Creating Thematic Displays

Thematic display is a wonderful tool for producing clear, straightforward visual presentation of nongraphic data as entities with a spatial dimension. Using the data structures that define the relationship of spatial and nonspatial data in the relational database, thematic display literally translates attribute data, for viewing purposes, into a spatial display. Attribute data are displayed as spatial data using the linkage between geometries and attributes stored in the data warehouse.

A thematic display is just what the name suggests. It is a graphic representation of geographic features, the display, based on nongraphic attributes, the theme. Thematic display symbolizes the attribute values of spatial features through the use of color or patterns. The ability to graphically display nongraphic data provides you with a powerful tool for visualizing the spatial relationships among the attribute values of a feature class or a query.

Furthermore, the representation of those relationships is maintained dynamically. That is, when warehouse data are changed, the changes are reflected (updated) in the thematic display automatically. The following illustration shows an example of a thematic display.

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Data Linkages That Make Thematic Display Possible

The characteristics inherent in the storage of spatial and nonspatial data in a relational database serve as the foundation of GeoMedia’s ability to generate thematic displays. The principles underlying thematic display are based on the relation of spatial to nonspatial data. This important relationship that exists in data warehouses can be viewed from two perspectives. First is the relationship between feature class and attribute values. Second is the relationship between the graphic and nongraphic characteristics of that data.

Thematic displays are easy to create in GeoMedia. However, to fully appreciate what is going on in thematic display, it is important to have a good understanding of data structure. The issues associated with underlying data structures are covered in many other sections in this book. However, because data structure is critical to understanding how thematic displays are created, it is briefly reviewed in the material that follows.

Features and Attributes
in Relational Databases

Thematic displays are built on a feature class, or on a query built on a feature class. Therefore, they are dependent on the relationships among feature class, feature, attribute, and attribute value. A feature class is an object stored in a database as a table consisting of rows and columns. The rows contain individual instances of the feature class (the features). The columns contain a heading corresponding to the name of the attribute that defines what the content of each cell represents. The content of each cell under a given attribute is referred to as the attribute value.

For example, suppose the feature class Parcels is one of several tables in a database named Land Information. The Parcels feature class (the table) contains one record for each parcel (the feature). Each parcel record contains information (attribute values) about a particular parcel, such as parcel ID number, the amount of land the parcel contains, the owner’s name and address, and so on. These pieces of information are stored for every parcel according to the rules of the column headings (the attributes) parcel_id, area, owner_name, and owner_address.

These relationships can be described another way. The feature class is a database table. It consists of individual features stored in the table. Each feature is a separate entry in the table and consists of a collection of attribute values that contains information specific to that feature. The column heading that contains the attribute name and its data type defines what the attribute values represent.

This type of information (parcel IDs, owners’ names, and so on) can be described as "textual" attributes that describe the various qualities of the parcel. They are, by definition, nonspatial or nongraphic. They tell you what, but they do not tell you where.

There is, however, a spatial attribute associated with textual information. Prior to GeoMedia, most systems (MGE, ARC/ INFO, ArcView, and so on) stored the attribute data in a relational database and the geographic (or graphic) data in a different file. In GeoMedia, both the attributes and the geometry are stored in the same database table, but in separate columns.

This association of "what to where" in the database allows the thematic display dialogs to transfer the "what" (the attribute values) to the "where," and to place them in a geographically accurate location in correct relation to other spatial entities.

For the purpose of display, thematics break up the selected attribute values of a feature class into categories. There are two operative concepts here. First, a thematic display is based on a feature class. Named queries can also serve as the basis of a thematic display, by virtue of the fact that they consist of features within a feature class.

Remember, queries are handled in the legend and elsewhere as if they are the feature class from which they are derived. For example, the thematic display in the previous illustration is based on a query that selected all counties in Virginia, North Carolina, and South Carolina. The thematic built on the query displays rainfall data restricted to the counties specified by the query.

Likewise, another point to bear in mind concerning feature class and thematics is that the display generated by a thematic is based on the geometry of the selected feature class. Controlling the display of map objects that constitute a thematic display is governed by the same rules that control the display of the underlying feature class geometry. In other words, point geometry can be viewed as dots, symbols, or bitmaps (see the following illustration). You can specify the display properties of area and compound geometry features in terms of area boundaries, area fills, and so on.

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The second concept is that thematic displays are structured on the data contained in the selected attribute column for the feature class. The values contained in that attribute column are used to calculate the categories for a range display or to assign symbology to attributes being viewed in a unique thematic display. Therefore, the categories themselves derive directly from the attribute values.

Range and Unique Thematic Displays

The process of generating a thematic display is one of clumping or reclassing attribute values into user-defined groups. Say, for example, you want to view average county rainfall by range. You would specify the number of ranges and the Map by Ranges dialog would calculate the range for each of the categories. Each feature would then be assigned to one of the ranges based on its rainfall attribute value.

In unique thematic displays, each unique attribute value stands alone and is assigned individual symbology that distinguishes it from the other attribute values. This thematic produces slightly different display output as a function of the attribute data used as input. For example, say you are generating a thematic display of the unique values based on the land use attribute for a parcel feature class. There are a total of 10 land use codes that are valid for each of the 100 parcel features in the table.

Each occurrence of a new land use value is assigned different display symbology, but all features that share that value are assigned the same symbology. In the resulting display, all Residential parcels are yellow, all Vacant parcels are blue, all Improved parcels are green, and so on. The 100 parcels would be divided into 10 "unique" categories. If the feature class road was selected, number_of_lanes might be the attribute used to define three unique symbols for two, three, and four lanes.

In a case in which the selected attribute value is truly a onetime occurrence, each category will contain one feature. If, for some reason, you wanted to build a unique thematic display based on the state name attribute in a United States coverage, the resulting map would contain each of the 50 state names, displayed in different colors. The availability of either type of thematic (range or unique) is a function of the data type of the attribute. Numeric data can be broken into either range or unique displays. Character data can be displayed using the unique option only.

Both unique and range displays are restricted to 300 categories. If the number of different attribute values exceeds 300, you are informed that the limit has been reached and that a thematic display cannot be generated for the attribute. As a general rule, however, people have a difficult time discriminating subtle differences in colors on maps. It is probably a good idea (when possible) to keep the number of categories down to under 10 or so.

Thematic Displays Are Dynamic

Thematic displays are stored in the GeoWorkspace as long as they are not deleted from the legend, and as such are reusable. Your thematic displays are affected by additions, changes, and deletions in the feature class. They are dynamic in response to changes in the data warehouse. Both range and unique thematics automatically update to reflect changes made to the underlying feature class. The display update behaves slightly differently, depending on which type of thematic has been generated for the feature class.

This is a very important concept. Suppose you have displayed a thematic map of the parcel values and someone in the Assessor’s office updates the information about a parcel to reflect a much greater assessed value. Your display of the map can change. Exactly when this change takes place depends on the type of warehouse connection you are using. A thematic display of data in a read-only connection is updated when the GeoWorkspace is opened. A thematic created from data in a read/write connection is automatically updated if the changes to the source data are made from the local client machine. If edits to the data are made on a different machine the thematic display will update when the GeoWorkspace is opened or when you select Warehouse > "Refresh with warehouse changes".

Making the Most of Thematic Displays

The "mix and match" relationships among feature class geometry and attribute values processed with range or unique methods allows you great flexibility in designing visualization tools with thematic displays. Thematics provide a tool for sub-setting your data (that is, for classing features that in reality are members of the same feature class) on the basis of an attribute value.

For example, assume you have a feature class consisting of road data, one of whose attributes is number of lanes or CFCC code. When you add that feature class to the legend, you are restricted to specifying symbology for all road graphics in the class. However, with a unique thematic display based on the number of lanes or the CFCC code attribute, each road type is sorted out into a separate category. You have then effectively "converted" each road type into a separate category with distinct display symbology so that interstate highways, four-lane roads, and two-lane roads each has its own color and weight.

Dynamic Thematic Display Updates

Thematic displays (like other displays) are dynamically linked to the data in the Access warehouse upon which they were based. If the data in the warehouse is changed, the display will update. When features are added to a class for which you have a unique thematic, the new feature is automatically added if the appropriate category already exists. The category count is then incremented. However, if a new feature introduces a new category, you need to access the Unique Values dialog and click on OK to get it added to the thematic.

Remember that an entry in the database that adds a new category will not be reflected automatically. You will need to access Unique Values manually. On the other hand, changes to the attribute value for a feature will cause the thematic display of the feature to be moved to the appropriate existing category. The category counts are adjusted to reflect a feature’s movement into another category, and the feature is redisplayed with the correct symbology for its new category assignment. Deleted features are removed from the category and the map window display, and the category count is decremented.

Updating range displays with newly created features works much the same way. A new feature is assigned to its appropriate range if one exists, the range count is incremented, and the graphic display is updated. Changes and deletions work as previously described for unique displays. Suppose you have a map displayed showing all the parcels in an area in four value categories, with four different colors. If the value for one parcel is changed to such a degree that it is in a different category, GeoMedia will update the display to show it in the new color.

One key aspect to remember is that updating or adding/ deleting feature attributes does not change the range boundaries. If the value for a new or modified feature falls outside the existing ranges, the feature is added to the Other values category. This is the significant difference in the update process for ranges versus unique displays. The new value will be assigned to the Other values category until you access the Map by Ranges dialog, reselect the Range technique, and click on OK. In the previous example, if the new parcel value were outside the existing ranges, it would be dumped into the Other values category.

Adding a Thematic Range Display

Thematic range display is a good tool for visualizing the spatial distribution of continuous data. It is appropriate for attribute values that can be categorized as falling within a subset of a range of values. The attribute used as input to the Map by Ranges dialog is restricted to attributes with a numeric data type. Character data cannot be displayed in ranges. The total number of ranges you can generate is limited to 300 categories.

Thematic displays are very easy to understand when you see their components in action. To that end, this section will step through the commands needed to build a thematic range display.

Thematic displays are added to the legend by clicking on Legend on the Main Menu bar and selecting Add Thematic from the drop-down menu. The Add Thematic Entry dialog is shown in the illustration that follows. From this dialog you select a feature class from the Feature drop-down menu. Notice that the connection name prefixes the feature class name. This is because the Prefix option was selected on the General tab of the Tools > Options dialog.

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Map by Ranges

The Map by Ranges dialog, shown in the following illustration, is packed with commands for customizing thematic displays. You can specify the range boundaries, legend entry properties, display symbology, and so on.

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Range Technique

The first field in the dialog allows you to specify one of four predefined ranging techniques. You can also set any specific range limits you desire. Equal Count places an equal number of features in each range. If you create a thematic map using the USSampleData included with GeoMedia and base it on population, the equal count will place 12 states in each of the four (default) ranges and set the range boundaries at the values needed to get this distribution. The Equal Range method builds ranges of equal value size, then distributes the features among them.

With the U.S. data, the maximum population is 29,760,021 and the minimum is 453,588, which gives a range of 29,306,433. Dividing this into four means that the each range should be 7,326,608 between the lower and upper range boundaries. The first range starts at the minimum (543,588) and goes to 7,780196 (or the minimum plus 7,326,608). Using Equal Range may mean that many features fall within one range. In the U.S. example, 40 of the states fall within the lowest range. With Standard Deviation, the standard deviation of all data values is computed and distributed across the number of ranges.

With Custom Equal Range, you can define the start and end values of an equal range distribution of values. Data values that do not fall into a defined range are placed in the Other values category. With the U.S. population data, you might be interested only in states with a population between 10,000,000 and 20,000,000. You would select Custom Equal Range and type in these numbers. Note that the minimum and maximum values for the data selected are preloaded in the dialog box. You can simply type over one or both.

Standard Deviation

When you select Standard Deviation, the mean of the value ranges is placed in the center range, and one standard deviation subtracted and one added to make the center range two standard deviations about the mean. Ranges above and below the center range each encompass one standard deviation. The mean state population of the U.S. data provided with GeoMedia is 5,134,262, and the standard deviation is 5,448,442. The selection of Standard Deviation and three ranges for this data means that the center range goes from -314,178 to 10,585,707 (or one standard deviation above and one below the mean). The lowest range is -5,762,620 to -314,178, and the upper range is 10,582,707 to 16,031,149. These represent one standard deviation ranges.

Setting Customized Range Boundaries

In addition to using the predefined range boundaries you may also enter your own ranges. You can do this two ways. You can simply use the mouse to select one of the range numbers, type in the number you desire, and press the Tab key. Both the entry you changed and the corresponding entry in the box above or below (if there is one) will update. Note that you can type over the entry and press the Return key. If you do this, the entry will update but you will be returned to the initial Add Thematic Entry dialog box. You can also interactively change the range boundaries by using your cursor to select the range boundary on the Count Bar, as described in material that follows.

Number of Ranges

The Number of Ranges drop-down menu allows you to specify from 2 to 22 ranges for the thematic. The default number of ranges is four, yielding a quartile map. There must be enough variation in your data to support the number of ranges you select. If there is not, Map by Ranges will automatically set the number of ranges to the maximum the data can support.

For example, if you choose six ranges but your data can only be divided into four, Map by Ranges will force four into the dialog. At a minimum, the selected attribute must contain enough variation to support at least two ranges. If it does not, the process will fail, giving you an error message. You are then prompted to select a different attribute for the display.

Count Bar

The Count bar on the far right of the dialog graphically illustrates the results of applying the combination of the selected range technique and number of ranges to your data. It is broken into the selected number of ranges and shows the size of the ranges relative to one another. It displays using the active color palette. If you wish to change the boundaries of any range, you can do that by placing the cursor on a range boundary on the count bar and moving the bar up or down. The counts in the dialog automatically update as you move the bar.

Color Selection

The are several approaches you can use to assign colors to your thematic displays. Color and symbology are important in thematic displays because they are the "expression" of the attribute values being mapped. The first method for assigning color is via the color selection tools provided in this dialog. The second method is to manually assign colors using the Style Definition dialogs.

The Color selection portion of the dialog consists of two buttons. On the left is the Rotate Color Schemes button and on the right is Ramp Assigned Colors. Clicking on the Rotate Color Schemes button cycles you through set of predefined color schemes. They are displayed on the Style buttons in the middle portion of the dialog and on the Count bar. When you click on the Ramp Assigned Colors button, new colors are assigned to produce a smooth color transition that starts with the top color on the style display and ends with the color assigned to the last range.

For example, if the first color were white and the last color black when you clicked on Ramp Assigned Colors, the intervening ranges would be assigned new colors scaling from white to black in increasingly dark shades of gray. Assigning colors to categories and then using Ramp Assigned Colors means that you can create any color scheme you want.

To assign your own color to a range, simply double click on the Style key (the small button to the right) for the range. You are taken to the Style Definition dialog for the appropriate geometry type. You have access to the complete functionality available through the Style dialogs. This allows you to not only assign any custom colors you have defined but assign styles such as symbols and bitmaps for your point type data, and special weights and styles to your line work.

Remember, GeoMedia can create thematic maps for line and point feature classes as well as areas. The thematic display in the second illustration in this chapter is an example of the use of those capabilities. The legend for the thematic contains increasingly large dots for the first four categories, a circle symbol for the fifth entry, and increasingly large circle symbols for the last three entries. It was produced by specifying the Map by Ranges settings shown in the following illustration.

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Using Styles in Thematic Displays

When you access the style options by clicking on the style button, you have a tremendous range of style options available to you. There is a complete description of the use of point, line, and area styles in Chapter 12. For area features, the first dialog box asks for area boundary styles. When you first access this dialog, it defaults to a single line boundary, and you can select line color, style, and width.

You may wish to have a more complex border, however, which you can choose by selecting the multiple line option for line type. When you do this, the lower portion of the dialog box becomes accessible and you can select the line width, style, and color for a second set of lines. The line you defined in the top of the dialog box will now have a pair of outer lines. Suppose you choose a thick, dashed red line from the first (top) dialog and a thin, solid black line from the lower one. All areas associated with this style would have a boundary that consists of three lines: two outer thin black lines between which is a dashed red line.

An area feature can also have a number of patterns applied. These include basic ones such as hatching as well as symbols. To access the patterning you must first click on the style button and then select the Area Fill tab. On the Area Fill dialog, patterns and symbols are selected as a Secondary Fill option. Clicking on the right arrow on the Secondary Fill dialog will display a series of patterns. You can choose any one. At the bottom of the list is the Symbol option. Selecting it moves you on to another dialog box that allows you to determine which symbol you want. The entire area of the area feature will be filled with the symbol.

A key aspect to decide is whether you want the symbol to scale or be scale independent. If you are not sure, there is a full discussion about area filling and how to use symbols and patterns in Chapter 12. Line features can also be symbolized using symbols. You must select the pattern option from the drop-down to access the symbols that can be used.

Statistics

The Statistics button in the top portion of the dialog gives you a read-only display of raw statistics for the selected attribute value. This display, shown in the following illustration, is provided to help you determine what the best range settings are for your data.

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Sort Order

The Sort Order portion of the dialog allows you to reverse the order of the range categories. Changing the order switches the order of the entries, including their assigned colors. The default sort order is ascending.

Settings

The Settings portion of the dialog is used to specify parameters that affect the appearance of the legend entries for the range display. The results of changes made with the Statistics and Label drop-down lists are displayed in the middle of the dialog box.

The Statistics drop-down allows you to select a method for showing your range statistics. This material appears on the legend when the Show Statistics box is checked on the General tab of the Legend Properties dialog. Your choices for statistics include Count, which displays the number of records included in each range. Count% displays the records in each range as a percentage of the total number of records. Range and Range% work similarly, based on the actual range extent. For example, if a category ranges from 52 to 10,430, the Range is equal to 10,378 and the Range% is the percentage of the total range represented by that category.

The Label drop-down contains two choices: Range Extent and Description. The selection for this field determines how the category titles display in the legend. Range Extent shows the starting and ending values for each range in separate fields. Description displays the same information but places it in one field. Although both of these label types are editable, when you need to display something other than the range extents in the legend, Description is the label type to use.

Collapse legend entry is a checkbox that allows you to display the thematic name in the legend, but suppress the display of the category headings. The Display checkboxes are used to turn off the display of the associated category. It is possible for range displays to contain ranges composed of no records. In this case you might wish to specify that the category not be included in the thematic display.

The box at the bottom of the dialog with the default entry "Other values" is the catchall category for any attribute values not included in one of the defined ranges. You can choose to keep the label "Other values" or enter a different label that is more appropriate for your map. This can result in cases in which you define custom ranges that do not encompass the entire range of attribute values. Records also get assigned to the "Other values" category when changes are made to features in the data warehouse. When records are modified or added, their values may fall outside the defined ranges. When this occurs, it is necessary for you to rerun the Map by Ranges dialog. This forces the recalculation of ranges and updates the thematic display to include the new values.

When dealing with floating point data in range displays, it is possible to have attribute values that fail to appear in your range display. This happens most often with highly precise data provided from servers, which do not allow control of the number of places of precision for floating point numbers. The starting value for a range is calculated by adding 1 to the maximum value of the previous range in the sequence. This works well for integers, but can cause problems with floating point numbers.

Floating point ranges are calculated based on the display precision of the field according to the metadata. For example, if display precision is two decimal places, and the first range is 1 to 10, the next range in the sequence will start at 10.01. If your data contains an attribute value of 10.005, it will fail to be included in the range from 1 to 10 and will also not fall within the 10.01 to 20.0 range. It will be dropped into the Other category. This situation can be resolved by reducing the precision of your data. If you have read/write access to the data and can afford to sacrifice that degree of precision you can (for example, in an Access database) change the data type from single to integer.

Working with Thematic Range Displays

Attribute data associated with feature classes or named queries can be graphically displayed in GeoMedia by means of a thematic display. The thematic display arranges all database values for a particular attribute according to user-defined specifications for ranges and symbology.

In this exercise you will create a thematic display of the number of housing units by census tract in Washington County. Connect to the Census database and the tracts_housing_stf3a feature class.

Adding a Thematic Unique Display

Unique thematic display is a useful tool for graphically representing discrete attribute data. This is data that does not easily lend itself to ranging techniques. Contrast, for example, an attribute that contains zoning codes to one that contains a population count. Population figures retain their meaning when grouped into ranges such as 1 to 100 or 101 to 200.

A set of attributes such as zone codes AI, AV, RI, CI, and so on are not amenable to such treatment. Unique thematic displays allow you to effectively handle the display of this type of data. Assigning distinct symbology to each of the zone codes allows you to visualize the spatial distribution of each unique type.

Unique displays can take either numeric or character type data as input. Like range displays, they are limited to a maximum of 300 categories. Unique thematic displays are set up in much the same manner as range displays. In fact, the Unique Values dialog contains fewer options than does the Map by Ranges dialog. Unique Values, like Map by Ranges, is accessed from the Add Thematic Entry dialog. The illustration that follows is a Unique Values dialog based on the attribute values in the zipcode field of the parcels database.

Color Selection

The Rotate Color Scheme and the Ramp Assigned Colors buttons work just as they do in Map by Ranges. The Rotate Color Scheme button becomes a very important tool when you are dealing with a large number of categories. In the previous illustration, there are 57 unique zip code values. Assigning colors to these by hand would be quite time consuming. It is much easier for you to use one of the schemes provided. An individual color in the scheme can be changed by clicking on it and modifying it in the Style Definition dialog. Again, the tabs available in the Style Definition dialog are dependent on the geometry type of the feature class to which the selected attribute belongs.

Sort Order

You can specify ascending or descending sort order for your attribute values by selecting the appropriate sort button.

Statistics

In the Unique Values dialog you are limited to two types of statistics for the legend entry for the thematic. These are Count and Percentage. Count tells you the total number of attributes that share a value. Percentage expresses the category count as a percentage of the total number of records in the feature table.

Label

The Label box contains two selections: Values and Description. The Values selection will place the attribute value in the legend as the name of the category. It is a read-only field and cannot be edited. When you need to enter category titles other than the actual attribute values, select Description and type in the new labels.

The bottom portion of the dialog is very similar to the Map by Ranges dialog. The display of any category can be suppressed by unchecking its display checkbox. The Statistics column is a preview of what the legend entry will contain if the General tab of Legend Properties has Show Statistics enabled. The Label field will contain either the attribute value or a user-defined label, if one has been entered. The Style key will take you to the Style Definition dialog. The workflow that follows takes you through the process of working with thematic unique displays.

Working with Thematic Unique Displays

Using Unique thematic displays, you can view the distribution of character type attribute data. In this exercise you will generate a map that displays all parcel usage codes for the assessor_data feature class. For this workflow you need access to parcels.mdb. Create a new GeoWorkspace and open a connection to the parcels warehouse.