1. Unit Four: Symbols In this unit… Types of Thematic Maps
Modifying Map Legends
This unit will introduce you to tools that help you to modify the legend.
The legend is a critical part of the design of any mapping product. Careful design of the colors, line weights, and choice of legend type will help you to make maps that clearly communicate the relationships between the various data you employ.
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2. Review Data Representation “Data Frame” “Layers” Organizes “Layers”
Multiple Data Frames possible
Yellow icon
“Layers”
Represents
symbolized spatial data
Located within Data Frames
Use “context menu” for managing properties
Draw in TOC order (bottom to top)
Data is organized in the Table of Contents (TOC) by Data Frames and by different types of Layers (e.g. point, polygon, line).
The order that the layers appear in the TOC is the order that they will draw as a map. Features “on top” may entirely or partially block those below. Manipulating the way these layers lie in relation to one another is a key component to making the map communicate effectively.
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3. Review Change the Legend from the TOC symbol Right Click Left Click
Click the symbol – not the name. This works ANYWHERE you see a legend symbol
Get this when you
Right Click
There are several windows behind the TOC. We have already opened the LAYER CONTEXT MENU, and from there, the Layer Properties dialog is available.
The LAYER CONTEXT MENU is opened with a right-click on the name of the layer. In contrast, the two dialogs above are opened by clicking on the symbol itself. Both are available indirectly by drilling down into the LAYER CONTEXT MENU.
Note that both the Symbol Selector dialog and the color palette shown above can also be opened from the Layer Properties dialog, but you have to drill down to get them.
clik!
Get this when you
Left Click
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4. Modifying the Legend
Types of Thematic Maps: There are several types of thematic maps as is illustrated in the AV 3.x help files:
Single Symbol
Chart
Dot Density
Unique Value
Legends are often dependent on the values found in the attribute tables.
Imagine a map of the nation that shows the larger cities as larger symbols.
In order to know which cities to show as larger symbols, ArcMap needs to see the population of those cities, or their area, or the average height of their buildings, or any other value that defines “larger” within the meaning of your map.
These values are found in the attribute tables of the data you are symbolizing, and because the features in the map are connected to that attribute data, ArcMap knows what features to make larger, or redder, or more numerous.
It is the link to the attribute table that allows GIS data to be represented in this way.
Changes to the attribute values are reflected in the geospatial data in the Display Area whenever they are changed.
Graduated Symbol Graduated Color
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5. Modifying the Legend Types of Thematic Maps: Unique Value(?)
(one layer)
Single Symbol
(three layers)
When you choose a legend type for any particular layer, the legend in the TOC displays the chosen symbologies.
You can’t really tell from the TOC representation of the legend whether the legend type is graduated color, graduated symbol or unique value…
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6. Specify legend type here.
Modifying the Legend
Layer Properties
“Symbology” Tab
Specify legend type here.
clik!
To alter these symbologies, you need to access the Layer Properties dialog. This is the window that is teeming with functions that you will need over and over again – in this case we access the Symbology Tab.
Remember that the Layer Properties dialog has a context of Layer… you need to right-click on the name of the layer (not its symbol) to access Layer Properties
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7. Modifying the Legend Layer Properties “Symbology” Tab
The dialog is very helpful in illustrating the difference between the legend types.
There are a host of features available in this dialog. The instructor will demonstrate these features.
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8. Your turn…
Chapter 4: Step 1 through 7
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9. Other Symbology Choices
Some methods for reducing symbol classes:
Removing the value/symbol from the legend
Making the symbol fully transparent
Redefining the layer altogether.
The symbology in the layer properties is only one of the ways to manipulate how data in the layer are seen.
The layer’s properties also allow the data available in the layer to be filtered by a tool called “Definition Query”
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10. Other Symbology Choices
Layer Definition Query:
a way to show only the results of the query
Definition Query: “NAME” = ‘Durham’
Only Durham will be visible in the layer
because the records for all other towns don’t meet the query
Definition Query looks a lot like the Query Builder.
With Query Builder, the result (features) is SELECTED
With Definition Query, the result (features) is VISIBLE
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11. Other Symbology Choices
Think about this…
Data Attributes:
[Area] & [Perimeter]
Properties:
Fill, Outline, Color
In Polygon data:
The “data” is about the area bounded by the outline.
Values related to “area” and “perimeter” are of primary interest.
Outline of the area is not “data” - except that it is simply the edge of the area!
In Arc data:
The “data” is about the line. The areas that might be bounded by that line are simply not relevant.
Data Attributes:
[Length]
Properties:
Line Weight, Color
A potentially confusing aspect of GIS data is how the type of data relates to the symbology. For example…
POLYGON data is used where the primary features of the data are related to the areas: e.g. lakes, towns, or soil units
ARC data is used where the primary features of the data are related to lines: e.g. roads, streams, or shorelines(!)
But some of these data may coincide (geospatially speaking), as when shoreLINES are a layer shown corresponding to the EDGE of lakes.
Lake data are polygons that have an outline that corresponds to the shoreline
But shoreline data can be delivered (as it is via GRANIT) as arc data.
Data that corresponds geospatially is said to “have topology”. You will have to keep track of the type of data that you are using and what it represents when you are making decisions about when to limit the data that is viewed using the techniques in this chapter… for example:
by layer definition which affects only “data” or
by manipulating the symbology which can affect either the “data” OR the properties used to represent it.
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12. Chapter 4: Step 8 through END
Your turn…
Chapter 4: Step 8 through END
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