Lab 3: Working with Attribute Data

Outline

• Purpose
• Get the data
• Explore the data in ArcCatalog
• Selecting attributes in ArcMap
• Calculating and Displaying Area
• Make a Classified Map of State Area
• Create a Bar Graph of State Area
• Conclusion
• To turn in

1.0 Purpose

In this lab you will strengthen your ArcGIS skills from previous labs and begin to work with attribute data. First we will download GIS data from the National Atlas, import metadata, define a projection and project the data to a new layer. Next, we will explore many aspects of attribute data in ArcGIS, including how to select records with queries, add new fields, calculate new values based on an existing field, and create classified maps and graphs based on attribute values.

2.0 Get the data

Make a Lab 4 work folder in your workspace, then go to the following web site: http://nationalatlas.gov/

The webpage may look different than the screenshot above because the National Atlas is a dynamic site, which changes its "What's New" section regularly.

Click on the "About" link below the search window at the top. Read through the description of the National Atlas and answer the following question.

4.0 Selecting attributes in ArcMap

We will now explore working with attribute tables in ArcMap. Load the countyp020.shp Shapefile in ArcMap.

For the purpose of this lab, we are only going to focus on the 48 conterminous states. We will create a new Shapefile by first selecting the 48 states. We can do this by opening the countyp020.shp attribute table. Do you remember how to do this? If so, open the table.

If you do not remember...you right-click the Shapefile and select "Open Attibute Table".

Right-click on the "Options" button at the bottom right of the table and select "Select by Attributes". You should see a new window appear:

Using your notes and lecture presentation on attribute data, build a query to select polygons from the 48 conterminous states. Use the default "Create a new selection" method. You can double-click the appropriate field name to get it to appear in the SELECT window. Once you select a field, you can press "Get Unique Values" to show all possible values for that field. Do not click Apply just yet. You will need a combination of Set Algebra (i.e., =, <, >, <>) and Boolean Logic (i.e.., AND, OR, NOT). It is easiest to build your query by selecting all polygons that are not conterminous states (Alaska [AK], Hawaii [HI], Puerto Rico [PR], and the Virgin Islands [VI]). Also exclude from your selection any polygon that does not have a county name (use "COUNTY" <> ' ').

You can save your query with the "Save..." button. Select the text in your query and do a right-click then "copy". Paste your query into the follow question box in your answers:

Go ahead and click apply. You should see polygons from the 48 conterminous states highlighted in light blue. Be sure that there are no polygons selected in the Great Lakes (polygons with no county name). If you did not get the query right, go back and try another query. You can always load your last query by clicking the "Load..." button in the query window (that is, if you remembered to save it before clicking Apply).

Now we can export the selected polygons to a new Shapefile. Right-click countyp020.shp and select Export Data

Leave the default "Selected features" option since we want to export only the selected polygons. We will use the "layer's source data" coordinate system. Give your output Shapefile an appropriate name.

Click "OK" and add the new Shapefile to the data frame. Turn off the "countyp020.shp" Shapefile. Zoom to the extent of the new Shapeifle by right-clicking your new Shapefile and selection "Zoom To Layer".

We will no longer use the countyp020.shp Shapefile. You can remove it from your ArcMap map document by right-clicking countyp020.shp and selecting "Remove". This removes the layer from ArcMap, but it is still physically present in your work directory.

5.0 Calculating and Displaying Area

We now are going to make a map that displays the area of each State. We do not want display the area values in the units of the Shapefile (see Question 7). Area in square units of the current coordinate system do not make much sense. Remember from Lab 2 that we can calculate the area in convenient units, like square meters, by projecting the layer to a planar projection. Remember how we did that?

We reprojected the shapefile by changing the data frame and then we selected "data frame" option for the output coordinate system in the data export window. There is an easier way to reproject a vector feature class. You can use ArcTools! Look in "Data Management Tools", then "Projections and Transformations", then "Features", then select the "Project" tool. (NOT "Define Projection" - that tool defines the coordinate system information if it was missing, like you did in ArcCatalog).

Either of these layer projection methods, ArcTools "Project" or using the Data Frame/Data Export approach from Lab 2, will give you a projected Shapefile. Chose a method and project your Shapefile to the "USA Contiguous Albers Equal Area Conic" projection that we used in Lab 2.

We could now calculate the area of the States field like we did in Lab 3 but there is one problem. We do not have State polygons. Instead we have polygons representing subdivsions of States (see Question 5). We will need to grounp polygons by State and dissolve their boundaries. This process will give us polygons representing states. We can do this with a very useful tool called "Dissolve". Open this tool in ArcTools. It is under "Data Management Tools", then "Generalization". Read the help to the right of the tool window to get a better understanding of the process. When you are ready, use your Albers-projected Shapefile as the input and give your output Shapefile an appropriate name. Click the "State" field as the "Dissolve_Field". Click "OK".

Close the tool and turn on your new dissolved layer. You should see that the small polygons in the input layer have been dissolved into larger polygons representing states.

Open the dissolved Shapefile's attribute table.

We now need to calculate the area for each polygon. First we need a new field to hold the area calculation. Go to "Options", then "Add Field". Type in "Area" for the Name and "Double" for the Type. Click "OK". This adds a new field to the table.

Use your skills from Lab 2 to calculate the Area field in the attribute table. Calculate area in square meters.

Make another field, but this time name it "Hectares" and set its field Type to "Long Integer". Calculate the area for each polygon in hectares. Remember that the Area column has units of square meters (m2) and there are 10,000 m2 per hectare. The calculation is done by selecting the new Hectares column and then selecting "Calculate Values...". Click "Area" in the fields window and then in the white window below the "Hectares =", type in "/ 10000" after the "[Area]". Then click "OK". You should have a new column with the area for each polygon in Hectares. Notice how the hectares values are rounded to the nearest whole integer. This is because we chose "Long Integer" as the field type, rather than "Double", which is a real number (floating point) data type with decimal precision.

6.0 Make a Classified Map of State Area

Right-click on the dissolved Shapefile and click on Properties. Go to the Symbology tab and select "Quantities", then "Graduated Colors". Select the "Hectares" field for Value. Click on the "Classify" button and chose "Natural Breaks " for Method. Notice that the classify window has statistics (e.g., minimum, maximum, mean, etc) and a histogram with class break points. Change the color ramp to a color scheme of your choice. You should experiment with the other methods for determining the classification breaks (divisions between classes). We will learn more about map classification in the next lab.

7.0 Create a Bar Graph of State Area

Finally, we are going to learn how to use the Graph tool in ArcMap. This tool allows you to make graphs of the data in an attribute table, much like you would do in Excel. In this exercise we will make a bar graph of state area in hectares.

Go to "Tools" in the main tool bar of ArcMap, then "Graphs", then "Create". Adjust the parameters so that they match the screenshot below. As you change parameters, notice how the graph in the right side of the window is affected. You can chose any color scheme that you want, but match the other parameters to this example. Here the color scheme is set to "Rainbow".

Click Next.

Change the title of the graph to something reasonable, for example "Area of the 48 Conterminous States". Match the rest of the parameters as shown in the screenshot, but take notice of what those parameters do. When you are done exploring, select "Finish".

Notice that the graph's Y-axis labels skip every other state. We need to change this so that every state is labeled. First start by stretching the graph window so that it is longer, as shown below. The extact size does not matter, but we want to make room for the extra lables. Now right-clicking on the graph and select "Advanced Properties...". Click the + next to Axis to open the Axis properties. Select the "Left Axis" (Y-axis), then select the "Labels" tab, then set the minimum label separation to 0%.

Feel free to explore the Graph Wizard's advanced parameters. You will find that there are many ways to make your graphs look professional and customized for your particular needs. When you are done exploring, click "Close".

You may have to stretch out your graph's window to see the graph's information properly. When you are have sized your graph, right-click on its window bar and select "Export". Export the graph to a JPEG (JPG) image format. Use the "Save..." button to save your graph image file to your Lab 4 directory.

Note: the final graph can be added directly to the map layout, so that you can have a map of geographic data along with the graph. This can be done by selecting "Add to Layout" instead of exporting to an image.

8.0 Conclusions

In this lab you learned how download data from a web page with GIS data, explore and define its projection information, and import metadata. You then learned how to select a subset of the data based on an attribute query, and then you exported the selected set to a new GIS layer. From there, you used ArcTools to project the new layer to a projected coordinate system. You also learned how to generalize the layer with the dissolve function. You learned how to add new fields for holding the values of custom calculations. Finally, you created a classified map and bar graph from your calculated values.

9.0 To turn in

• The question sheet, with typed answers (Word document)
• countyp020.prj file
• map of the 48 states area with a natural breaks color classification (pdf format)
• bar graph of the 48 states area (jpg image format)
  

Submit electronic files via email to klacefie@sonoma-county.org, with the subject "G387, Lab 3, [your last name]".

Remember to put your last name before each file name (e.g., clark_county020.prj). We will deduct 0.5 points for each file not properly named.

Be sure to save all of your Shapefiles in a safe place! We will continue to work with this data in future labs.

This lab was designed by Matthew Clark, Geography and Global Studies Department, Sonoma State University.

This page was last modified on Sep. 3, 2008 by Matthew Clark