Today we will use ArcMap and ArcToolbox to manipulate vector-based geographic data. The results of these simple analyses will allow us to visualize complex spatial relationships. Notice that some words are italicized throughout the text. These words comprise the list of pertinent vocabulary that appears at the end of this exercise.
Learning Objectives:
• Simple overlay analysis • Basic proximity analysis • Advanced selection techniques • Data display priority
Creating Buffers
A buffer is a zone of specified distance around features in a geographic layer. Buffers can be set at constant or variable distance based on feature attributes. For instance, we could use buffers to visualize “late night noise zones” around bars in New Brunswick. A more complex buffer application might consist of creating concentric buffers of increasing distance around each bar in order to rank areas by likelihood of late night noise. Suffice it to say that creating buffers or buffering is a common and simple technique of spatial analysis
Buffering point, line, and polygon features will allow us to produce a map of wetlands that may be at risk of pollution by highway and contaminated site runoff in Readington Township, Hunterdon County. Point Data
1. Launch ArcCatalog, then navigate to the following location using the directory tree on the left side of the screen:
\\ad-rsc\data\teach\envgeolab\class_data\nj
§ Highlight the shapefile called “kcsl2005.shp.” “KCSL” stands for “Known Contaminated Sites List.” As you might expect, this point file represents sights that are known to have or are suspected of having contaminated soil or groundwater. It also contains sites that have undergone remediation and are therefore no longer contaminated.
2. Now, click the “Preview” tab above the viewing panel. Note that “kcsl2005” is a statewide layer. We need to isolate the sites within Readington Township in order to continue this lab.
3. Launch ArcToolbox from the main menu under “Geoprocessing”. Expand “Analysis Tools.”
Expand “Extract,” then double-click the “Select” tool. Click “Yes” on any error windows that
appear.
§ Choose “kcsl2005.shp” as your “Input Features” by using the file folder on the right to navigate to the shapefile or by dragging and dropping the file in the space provided.
§ Designate the “Output Feature Class” by using the file folder after the space provided to navigate to your directory in your section’s folder. Name the file “readkcsl05.shp,” then
click “Save.”
§ Now, we will perform an attribute query in a slightly novel environment. Click the “SQL” button that follows the space provided for optional “Expressions.” Choose “MUNIC” from the attribute list in the Query Builder. Add an “equal to” symbol to the query. Click “Get Unique Values” above the query window, and choose “Readington Twp” from the list. Your query should appear as follows:
"MUNIC" = 'Readington Twp'
Click “Okay” to close the Query Builder, and then click “Okay” once more to perform the
selection.
§ Close the ArcToolbox panel once the selection is complete. We will use it again within the ArcMap module, shortly. Now, navigate to your folder in your section’s directory, and preview the results of your selection. (Highlight “readkcsl05.shp,” then click the “Preview” tab above the viewing panel). The “Geography” preview reveals a far smaller subset of the statewide layer. Switch to the “Table” view. Make sure there are 45 records, and that they all belong to Readington Township.
4. Now, we are ready to buffer the contaminated sites. Open ArcMap, add “readkcsl05.shp” to your view, and launch ArcToolbox. Expand “Analysis Tools.” Then, expand “Proximity” and choose the “Buffer” tool.
§ Choose “readkcsl05.shp” as your “Input Features” by using the file folder on the right to navigate to the shapefile or by dragging the layer from the TOC and dropping it in the space provided.
§ ArcMap may decide an output file name and location for you, but that might not be the location you are expecting. Specify the location for the “Output Feature Class” by using the file folder after the space provided to navigate to your directory in your section’s folder.
Name the file “readkcslbuf.shp,” then click “Save.”
§ Now, we must decide the distance at which we will buffer the sites. Select the “Linear unit” option by clicking the radio button that precedes it. Enter “0.25” in the space provided, and
change the units of measure to “Miles.” [Note: I will know you didn’t read the lab carefully
if we cannot see your buffers. The unit of measure was most likely “Feet,” and you buffered the site by 3 inches instead of ¼ mile].
§ Set the “Dissolve Type” to “ALL.” This will merge any overlapping buffers into a single feature. Click “OK,” and watch as the resulting data is added to your view. If it looks like mitosis, you have performed these steps correctly.
§ Save the work you have done. Select “Save as” from the main “File” menu. Navigate to your folder in the class directory (\\ad-rsc\data\teach\envgeolab\yoursection#\). Use of consistent naming conventions is highly recommended. Save the file as
Lines
Stormwater runoff from heavily traveled roads contributes to degraded surface water quality. Perhaps, then, we should buffer the roads of Readington Township. We will restrict ourselves to highways since Readington is a fairly rural municipality.
1. Load “njdotrdssp” from the following location:
\\ad-rsc\data\teach\envgeolab\class_data\nj
Note that this is a statewide highway layer. We should isolate the roads that run through Readington Township for the sake of efficiency. This is a prime opportunity to introduce another tool of
extraction/overlay.
2. Data that reach beyond the extent of our interest is like an amount of dough that exceeds our need to create a single gingerbread person. Both problems are solved by the use of a “cookie cutter” to clip the excess. In the case of geographic data, a layer that represents our area of interest can serve as the “cookie cutter” or template for reducing the input data. This process is called clipping, and is actually a form of overlay analysis, though ArcGIS includes it among tools of extraction. First, we must build our template.
§ Return your attention to ArcToolbox. Expand “Analysis Tools,” then expand “Extract.”
Double-click the “Select” tool. Repeat the procedure explained in step 3 on page 2 in order
to create a “cookie cutter” that is shaped like Readington Township. Your input and output features are as follows:
• Input: \\ad-rsc\data\teach\envgeolab\class_data\nj\stmun.shp • Output:
\\ad-rsc\data\teach\envgeolab\section#\(username)\readtwp.shp • Query: You’re smart. Build it yourself!
3. Now, let’s reduce the road coverage to the extent of your newly created “readtwp.”
§ Expand “Analysis Tools,” then expand “Extract.” Double-click “Clip” in the Toolbox. The illustration on the right side of the “Clip” dialog serves as an excellent guide for this
procedure.
§ Add the “arc” feature class from “njdotrdssp” as the “Input Feature.” § Choose the newly minted “readtwp” as your “Clip Features.”
§ Designate the “Output Feature Class” by using the file folder after the space provided to navigate to your directory in the “section1” folder. Name the file “readrds.shp,” then click “Save.” Click “Okay,” and add the finished product to your view. Remove the statewide roads layer from your view.
4. Activate the Buffer tool just as we did before, and buffer the roads at a distance of 0.25 mile, remembering to dissolve all. Save this layer as “readrdsbuf.shp.”
Polygons
Wetland ecosystems are very rich, but they are also very susceptible to disturbances like polluted stormwater. Let’s locate the wetlands of Readington Township in order to determine what risk might be posed by runoff from roads and contaminated sites.
1. Add the “hunlc.shp” layer to your data frame from the following location:
\\ad-rsc\data\teach\envgeolab\class_data\hun\
This layer contains major land cover classes in Hunterdon County.
2. We are only interested in the wetlands that fall within the boundary of Readington Township. § Choose “Select By Attributes” from the “Selection” menu on the main toolbar. Set the
“Layer” to “hunlc,” and the “Method” to “Create a new selection.”
§ Choose “TYPE02” from the attribute list in the selection dialogue. Add an “equal to” symbol to the query. Click “Get Unique Values” above the query window, and choose “Wetlands” from the list. Your query should appear as follows:
"TYPE02" = 'WETLANDS'
3. Now, use the Clip tool to create a layer containing only those selected features that are within
Readington. A convenient feature of ArcGIS is that only those input features that are selected will be clipped. Carry out this operation, and save your output in your directory as “readwet”.
4. Remove the Hunterdon County land cover layer from your view. Visualizing Spatial Relationships
We have amassed all the necessary data for our map. We just need to figure out a way to make it look reasonable. We first need to ask ourselves what is the purpose of the map. We wanted to identify wetlands in Readington Township that are at risk of contamination. That means that our map should emphasize the wetlands and buffers that identify the at-risk areas. We should also show the highways and contaminated sites that were buffered to give meaning and context to the buffers. Finally, we should also include the extent of Readington Township.
We could simply draw wetlands on top of the buffers. The resulting map is, however, less
meaningful. The true proximity of the wetlands to the buffered features would be obscured by the wetlands. We can solve this problem by adjusting the transparency of our buffers, and ensuring that they draw over the wetlands.
§ Assign different colors to each buffer. Open the layer properties for one buffer layer at a time. Select the “Display” tab, and set the transparency to 40%. The higher the percentage the more transparent the layer appears. Repeat this process for the second buffer layer. Now look at your map. Make sure your buffer layers are drawn over the wetlands. Once you do this, you should be able to see the wetlands beneath the buffers. Adjust the transparency levels until you are satisfied
with the appearance. Display highways and contaminated sites over both of their buffers. The color you choose to represent them should be a darker shade of the color you chose to represent the respective buffer.
Homework Assignment
The work you did above identifies wetlands that are situated within a quarter mile of potential pollution sources. But what if we want to identify those highway segments and contaminated sites that are situated within one-eighth of a mile (0.125 miles) of wetlands? This means buffering the wetlands, rather than the highways or contaminated sites.
Using the techniques you learned in this lab (buffering, clipping, selection), create a map that shows wetlands with a one-eighth mile buffer, and that indicates which road segments and contaminated sites are within that buffer. When buffering the wetlands, buffer the outside of the wetland polygons while also including the polygon interior. (Remember: “Dissolve” by “All”). Add all the usual suspects to the map, and make sure you can distinguish between the road segments that are within and those that are outside the wetlands buffer.
Extra credit challenge: Create a map to display those roads and contaminated sites that are inside the
wetlands buffer as a different color than those outside the buffer. You should be able to do this with the techniques you learned in this lab and some critical thinking. Don’t be afraid to use the ArcGIS Desktop Help.
Due Date: Your printed map is due at the beginning of next class. Vocabulary
Geographic information science has a language of its own like many other areas of specialization. Whether you are just visiting or decide to live here, it is polite to learn the language. We will make a list of pertinent vocabulary each week. The following list contains words you should be comfortable with at the end of this exercise.
GIScience-related Buffer, buffering Concentric Metadata Overlay Proximity Software-related Clip, clipping Extract, extraction Transparency
