Geography 281 Map Making with GIS Project Three: Viewing Data Spatially

Transcription

1 Geography 281 Map Making with GIS Project Three: Viewing Data Spatially This activity introduces three of the most common thematic maps: Choropleth maps Dot density maps Graduated symbol maps You will learn how, when and why to create each type along with some of the pitfalls to avoid when making them. Most of the work will take place within the Symbology tab of the Layer properties form in the ArcMap Data View. In ArcMap Layout View, you will learn how to Set Relative Paths Modify legend text The first part of the activity guides you though the steps and decisions needed to create a series of maps illustrating the spatial relationship between city population, state population and area in the lower 48 states. The On Your Own part lets you apply what you've learned to create a new map that combines information from two previous maps. The data needed for proj3 is located in the \\Geogsrv\data\geog281\proj3 directory. Project 3 files: Description: Feature Type: counties48 shapefile base map of US counties polygon us48_major_cities shapefile Major U.S. cities point us48_states shapefile base map of lower 48 states polygon Visual Variables Maps use symbols to portray data. By selecting the right type of symbol you can create an intuitive map that is easy to read. Choosing the wrong symbol can mislead and confuse the map reader. Depending on the data type (qualitative or quantitative) and the type of feature (point, line, or polygon) you are mapping, you have many choices with regards to selecting symbol color (hue and value), size, shape, and texture. Y R G B Page 1

2 After copying the data from the server to your local working directory, you are ready to begin Project 3. Open the Proj3 map document file Do you see a map of the lower 48 states or a blank data view? What happened to your map? Remember, map documents do NOT store data. Map documents only point to the data using the saved path. Let s find out where the map document is looking for the Proj3 data. Open the property form for us48_major_cities. Under the Source tab, look at the location of the data. This is where the computer is looking for the data. How can you tell that the computer cannot find your data? What symbol appears in the Table of Contents? Draw the symbol. You can repair the link to your data, by resetting the data source. Click on the Set Data Source button. Navigate to your working directory and select us48_major_cities.shp Repeat this process with the remaining two layers. If you ever see the red exclamation points when working with your map documents, you will now know how to correct the problem. Now let s look at preventing the problem. In the example above, Proj3.mxd was pointing to the data using absolute paths. The computer could only find the data at the following location: C:\ClassData\Proj3\Data. One way to avoid losing your data is to store the path as a relative path instead of an absolute path. Close Proj3.mxd- Do not worry about saving it. Open Proj3a.mxd Do you see a map of the lower 48 states or a blank data view? You should see a map because the Proj3a map document stored the path to the data as a relative path. Consider the following file structure of Project 3: Page 2

3 The data for Proj3 was found under \Proj3\Data\. We will use the counties48 shapefile as an example. The Proj3a map document stored the path to the data as..\data\counties48.shp. The.. symbol tells the computer to go up one directory level from the current directory (MapDocuments) and find the directory called Data within which will be a shapefile called counties48. As long as the data remains in the same location RELATIVE to the map document, the computer can find it. Open a new map document File New Blank Map Add the following Layers to the Data View counties48.shp us48_states.shp Remove the visibility check mark next to the counties48 layer in the ArcMap Table of Contents. Only the us48_states layer should be visible. Set the map document to use relative paths. Under File Map Document Properties, check the box next to Store relative pathnames to data sources. Click Ok. Save the map document Save this map document as Proj3_Choropleth. You should be saving the Proj3_Choropleth map document in C:\Temp\Proj3\MapDocuments. If you saved the map document in the correct location, proceed to the next page. If not, go back and try again. Map 1- Choropleth Map ( Graduate Color Map) of Population A choropleth map portrays data collected in enumeration units- such as counties and states. The numerical data is grouped in categories and each category is assigned a color value. Key elements of a choropleth map: Feature = Polygon Data Type- Quantitative Normalized Data Values Visual Variable- Color Value To illustrate why choropleth maps use normalized data values instead of total numbers, let s make a choropleth map using state population totals. To do this, you need to assign appropriate symbols to population ranges in the us48_states layer: Right-click on the us48_states layer name in the Table of Contents and choose the Properties option. Click on the Symbology tab. In the left-hand frame under Show choose Quantities Graduated Colors. Set the Fields Value to the Pop2000 (2000 census population totals) field. Press OK. Before clicking on OK, compare your property dialog box with the graphic. Page 3

4 Note that California, the second largest state in area among the lower 48 states, occupies the highest population category while Texas, the largest state, occupies the second-highest category. But are these states rated this high because they are more densely populated than other states or simply because they are larger than the other states? If we want to know where people reside in space, we must divide the population of each state by the state s area and map the resulting population density values. Reopen the Symbology section of the Layer properties form and change the Fields Value to Pop00_sqmi (2000 census population totals divided by the state area). Press OK. Now you can see that California falls to the middle-category while Texas shows up as below average. Although these are states with large populations (political-demographic measures), they have average or below average population for their size (spatial measure). Since we use maps to measure spatial relationships, not political-demographic ones, the general rule is that you never use a choropleth map with total numbers. The data must first be standardized (normalized) by area, percent or by some other ratio. Symbolizing a Choropleth Map Now let's explore some of the options for symbolizing a choropleth map. First, take a look at a monochromatic color ramp: Reopen the Symbology section of the us48_states Layer properties form. Note: To list color ramps by name instead of visually seeing the colors, Press the down arrow next to Color Ramp and note the choices. right click on the down arrow next to the Color Ramp and remove the Highlight the Green Light to Dark color ramp. checkbox next to Graphic View. Press OK. A monochromatic color ramp preserves the value gradient of the data by gradually varying one color in regular intervals from a lighter shade (low values) to a darker shade (high values). If you want to suggest a gradual shift from low to high values, this is an appropriate color ramp to use. Next look at a three-color ramp: Reopen the Symbology section. Press the down arrow next to Color Ramp. Highlight the Yellow to Red color ramp. Press OK. This approach provides a more dramatic sense of change in the underlying data by varying the amounts of two colors one a low intensity color (yellow) and the other a high intensity color (red). If you want to emphasize the magnitude of change in the data (from very low to very high) this is an appropriate color ramp to use. Open the Layout View Now you are ready to open the Layout View so you can work with the map legend and title. First, open the Layout View and set the page properties to Landscape using the techniques that were introduced in Project 2. After resizing the map frame, you may notice that a solid line marks the border of the map frame. To remove this from your map: With the Select Elements tool still active, right-click inside the map and choose Properties. Click on the Frame tab. Press the down arrow under Border, scroll up and select None. Press OK. Next, insert a legend, again using the techniques that were introduced in Project 2. After inserting the legend, position it on the page below California and Arizona. Page 4

5 Note that the default legend contains a meaningless title Legend and some obscure and unnecessary references to the layer name (us48_states) and the data field name (pop00_sqmi). You should modify the legend as follows: Right-click on the legend and bring up the legend Properties form. If necessary, click on the Legend tab. Because the title Legend conveys no useful information, highlight it and replace it with Persons per Sq. Mile. Next, press the Items tab. Confirm that us48_states appears under Legend Items (If any other layers appear, highlight them and press the left arrow to remove them as legend items). Right-click on us48_states under Legend Items and highlight Properties to bring up the Layer Legend Item Properties. Click on the General tab. Remove the check marks from Show Layer Name and Show Heading. Before pressing OK twice, check your legend property dialog boxes with the two below. A clear, simple legend is all you need for a thematic map such as this one. The final step is to add a title to your map: From the menu, choose Insert Title. Enter the title text: Population Density. Double-click on the title to bring up the title Properties. Press the Change Symbol button. Set the size to 24 and press OK twice. Note: To modify the Title text, open the Map Document Properties found under the File Menu. You ve now completed a simple choropleth map of U.S. population density. In a subsequent exercise, you ll learn more about choropleth maps including different approaches to classifying data. Before moving on, save your work to the Proj3_Choropleth.mxd file. Page 5

6 Map 2- Dot Density Map Population distribution may also be mapped using dot density maps. A dot density map uses a dot to denote the specific number of phenomena being mapped, in this case population. The dots create a visual impression of density - dots that are closer together have higher densities and dots that are farther apart have lower densities. Key elements of a dot density map: Feature = Polygon Data Type- Quantitative Total values To begin create a new map document file. This will enable you to create a dot density map (Proj3_DotDensity.mxd) without destroying the choropleth map saved in Proj3_Choropleth.mxd: From the menu, choose File Save As. If necessary, navigate to C:\Temp\Proj3. Enter the new file name: Proj3_DotDensity.mxd. Press Save. Create the dot density map: From the menu, choose View Data View. Right-click on the us48_states layer name and bring up the Layer properties form. Click on the Symbology tab. Under Show, highlight Quantities and Dot Density. Under Field Selection, highlight pop2000 (unlike choropleth maps, dot density maps use raw numbers (i.e., total numbers), not density data). With pop2000 highlighted, press the right arrow. In the right-hand frame, double click on the dot to the left of POP2000. Set the Color to Mars Red (2 nd Column, 3 rd Row in color grid) and the Dot Size to 1.0 and press OK. Set the Dot Value to 20,000. Before pressing OK, compare your property dialog box with the one on the right. You should see a problem immediately: dots are distributed evenly throughout each state. This is because ArcMap randomly places the dots within the enumeration unit, in this case the state polygons. The overall effect is little different than that of a choropleth map. In fact, it can be misleading in large states like California and Texas where populations are unevenly distributed across the state. A better approach for making a dot map is to use detailed geographic boundaries to set the dots and more general boundaries to orient the viewer. Let s try it again using county population instead of state populations. Remove the check next to us48_states and add a check next to counties48 so that only the counties48 layer is visible. Right-click on counties48 and bring up the Layer properties. Set up a county population dot density map with the same parameters that you used for the state population dot density map. Page 6

7 Note how the county boundaries make it difficult to see the resulting map. To remove the boundaries: Bring up the Symbology form for the counties48 layer. Click on the line symbol under Background. Click on the down arrow next to Color and select No Color. Press OK twice. Now you can see the dot density pattern more clearly, but your map lacks any useful spatial reference. To fix this, overlay the dot density map with state boundaries: Change the drawing order by pressing the mouse on the name us48_states and dragging it up above the name counties48 (you may need to click on the List By Drawing Order icon in the Table of Contents before ArcMap will let you change the drawing order). Make the us48_states layer visible and bring up its Symbology form on the Layers properties form. Under Show, choose Features Single Symbol. Click on the color rectangle under Symbol. Select the Hollow symbol and press OK twice. This is a much more accurate and readable map of U.S. population distribution than the previous one that distributed the dots evenly across each state. To complete your map, return to the Layout View and use the Select Elements tool to modify the title and create a custom legend: Double-click on the Title and change the text to Population Distribution. Click on the Legend and press the Delete key to remove it from the map. From the menu, choose Insert Text. Modify the text to read: One Dot = 20,000 Persons. Drag the text box to a new position below California and Arizona. Double-click on the text box to bring up its Properties form. Click on Change Symbol, set the Size to 12, and press OK twice. You ve now created a simple dot density map of population distribution. Before moving on to a third type of population map, you should save the Proj3_DotDensity.mxd file. Map 3- Graduated Symbol Map A graduated symbol map portrays point data. Point data includes specific locations (point features) and conceptual points (center of polygon features). The numerical data is grouped in categories and each category is assigned a different size symbol (usually a circle). Key elements of a graduate symbol map: Feature = Point (either specific locations or conceptual points) Data Type- Quantitative Total Values Visual Variable- Size In this part of the exercise, you will create a graduated symbol map of major cities in the U.S. You will assign different size symbols to the categories containing city population data. Before starting, you will need to create a new map document file called Proj3_GraduatedSymbol.mxd for the graduated symbol map. Click on the checkbox next to counties48 to remove the counties48 layer from the map. Click on the Add Data button. If necessary, navigate to the C:\Temp\Proj3\Data directory. Highlight us48_major_cities.shp and press Add. In the layer list, ensure that US48_major_cities is at the top of the list so it draws last. Page 7

8 Right-click on US48_major_cities and bring up its Properties form. Click on the Symbology tab. Under Show, click on Quantities Graduated symbols. Under Fields, set the value to Pop2000. Now set the symbols: Click once on the Template button. Highlight the Square2 symbol. Press the down arrow next to color and select Mars Red (2 nd Column, 3 rd Row in color grid). Click OK. Before clicking OK again, compare your property dialog box to the one pictured on the right. Looking at the map, you might decide that the symbols are too small. To change the size of the graduated symbols: Bring up the Properties sheet and click on Symbology. Set the Symbol Size from, so it reads 8 to 18. Press OK twice. We ll discuss symbol selection in more detail in a later activity. For now, it s enough that you know how to create a graduated symbol map and to edit the symbols. Before continuing, save your map document file. On Your Own Create a map that combines a dot density map of county population with a graduated symbol map of major city populations. Do not use the same colors from the sample maps above. Create a layout and legend for your map. Ask the lab assistant or instructor to check your work. Print a copy of the On Your Own map and give it to the lab assistant. Do not put your name on the map; the lab instructor will assign it a number. Save your work to Proj3_OnYourOwn.mxd and back up all your work on your Flash drive. Page 8