FOR557 – 04
Due 11/23
Erosion Potential
Exercise 7c
Overview
The town of Martinsburg Environmental Quality committee has noticed that there is obvious
erosion taking place on some of the agricultural and residential parcel lands. Your job is to find
these parcels and produce a summary Letter to the committee with a map or maps showing the
parcels that have any areas with the steep slopes.
This exercise is also an introduction to Grid processing. The first part holds your hand while the
second part is on your own. For the first part you have to
 Create metadata for the Martinsburg DEM
 Create a 10’ contour map just so you can say you did it
 Computer the slope in percent of each pixel in the DEM
 Smooth the slope grid
 Reclassify the grid into Slope Classes
 Convert it to polys of slope class.
What gets turned in


A letter summarizing your results
A map showing the parcels that have steep slopes that should be inspected for
erosion.
Grading
Right answer (table) 20pts
Map 1
20pts
Map properties
Name and date
Border or fill or some other way to tie map components together
Symbolization
Legend content
Legend quality
Composition (balance, white space, etc.)

The Assignment
1) Open ArcCatalog
a) Make a copy of the grid dem_lewismart and call it martdem
b) From the toolbox
i) Project martdem to match the other data in the Martinsburg GDB. There is NO
define projection for rasters. Use Project under Rasters in the Toolbox. In the Project
dialog use import to set the projection the same as that for the other Martinsburg data.
ii) Build pyramids for martdem (why I don’t know, but ArcGIS usually wants to do
that).
2) Open ArcMap
a) Drag martdem and the tax parcel shape to a new, empty map in ArcMap
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FOR557 – 04
Due 11/23
Erosion Potential
Exercise 7c
b) In order to process grids you need to activate Spatial Analyst
(SA). SA has to be turned on in two (2) places. If the SA
Tool Bar is not present then you have to turn the toolbar on
by Right clicking in a empty space in the Tool Bar area
(usually found at the right end of the toolbar area- where the
star is in the image to the right). In the dropdown list that
appears select Spatial Analyst. This will put the SA tool bar
into the header area.
If the SA tool bar is present but the Task area is grayed out then you have to activate the
toolbar by selecting Extensions/Spatial Analyst from the Tool menu.
c) Martdem should be the only layer that appears in the layer window of SA
d) Use some of the tools found in the SA dropdown menu under Surface Analysis to
compute…
i) 10 foot contours
ii) Calculate a slope in percent grid. Select the slope function and in the dialog make
sure the percent radio button is clicked on. Then make sure that you browse for the
location where you want the percent slope grid stored and give it a name that makes
sense to you. Click OK.
iii) The resulting grid has lots of different values for neighboring pixels. Investigate this
using the “i” tool. This is not good since when you make the grid into features you
will get thousands of polys with slightly different values. Since slope calculations are
not that accurate anyway you will do a couple of operations to smooth things out. A)
Smooth the slope grid using a 5x5 moving averaging window over the surface – this
is like sanding a rough surface (details below). B) Symbolize the grid to find a
symbolization that further reduces the amount of “frass” (the number of scattered
single pixels with different values visible in the map – like some insect left some
presents in your grid)
iv) Smoothing: Select Neighborhood Statistics from the SA dropdown
(1) Make sure the input is your slope grid, the field is <value>, the statistic is Mean,
and that the Neighborhood is Rectangle.
(2) In Neighborhood settings make the window height and width both 5
(3) Cell output size = 10
(4) Make sure you tell the system where to store the output. If you don’t it will try to
store it some odd place and you don’t need that aggravation.
(5) Click OK
v) Symbology: Go the Layer properties and the Symbology tab. Use manual
classification to display slope classes of 0-5, 5 -15, 15-20, 20-30, and 30-“rounded up
value of maximum value” Zoom in a observe the grid in the western part of the town
to see what the grid looks like. Notice that there are still lots of neighboring pixels
that have different values (use the “i” tool) and if we were to convert this to features
(polys) there will be thousands of them. One of the reasons I picked 0-10% is that
that range greatly reduced the number of neighboring pixels that are different colors.
Too much frass! Change the first two classes to 0-10, 10-15. Note: changing the
symbology DOES NOT CHANGE THE DATA in any way and you need to
change the data into the above classes for a planning problem.
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FOR557 – 04
Due 11/23
Erosion Potential
Exercise 7c
vi) Too make a feature class of slope classes you first need to reclass the data in the
smoothed slope grid into classes of slope – so the data is actually in integer numbers 1
to 5 that represent slope classes. Here’s how.
(1) Select Reclassify from the SA dropdown menu.
(2) The dialog should open with the same “New Class” values that you used to
symbolize the smoothed slope image.
(a)
The input raster should be SlopePercent (or whatever you called it) and the
reclass field = <value>
(b)
Set the Output raster to SlopeClass or some name that makes sense to you.
(i) Make sure the file is going to be stored where you want it stored
(ii) Click OK
(c)
A new grid layer is added in which the raster values are slope class and not
slope percent! Investigate using the “i” tool. Change its symbolization to
something that is easily understood. Usually green – yellow – red works well
since the lower classes are ok for development while the high values (4 & 5)
are not.
e) Now you are ready to convert the grid to a feature class.
i) From the SA dropdown menu select Convert/Raster to Features
ii) In the dialog that opens make sure the correct grid is listed in the input raster, that the
Generalize Lines check box is checked, that the output geometry type is polygone,
and that the resulting feature class will be stored where you want it stored.
iii) Click OK
3) OK, now you are ready to find the parcels that have a landuse category of residential or
Agriculture and contain any area that is in slope class 4 or above.
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