Vector Data Analysis I: Buffering 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. Creating Buffers Learning Objectives: Simple overlay analysis Basic proximity analysis Advanced selection techniques Data display priority 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 yourlastname_lab05.mxd. Remember to save often after this step! 2
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. 3
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 4
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 5