In the Part II of this lesson you investigated classification methods using ArcGIS's tools and probably used default color schemes. I'd like to revisit (and expand) some of the color topics we touched on briefly in Lessons 1 and 2. In addition, you will see how to create and use Layer files in ArcGIS.
A. Working with Layer files
It is important to remember how ArcGIS stores information. As you know, an ArcMap (*.mxd) document does not store the underlying data compiled in your map. Instead it records a pathway (either full or relative) to the location of the data. Other map elements like a legend, north arrow, or scale bar are saved as part of the *.mxd. Similarly, all of the symbolization and classification changes you make are saved with the map document. You may have experienced the process of remaking a map - when you start over all of the steps taken to give your map a certain look have to be repeated. A Layer File is a way of saving your classification and symbolization choices as a stand-alone file that can be used on other maps.
- Open your lesson4.mxd map document.
- In the Table of Contents, right-click on your Equal Interval layer and choose Save As a Layer File. Save the new file to your Lesson 4 directory and call it Equal_Interval.lyr.
Layer files can also be created from groups of map layers.
- In the Table of Contents, Cntl-click on your pa_stateroads-philadelphia_2004 and hydrology layers. With both selected, right-click on either and choose Group. Notice that the two individual layers now fall under the heading, New Group Layer. Change this name to Roads and Rivers.
- As you did in step 2, save Roads and Rivers as a layer file in your lesson 4 directory. Call it roads_rivers.lyr.
Let's take a moment and look at the results in ArcCatalog.
- Open ArcCatalog and navigate to your Lesson 4 directory. Click the Preview tab.
- First, look at the preview of the tracts2000, pa_stateroads-philadelphia_2004, and hydrology shapefiles. Notice that polygons are filled with yellow and lines are displayed in blue.
Layer files are given yellow, diamond shaped icons.
- Now click on the previews of the two new layer files. Do they look as they did on your map?
It's likely that your roads_rivers.lyr appears unprojected - like the roads shapefile. This is because of the way in which you grouped and saved the *.lyr file. The resultant group inherited coordinate system information from the roads because they were arranged above the rivers in the table of contents. Let's have a closer look at the *.lyr files.
- Open the Properties for the Equal_Interval.lyr file. You should notice that the Properties window looks just like one from ArcMap and NOT like an ArcCatalog Properties window for a shapefile.
Like a *.mxd file, a layer file does not store the actual data - only the appearance information. This means that delivering a layer file to a colleague is only useful if you both have access to the same datasets.
- Open the Properties for the roads_rivers.lyr file. Because this is a grouped layer file the information is organized a little differently.
- Click on the Group tab. Choose hydrology from the Layers list and then click the Properties button. The window should now look like what you saw for the Equal_Interval.lyr file.
You will add the two new layer files to a new map shortly but first let's review some material about appropriate color schemes for different mapped data and constraint on color selection.
B. Color schemes for maps
Learn more about color schemes in the Concept Gallery.
In the Lesson 4 Concept Gallery, we discussed common classification methods and color schemes. Before you start creating any custom colors and ramps, it is worth taking a few minutes to think about these topics again. Below, you will find three maps of different census data (2000 Census). For each map there are nine different color schemes. After reviewing the alternatives, decide which option is the most appropriate for the mapped data. Click the Best Choice link in each caption to see results and comments.
The first set of nine maps (Figures 4.1.a through 4.1.i), each of which uses a unique color scheme, depicts the percentage of people under age 18 identifying themselves as two or more races. The data are aggregated to counties and classified identically in each example. You may click on each individual map to see an enlarged version of that map.
View the best choice.
The second set of nine maps (Figures 4.2.a through 4.2.i), each using a unique color scheme, present the percent change in population from 1990 to 2000. The data are again aggregated by county. The U.S. rate of change was 13.2%. You may click on each individual map to see an enlarged version of that map.
View the best choice.
The third set of nine maps (Figures 4.3.a through 4.3.i), shows religious affiliation. Counties are classified by the denomination with the highest percentage of religious adherents. [Additional source: Gaustad and Barlow, 2001, New Historical Atlas of Religion in America. Oxford Press, New York.] You may click on each individual map to see an enlarged version of that map.
View the best choice.
C. Constraints on color selection
Most of us take color for granted. We see the world in vivid hues and with subtle variations. As map designers, we also need to be cognizant of those occasions when maps need to be read without color - by choice or because of color blindness. Most people who are colorblind are still able to distinguish differences in lightness and see many hues. Color confusion tends to be exacerbated when desaturated colors are used.
The nature of colorblindness has been extensively researched as a matter of physiology and perception. It has also been modeled in numerous color spaces. While the variety of stable color combinations is lengthy (especially when described as luminosity measurements), we can generalize a list of ten color-pair combinations that are clearly distinguishable to people with common color vision impairments.
To visualize how these hue-pairs were determined, imagine a 3D cube (see Figure 4.4, below). Once the cube is flattened, we can arrange the hues in spectral order around the perimeter and create lightness variation by placing white in the center.
Using this arrangement, we can create regions of colors that are indistinguishable by drawing colorblind confusion lines through the space. The lines drawn on the figure below were approximated based on the CIExyY color space.
Colors within the same or neighboring regions that share similar lightness will be confused. A good rule of thumb is to choose colors that vary in lightness and that are separated by at least one region. In Figure 4.6, below, a diverging color scheme is created by choosing two, three-color lightness ramps from regions that are appropriately distant.
In the previous example, we have discussed color choices in terms of diverging color schemes. All types of color schemes can be modified to be colorblind safe. The basic task is to choose colors that vary distinctly in lightness. An easy test for color stability is to use a black and white photocopier. If your map uses hues or lightness specifications that are too similar, the resulting photocopy will appear as a uniform gray mess.
D. Creating and managing custom colors and color ramps
Let's get back to ArcGIS. For the rest of this part you will use ArcMap to specify and save custom colors and color ramps. To date, all of the custom colors you have created are part of each respective map document. If you specified a great shade of blue for use in one map, it is not available in the next unless you repeat the specification. In this step, you will learn to save colors and color ramps so they are independent of a specific map document.
- In ArcMap, open a new map document. Save it as color_schemes.mxd in your Lesson4 directory.
- Add the Equal_Interval.lyr file to your map.
- Add the roads_rivers.lyr file. In the Table of Contents, right-click Roads and Rivers and choose Ungroup.
Let's create some custom colors for the road features.
- In the Table of Contents, click on the Interstate symbol. This will open the Symbol Selector window. Click the Edit Symbol... button to open the Symbol Property Editor window.
- The default interstate symbol is comprised of three elements - a heavy black line, a medium yellow line, and a thin black line. In the Layers field, select the thin black line with a single click and then click the stylized X button to delete this element.
- Select the yellow line and change the Color to a light orange and the Width to 2.0.
- Select the thick black line and change the Width to 4.0. Click OK to dismiss the editor window.
- Back in the Symbol Selector window, click the Save As... button.
- In the Item Properties window, name the new road symbol my_Interstates and leave the Category field blank. Click Finish.
Notice that the new symbol is now listed in the symbol menu (it should be right at the top). Included with ArcMap are a wide variety of purpose- and industry-specific symbol sets. By saving your interstate symbol in the last step, you have added to these. By default most are turned off when ArcMap launches.
- Click OK to apply the my_Interstates symbol and then click the US routes symbol. In the Symbol Selector window click the Style References... button.
Your custom symbols are stored under the user ID you use to log onto your computer. Notice that this set and the Esri set are checked (by default).
- Take a few minutes and turn some of the other symbol sets on and off. The new symbols sets will appear in the scrolling symbol list.
Symbols sets vary for point, line, and polygon features. Because you started this process by selecting a line feature, you will only see line symbols.
- Check the Transportation symbol set to make it active. Scroll down the symbols list and choose A23 for your US Routes.
The alternative to choosing-customizing-saving a symbol is to create one from scratch using the Style Manager. The Style Manager is a tool window that provides you access and control over the look of predefined and custom symbols and elements. Any custom symbol you create is stored within your Windows profile in a file called <username>.style.
- Click OK to apply and dismiss the Symbol Selector windows.
- On the Main toolbar, click Customize > Style Manager...
On the left hand side is a list of the active symbols sets. Notice that yours is listed first (described as C:/Documents and Settings/... /<username>.style), followed by Esri and then any others you activated in the previous steps.
- Expand your folder by clicking on the + sign next to the folder. Symbols are organized into subtypes. Sub folders with white icons are empty. You should see a yellow folder next to Line Symbols. Click on the folder on the left hand side to see your my_Interstates symbol on the right.
- Click on the Color Ramps folder on the left. Then in the right hand pane, right-click in open space and choose New > Algorithmic Color Ramp...
- In the Color Ramp window that opens, choose a light pale green for Color 1. Click the Color 2 radio button and pick a dark blue.
- Experiment with the different Algorithm choices and the Black and White sliders until you are happy with the ramp.
- Click OK and then name the new ramp A Ramp.
Before you leave the Style Manager, let's create another custom ramp. This one will be a colorblind safe, multi-part, algorithmic, diverging scheme. Can you guess how to make it?
- Right-click in the right pane and choose New < Multi-part Color Ramp. Click the Add button twice, choosing Algorithmic Color Ramps each time. Each portion of a multi-part ramp has its own properties.
- Click on the top ramp and then the Properties button. Choose a Dark Brown and White for Colors 1 and 2, respectively.
- Repeat this process for the second ramp using a Dark Bluish Purple (or Dark Blue) and White. Does white appear in the center of the diverging ramp? If not, you will need to swap Colors 1 and 2 in the second ramp to fix the orientation. Save the new ramp as AA Ramp. Close the Style Manager.
- Open the Properties for the Equal_Interval layer. Look at the choices under the Color Ramp drop-down menu (Symbology Tab > Quantities > Graduated Colors). The color ramps are organized alphabetically (even though the names are not listed), so your ramps should be the first two in the list.
Because you are applying a continuous color ramp to classed data, the legend will appear as distinct steps. If you were using your ramp on unclassed data (like elevations), you would see a smooth transition along the ramp.
- Make a duplicate of the Equal_Interval layer. Give the new layer a unique name.
- Of the classification methods you have used in this lesson, which one is commonly symbolized with a diverging color scheme? ______________________________
- Re-classify the new layer with this classification method. And use your AA Ramp as the color scheme.
E. Save the map document
That is it for the walk-through instructions for Lesson 4. See the deliverables page, next, to see what is due as the assignment for this lesson.
Next week, using the same data, you will learn about map representations other than the choropleth map.
If you have any questions, please post them to the Lesson 4 Discussion Forum.