GIS Virtual Workshop: Buffering

Transcription

1 This workshop will teach the different methods of buffering data. They will include: Basic buffering of data Merging buffering zones Clipping the buffer Concentric rings around the object You will find multiple types of information available on the website which will include sample data, video and web based content. buffer/pages/default.aspx. The starting point for this module is that you should have ArcMap open and have loaded five data sets: KCTCS college locations (point), Kentucky Roads (line), Kentucky State Boundaries (polygon), Kentucky County Boundaries (polygon), Kentucky Zip Code Boundaries (polygon). You can download these shapefiles from the website, they will be in a compressed format (zip). You can choose geographic information of your own region if you would like, but you need a point, line and at least two polygon shapefiles of the same coordinate and projections. Turn off all layers except state boundary. The color of your layers may look different. 1 1/24/2008

2 Double click on the State Boundary Layer Color Box to display the window like the one to the right. We want the color of the fill for the map to be hollow, so that other layers can be displayed through this layer. Double click on the hollow icon, then click on the properties button to display a window to change other properties. 2 1/24/2008

3 Once you selected the hollow fill it created a black outline around the state. In this step you will change the outline to be a standard state border. The window displayed to the left is only a part of the window that will open. Click on the outline button to open a window to change the property of the outline for the state of Kentucky shapefile. Select the Boundary, State from the list of line types. Note there are many other types of lines that might be used in other parts of this module or other modules. 3 1/24/2008

4 The white area of the map is really hollow, but since all other layers are currently turned off it appears white. The next step is that we want to display the county boundaries on this map. Click the check box next to the county boundary layer to turn it on. Once the county boundary layer is turned on it should look something like the map to the right. The color of the fill may be different. We want to make the fill for this map hollow and the boundaries represented with the standard county boundary representation. You will need to do the same steps as was previously done for the entire state. The one difference is selecting the boundary as county boundary instead of state boundary. 4 1/24/2008

5 The resulting map should have an appearance similar to the one on the right. At this scale it is a little difficult to see the difference between the state and county boundary information, but when you enlarge a region you will be able to observe the differences. Next turn on the zip code layer by clicking the check box. It should look like the map to the left. The fill color maybe different. Again we want to make the fill hollow and change the boundary lines to being neighborhood. You will follow the same steps as done for the state or county layers to complete this operation. 5 1/24/2008

6 The results of changing the fill and boundary type for the zip code layer should produce a map similar to the one at the right. Next you will turn on the KCTCS layer. This is a point shapefile and it maybe difficult to see the dots for each campuses at this scale. The campuses are represented by a dot and we want to change the symbol to a school symbol with a larger size. Click on the dot symbol in the layer menu for the KCTCS layer to open the following dialog box. 6 1/24/2008

7 Select the School 2 symbol and keep the color with the default green color and the size as 18. You can change these later by clicking on the symbol to open this window again. The results of this operation can be seen in the map to the right. You can now see the locations of the KCTCS campuses and see that a more appropriate symbol is being used. The referenced point is the center of the symbol. 7 1/24/2008

8 Next we want to turn on the road shapefile (line). There are many different types of road classifications. They are in general classified by a code known as, Census Feature Class Codes (CFCC) a description of these can be found at appendxe.asc. The road file that you have loaded has only the major roads and does not include city and county roads, but does include state and interstate highways. For our work we will not worry about classifying the roads as different types. Click on the road symbol in the layer menu to select the roads display properties. Click on the Major Road symbol, leave the color and the width at the default levels. You may wish to change the width later so that the display is easier to view. 8 1/24/2008

9 We have now modified the data appropriately from the initial load of information. If we are viewing the entire state there are far to many layers turned on for any meaningful information to be obtained. Some of these layers are more useful when we are at a zoom level of one or a few counties. If the map was being plotted on a large scale plotter than all layers could be left on, but for a desktop size map we need to turn off some of the layers as we view the entire state. Turn off the road and zip code layers, do not remove the layers from the map. Click on the toolbox symbol. Next click on the Analysis Tools and the Proximity tray to show the Buffer tool. In a later part of this module we will also use the multiple ring buffer. Select the buffer tool. Note in ArcMap 9.1 the buffer tool was also in a menu known as the Tool Menu. The view to the right is only part of the full screen, you should see your map to the right of the part of the screen currently displayed. 9 1/24/2008

10 The pull down arrows next to the boxes allow you to use files that are open on your map. The open folders icon are used for files that are not open on your map. The input feature will be the KCTCS Campus layer. The output feature will be a file that is created by the process of buffering, you need to note the name of the file and the storage location, you can change both of these items before the process is completed. The distance is a linear distance and we will select 25 miles for this example. All other items will remain at the default levels. In the image below we are using the pull down arrow and the list shown is of all layers currently loaded on our map. We will select the KCTCS layer. 10 1/24/2008

11 Note the output feature that has been placed in the second window. We have inputted the distance of 25 and selected miles from the pull down arrow. Click on ok to execute the creation of buffers. A progress window will open and this may take several minutes to complete depending on the speed of your computer and the amount of information to buffer. The KCTCS file is constantly updated so your final results may look slightly different. Note many of the buffers extend outside of the state boundary. 11 1/24/2008

12 To make the circles remain within the state boundaries we will use the clip feature. You may wish to refer to the module on Clipping for more details. The clip tool is located in the Toolbox under the Analysis Tools in the Extract Tray. Our file will be the recently created buffer file and the boundary will be the state boundary file. Since both of these files are loaded you can use the pull down arrows to select. The third window is the location in which the new file, the clipped buffer file will be stored and the name that the system assigns to it. Make sure the name is appropriate and that the storage location is part of your tree structure. The remaining items in this window will use the default values. The clip window requires a file and a boundary polygon to clip with. The results of clipping against the state boundary can be seen in the image at the left. Your county boundary file should be a higher layer in the layer menu than the new clipped buffer file, so that the county boundaries display on top of the buffer. 12 1/24/2008

13 In the next part we want to dissolve the boundaries between the circles. To do this we are going to create a new buffer around the KCTCS campus locations. Therefore, you need to go back to the toolbox and open the buffer window again. We have expanded the buffer window to see all of the different options that can be used. Near the bottom of the window you see Dissolve Type. The default is None. We will change this value to All. We will also need to input the layer for buffer to be defined about which is the KCTCS campuses. Remember a new shapefile will be created and stored with a default name and note the location unless you changed the information in the second window. You will also need to input a distance for the buffer with units. The window to the right shows the values filled in. 13 1/24/2008

14 The results of the dissolve is that circles that touch have been dissolved. The country boundary shapefile needs to be placed above the buffer layer. Note the information extends outside of the state boundary because we have not clipped this layer. If you wish you can do a clip, as was done previously. The final part of this module will be to create concentric rings around the campuses of different distances. You will need to go back to the Toolbox and select Multiple Ring Buffer this time. Remember buffering can also be done with lines and polygons, in this module we have only looked at point shapefiles. You have already loaded road and zip code information and you should try to buffer these layers to see the results of buffers around these geometries. 14 1/24/2008

15 The multiple ring buffer window looks similar to the standard buffer window and the first two boxes are the same. Select the layer to be buffered using the pull down arrows. Again we will buffer the KCTCS campuses. The output feature will be automatically filled in, make sure that it make sense both in storage location and name. Input the first distance and click on the plus button. Repeat this for two additional distances. In this example we have used 10, 25 and 35. Next select the unit, for this example we will use miles. We have also turned on the dissolve option to ALL to dissolve overlapping rings. In the window to the right you can see all of the selected and inputted values. 15 1/24/2008

16 The image to left is the results from the completed operation. You will note that the rings are dissolved, they extend beyond the state boundary since they have not been clipped. To better understand the results it would be good if each distance was represented by a different color. To do this you will need to open the property window and use the symbology tab. Select quantities and then select the field which has the distance measurements, select a ramp color. You should have three classes since we used three different distances. For a final map you should also move the country and state boundaries to a higher level and clip about the state boundary. 16 1/24/2008