Lab 4: Digitizing and Data Automation
Geographic Regions. The region is one of the major spatial building blocks of GIS. Regions can be represented as vector polygons or raster clusters. Regionalizations are of great interest partially because there is generally disagreement on what the boundaries, extents, and locations of many regions actually are. This has led to a great deal of research and commentary both on the nature of regions and the techniques for defining them. In this lab you will create a regionalization of the United States based on your own understanding of the divisions that are present within the country.
Creating New Spatial Data by Digitizing. Digitizing is the process of copying an existing map to create a digital version of the map. During the digitizing process, maps can be selectively digitized and generalized to create a simplified version of the original. Digitizing can be performed either using a digitizing tablet to copy a paper map or using "heads-up" digitizing which involves digitizing from an existing digital data set on screen. We will be doing "heads-up" digitizing in this lab. We will set up our newly digitized layer as a shapefile, and edit the table to make each newly created polygon a unique unit.
This lab will differ from previous labs in that there will be fewer questions for you to answer, and a more difficult mapping assignment for you to turn in. Be aware that good digitizing takes concentration and time; plan accordingly.
Start ArcMap and add the States.shp layer. You will use this theme as the template for your own digitizing. When digitizing in ArcMap, you must digitize into some predefined geographic space such as the states theme. You should now be looking at an ArcMap data view with a map of the United States including Alaska and Hawaii. Now clip out the lower forty-eight states. Use the Select Features tool to drag a box around the lower forty-eight states.
The selected states should be highlighted in blue.
Now right click on the layer name, select Data -> Export data. This will export the selected features to a new shapefile. Name the new layer lower48. Add the new data layer to your map. Remove the States layer. Use the Zoom to Full Extent button to center the map in the view.
Now we need to prepare for digitizing. We will be digitizing into a new shapefile. We will use ArcCatalog to create the new shapefile to hold our digitized data. Open ArcCatalog.
Navigate to the folder with your data for this lab. If you cant find your folder, you may need to use the "Connect to Folder" button (we learned about this feature in Lab 1).
When you have selected your folder, right click in the contents window of ArcCatalog and select New -> Shapefile.
In the Create Shapefile window that opens, name your shapefile something that you will remember (for instance, "myregions"), and select Polygon as the Feature Type. Notice that the current coordinate system is unknown. We will need to set a coordinate system. Click on the Edit button. In the Spatial Reference Properties window Import the coordinate system from your lower48 shapefile.
We now have a new shapefile into which we will digitize our regions of the United States. Before we begin digitizing, however, we need to finish setting up the table we need to create a field so that we can add data about our regions.
Open the properties window for your new shapefile, and add a text field "Name" to the table (we did this in the first lab of Chapter 3).
Once this is finished, we should be ready to digitize.
Back in ArcMap, add the new myregions shapefile. Edit the color of the symbol so that it is different than the symbol used for lower48.
Now we can start digitizing.
Turn on the Editor toolbar. This is found under Tools -> Editor Toolbar. With the toolbar on, click on Editor -> Start Editing. Select your new shapefile (not the lower48 shapefile!) as the Target.
Click on the "Create new feature" tool on the Editor toolbar. This tool can be extended by clicking on the down arrow to the right of the tool. Notice that there are several tools that can be selected including one to draw Bezier curves. Select the Create New Feature tool. Make sure that the Task is set to "Create new feature."
Now draw a large polygon by left clicking in the view. To end the polygon, double left click.
If you make an error in digitizing you can put the digitizer "crosshairs" over the vertex you want to remove and right click. The digitizing options window will pop up. It allows you to do simple editing. Experiment with this window. You will need it.
If you right click while not directly over a vertex, you will see a different set of options. This allows you to modify some of the properties and relationships between the segments that you are creating.
When you are digitizing, often you will need to zoom in or out, pan, or otherwise change the scale of the view. Even if you are in the middle of digitizing a feature, you can use the zoom, pan, etc. icons. Try this out so that you feel comfortable using these features. You will need them.
Now we will append a polygon to your first polygon. Change the task from Create New Feature to Auto Complete Polygon. Left click inside the polygon you just drew, and then draw a polygon adjacent to your original polygon. End by double left clicking inside the first polygon. It is very important that your starting and ending points be inside an existing polygon if they are not, the polygon will not append and your new polygon will disappear!
When digitizing in ArcMap you can turn on "snapping" to make your digitizing easier and more accurate. Open the snapping window by clicking on the Editor button and selecting Snapping. In this window you can select which, if any, layer you would like to snap to, and whether you would like to snap to the vertices, edges, or ends of features. Set snapping for vertex, edge, and end for the lower48 shapefile.
Now zoom in to an area on your lower48 shapefile. Digitize a new polygon and notice the effects of the digitizing crosshairs when you are near a vertex, edge, or end of a line in the lower48 shapefile.
Question: What does snapping do? Do you think that this is useful? Why or why not?
Question: If you digitize into a reference map in the geographic coordinate structure, what will be the projection of your newly digitized map?
Now that you have practiced with the digitizing tools you will create a regionalization of the United States. The digitizing in the exercise will consist of two problems. The first will be copying the relatively fine line work of the continental coastline. You do not need to digitize islands. The other will consist of creating more general internal boundaries around regions.
Your assignment in this exercise is to regionalize the lower 48 states of the U.S. into at least 5 regions. These regions can be based on any concept you have of the U.S., with the proviso that you MAY NOT use any existing state boundaries. Delete your practice digitizing from the regions shapefile that we created earlier. Digitize your new regions. Fairly frequently, you will want to save your work Editor->Save Edits. If you do not complete your work in one session you will need to save your edits, and select Editor -> Stop Editing. When you return to work, open the project, and start editing again make sure that your regions shapefile is the Target. You may then continue your digitizing project. When you are finished, set your new theme to "No Color". You should see only the outlines of the polygons. Make the outline a dark contrast to the polygons underneath. Zoom in on the coast of Maine so that your lines and the original lines are clearly visible. The quality of your linework on the coasts is important. Print and turn in a layout of this map with your lab.
Zoom out to the active theme. Print and turn in a layout of your regionalization with your lab.
Now we will work with the table for the new regions theme to build a simple database, and make each object unique. Right click on your region shapefile and select Open Attribute Table.
Question: Draw the table for Myregions.shp. How does this table differ from other tables you have seen? Can you explain why this might be?
Start editing the file. By clicking in the ID field for each record you can type in new data. Assign each record a unique number.
Select the first record, notice which region is highlighted in your view. Enter an appropriate name in the Name field. When you have named all of your regions save your edits and stop editing.
Save your work and stop editing. Now open the Properties window for your regions and select the Symbology tab. Select Categories -> Unique Values as the Legend Type. Select Name as the Value Field. Click on the add all values button to add all of the unique values from your table. Click Apply.
Now with no polygons selected, right click on the myregions layer and select Label Features. Examine your view. If you do not like the way that the labels have appeared on your map, you can change the labeling options in the theme Properties under the Labels tab.
Select an appropriate projection, and project your map. Use layout to create a map composition. The map should include at least a legend, a title, and a neatline. Print and turn in this map with your lab.
Question: Discuss the validity of the region concept. Do you think regions exist? Were you uncertain of where certain boundaries ought to be?
In this lab you have digitized a new theme, created a useful table, used an auto-labeling algorithm, and created a final map composition. In addition you have created a regionalization of the United States based on your own personal knowledge. This should have given you the opportunity to reflect on the concepts of regions, and the wide variety of views that individuals might have of regions and their creation.