods
via a map for a) and map and table for b).
Making the slope image useful
20)Symbolize the slope layer so that it is red and green. Red for areas that have slopes >= 15%
green for lower slopes, so you can see what you are doing (Figure 7b.4).
21) Use RECLASSIFY (SPATIAL ANALYST TOOLS/Reclass/Reclassify) to actually change the values of
the red and green slope raster. Since this procedure changes the data anyone with ½ a brain
will make a copy before proceeding!
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Exercise 9b
a) Reclassify will happily use the Classification you made on the slope image to do the
Reclassification (note that Reclassify and Classify are different ideas). Thus when you
input your slope raster it should open to something like that shown in Figure 7b.5.
Slope_DEMfil1 is the name of my slope raster.
b) The reclass field Value is the default name of the slope field in the raster but it could be
anything. The Old values are exactly that – the values that are currently in the
classification of the slope data. Since we are not interested in the areas that have slopes
<15% change the New Value field to No Data as is shown in Figure 7b.5.
22)Inspect the slope raster to be sure what is done is correct. Note that there are many small
areas of steep slope!
23) Convert the raster image to polygons (areas) using the Raster to Polygon Tool (search for
it). In this tool you have the option to simplify the resulting polygons. Run the conversion
tool both ways – simplified and not. Which result has the larger total area?
24)The next step, as was alluded to above, is to find all the tax parcels that have Agricultural or
Residential landuses AND contain slopes greater than 15% AND the areas of steep slopes
are within 50 meters of a stream. The parcels that meet these requirements can then be
inspected for erosion.
a) The easiest way to get this done is to convert the slope raster to a Feature Class of
areas with slopes >= 15%.
b) To do this the classified slope image has to be RECLASSed. Remember that
symbolization does NOT change any data values and that RECLASS does change data
values in the input layer. Make a copy of your slope raster before using RECLASS.
 Open the RECLASS tool
 A dialog opens that is very similar to that in the Symbolization dialog. Set the class
for cells that have a value >=15% to
2 and all others = to “NoData”. See
Figure 7b.5. In fact, if your slope
Grid had the red and green
symbolization like that above the
RECLASS dialog will open with the
two classes – red and green. The
result should look like that shown in
Figure 7b.6 after the tax parcels are
added to the map and the fill color
was “none”.
25)Build the Project Model
Figure 7b.5. Reclassify dialog used to produce the
a) Use the Raster to Polygon to convert the
raster shown in Figure 7b.6.
raster version of the slope areas to
polygons. Be sure to uncheck the box for
simplification (or try it both ways). Why?
It seems to me that if you start the analysis with a raster of square pixels (not all are
square) you should keep that property. If you leave the simplify box checked you will get
a lot of triangular things. And the resulting area will be less than that produced if you
uncheck the box and get only things that have the square jaggies. See the Appendix!
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Exercise 9b
b) In this model you will have to use the Select layer by Location tool and it appears that
that is a royal pain. I spend several hours
trying the use of this tool work. You are
supposed to be able to make successive
selections (using the equivalent of the
“select from the currently selected
features” Selection Option in the main
menu’s Select by Location and the s
c) Selection Type of “SUBSET_SELECTION” in
the Select Layer by Location Tool) but I
could not make it work in the tool ). The
only alternative is to use the Select Layer
by Location tool and then convert the
Figure 7b.6. Parcel layer under the reclassed
resulting layer to a Feature using the
raster layer of cells with value >= 15%
Make Feature Tool to create an ordinary
feature. Doing this for a series of selections makes for a very messy model.
d) But remember that a tool you make in the model builder is not different than a system
tool. You can put your tool into a model and it will work just like a subroutine. More on
this later in the class. However you have to arrange for a variable to passed to your
model. There are a couple of ways of doing this that I will be posting at a later date.
26)What is due and when.
a) April 4th
 A work flow for how you are going to do
(i) Part 19 a)
(ii) Part 19 b)
b) April 12th
 A final report containing
(i) A cover sheet
(ii) Your two maps
(i) Your final work flow
(ii) Your model(s) (and you might want to use several for this exercise)
27) The first part of this exercise will be an Ex in the grade book while the second part will be
an EX+ which counts 2x an Ex.
Appendix
The graphic on the left is the results of converting the slope >=15% raster to polygons with the
simplify box checked while that on the right is with the simplify box not checked. On the left
there are lots of triangular polys while on the right the polys reflect the square structure of the
raster data. Also note that there are a larger number of wee triangles than square polys. Not
sure why.
There is no tool for rounding cell values. Round up and Round down do not do the job – look at
their helps. The way to do a true round up is to add a raster 0.5s to the data raster and then
Truncate (just lop off the fractional part. With a value of a cell of 5.7 adding .5 -> 6.2 and
truncating that -> 6. With a value of a cell of 5.3 adding .5  5.8 and truncating that produces a
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Exercise 9b
value of 5. If you needed to do a lot of rounding then you could make your own Roundit Tool
and add it to a model as many times a needed.
The End
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