VIRGINIA TECH - CARDI JAMAICA
INFORMATION SYSTEMS FOR COLLABORATIVE RESEARCH
GIS WORKSHOP
August 18 - 20, 1998
Make sure the Identify tool is selected and examine the identity and various characteristics of the tracts on the map.
Right now, the theme Stcath_s is displayed uniformly throughout our map. We know where the various tracts for Stcath_s are located--we can see their boundaries--but the map currently does not tell us anything else about the attributes associated with each tract. For example, by looking at the map we do not know anything about the spatial distribution of values for fields such as "Landuse" or "Capability."
But we also know that we have such data because when we looked at the Identify Results table above, information on various values for "Landuse" and "Capability" were in the table. How do we map the spatial distribution of such variables?
In the Identify Results table, we saw that each tract had several "fields," or variables, associated with it. While all the tracts on our map have the same fields, they differ in relation to their values for each field. For example, while all the tracts have the field "Landuse," each tract can have a different landuse. We can thus classify and map areas according to their respective values for a particular field, such as Landuse.
The manner in which we display values depends upon their characteristics.
Double-click on the theme name "Stcath_s" in the Table of Contents to bring up the Legend Editor dialog box again.
Let us look at the Legend Editor dialog box closely. There are two small boxes at the top entitled "Theme" and "Legend Type," each with a small button at the right with a downward pointing arrow that is used to bring up a drop-down list.
Right now, the "Theme" in the Legend Editor dialog box is "Stcath_s" and the "Legend Type" is "Single Symbol". It is "Single Symbol" now because we have just one color (green) representing the theme Stcath_s.
Click on the down-arrow next to the box for Legend Type to bring up the drop-down list.
There are several choices in mapping in ArcView besides "Single Symbol."
The same variable can often be represented in a variety of ways. For example, pest populations can be illustrated by all of the above types of maps.
Note that when you selected "Graduated Color" from the drop down list, the Legend Editor dialog box changed with new choices appearing.
Two new boxes can be seen, "Classification Field" and "Normalize by" and both have the word "None" inside them. There is also an empty table below them, which we will use shortly.
Move your cursor to the arrow pointing down on the right side of the "Classification Field" box and press on the arrow to reveal the drop-down list and scroll down and select "Area" from the list.
The empty table in the Legend Editor dialog box is now filled with five categories of Areas.
ArcView classified all the values in the field "Area" into five different classes (in square feet) using a default of natural breaks in the data. For example, in this classification, the last (or fifth) class in the table, represented by the darkest red color, includes tracts with Areas between 26186400 sf and 10870208 sf.
(Note: You can use other classification schemes as well. To do so, click on the Classify button on the right in the Legend Editor dialog box to bring up the Classification dialog box.
Here you can change the "Type" of classification by selecting a different choice from the drop-down list.
You can also change the "Number of classes" to be displayed by selecting the number of classes that you want in this window. The default for the number of classes in ArcView is 5.)
The five different area classes will be automatically assigned distinctive color symbols of varying shades of red because ArcView chose the "Red monochromatic" scheme to classify the data.
The name of this color scheme is listed at the bottom of the Legend Editor dialog box in the small box to the right of the category "Color Ramps."
Look at this table. There are two column headings, "Value" and "Label."
The information in the "Value" column is what ArcView uses to classify your data after it makes its calculations. You can also change the numbers in the classes to produce your own categories. For example, if you were examining slopes, you would want "Value" categories such as 0-10, 11-20, 21-30, etc., not categories such as 0-8.654, 8.655-11.124, etc. that ArcView might produce.
The data in the "Label" column are what will appear in the map in the Table of Content to the left of the map (its legend).
We should provide better labels to the data, such as adding commas and "sf" (for "square feet") and deleting decimal places. This would give a more professional appearance to our map, so go ahead and do this step by typing them in as I have illustrated below.
(NOTE:When you click on the data in one of the labels to edit it as instructed above, the entire label is highlighted in black. Do not start typing right away as this will delete the data in the label. To edit the label without deleting the data, click on it a second time to remove the black highlighting!)
Your Legend Editor should look like this when you finish ("sf" at the end of the lines is obscured in the picture).
(Note: You could also change the Labels to non-number categories such as "low," "medium," etc.)
You can also change the color scheme used to represent the five classes of Areas by using the "Color Ramps" at the bottom left of the Legend Editor dialog box. Click on the arrow to the right of the "Color Ramps" box (which now has "Red monochromatic" in it) to reveal the drop-down menu and scroll down and select "Purple monochromatic".
You can experiment with other color schemes available under "Color Ramps."
(Note: you can also change the color of any of these five symbols individually by double-clicking on its color under the heading "Symbol" in the same way that you changed theme colors before.)
Now click on the Apply button on the bottom right of the Legend Editor dialog box to apply this new color classification scheme to our map. Close the "Legend Editor" dialog box to reveal the new color scheme on the St. Catherine map, which should now look like this, with the spatial distribution of values for areas in tracts clearly illustrated.
Remember that before in the Table of Contents we just had the term "Stcath_s" with no other information. Now the five classes of Areas have been added below the term "Stcath_s," with both the five colors and the five categories of values of areas in square feet.
Note: on your screen, you may not at first be able to see all the numbers in the five new categories of Areas under the Stcath_s theme as you see here. If you want to reveal the complete numbers, you need to drag the right hand border of the Table of Contents to the right with your cursor.
There is still one problem here. The current theme title--Stcath_s--no longer reflects the new categories that we have created. We need to change the title of our theme in the Table of Contents to "St. Catherine Tract Areas" to reflect the new values.
To change the title, go to the buttons at the top of the window
and click the Theme Properties button 
,
which brings up the Theme Properties dialog box.
At the top, where it says Theme Name it now has "Stcath_s." Delete that title with your cursor and type in "St Catherine tract areas."
Then click on the OK button and notice that the new theme title appears in the Table of Contents.
We could change the theme "St Catherine tract areas" to create a map of soil texture by using the Legend Editor dialog box and changing the "Classification Field" to "Texture."
But we want to keep our theme "St Catherine tract areas" in the Table of Contents in order to save the changes that we have made to its legend; if we leave it in the Table of Contents, we can quickly display it later when we want to do so.
To keep "St Catherine tract areas" in the Table of Contents and add another theme from "Stcath_s" showing the distribution of soil texture, click on the add theme button and select "stcath_s," which will appear at the top of the Table of Contents.
Now uncheck the box next to the theme name "St Catherine tract areas" in the Table of Contents so that the theme is no longer displayed in our map window.
Double-click on the name Stcath_s in the Table of Contents to bring up the Legend Editor dialog box and click on the drop-down list for "Legend Type" and select "Unique Value" from the drop-down list.
From the "Values Field" drop-down list, select "Texture."
The Legend Editor dialog box should now look like this.
Note that in the dialog box there are several categories. We could change any Symbol individually, as discussed previously.We can edit items in the category "Label" just as we did above (in this case, though, it is not necessary). There is also a column labeled "Count" which provides information on the number in each category. We could also change the "Color Scheme" specified in the bottom of the Legend Editor dialog box if we wanted, as we did previously.
Click on the Apply button and change the title of our new theme to "St. Catherine Soil Texture." Our map should look like this ( be sure to click on the box to the left of our new theme in the Table of Contents):
First, click on the check box for the theme "St. Catherine Soil Texture" in the Table of Contents so that it is not displayed on the map.
We are now going to focus on the Bushy Park area. Add the theme "pestcount.shp," make this theme "active," and click on the "Zoom to Active Theme(s)" button. Also click on the box for Bpkcrops.shp in the Table of Contents to make it appear on the map as well, which should now look like this:
Double-click on the theme name "pestcount.shp" in the Table of Contents and select "Graduated Symbol" from the "Legend Type" drop-down list.
From the "Classification Field" drop-down list, select "Pest_count."
Now we have graduated or proportional circles to display values, in this case the number of pests, at particular points.
Note that at the bottom of the Legend Editor dialog box is the category "Symbol." If you double-click on the circle symbol inside the box next to "Symbol," you can change the shape and size of the symbol using the Marker Palette window.
(You can also change the color of the circles using the Color Palette window, as we did previously with polygons and lines.)
My map now looks like this, with the distribution of values at particular points clearly displayed.
Now, go to the Table of Contents and scroll down to "Bpkcrops.shp". Double-click on the theme name "Bpkcrops.shp" to bring up the Legend Editor dialog box. Classify the tracts according to the variables "Capability"
and then "Landuse."
Which variable or field seems more closely associated with the spatial distribution of pest-count scores? This is a very simple example of using a polygon theme and a point theme together to examine possible spatial relations.
You can also use graduated symbols with line themes, such as roads and rivers. The thickness of the lines representing them would be proportional to the type of road or the size of the river.
You create a dot map by bringing up the legend editor dialog box and selecting for the "Legend Type:" "Dot".
To portray a dot map on top of a polygon map--which allows us to see the spatial relationship between the two--you need to load the same shapefile twice. Thus, in the case below, I loaded "bpkcrops.shp" twice for each map. The polygon theme below in the Table of Contents represents landuse for "bpkcrops.shp". The theme above it portrays a dot map for pest populations created from "bpkcrops.shp" so that you can clearly see the association between the dots for pest populations and landuse.
When you create dot maps, you need to click on the Calculate button in the middle right of the Legend Editor dialog box to allow ArcView to determine what amount each dot on the map will represent. The Density Field drop-down list in the middle of the dialog box contains the various fields in the theme that have quantitative data that can be represented by dots; you need to choose the field that you want to portray.
The two maps below represent the comparative growth of the pest populations during two time periods in relation to landuse. Note that in the Table of Contents, the value of each dot is provided as a key. In this case, each dot equals 5 insect pests.
The greater the density of dots in an area, the higher the population in that tract. Which types of landuse are associated with the most rapid rate of pest population growth?
Now we are ready to print our first map.