Mapping Earthquakes in Northern California

Table of Contents

  1. Mapping Earthquakes in Northern California
  2. Setting Up Your Workspace
  3. Downloading Data from Natural Earth
  4. Download Data from the Northern California Earthquake Data Center (NCEDC)
  5. Skill Drill: Download Data from the United States Census Bureau
  6. Adding California as a Basemap Layer
  7. Skill Drill: Adding the Humboldt County Boundary as a Basemap Layer
  8. Adding XY Data
  9. Changing the Map Projection of the Data Frame
  10. Representing Earthquake Magnitude Using Graduated Symbols
  11. Changing the Map Size and Position
  12. Creating an Inset Map
  13. Inserting A Map Title
  14. Inserting a Map Legend
  15. Inserting a North Arrow, Scale Bar, and Acknowledgments
  16. Exporting your map as a PDF file

Inserting a Map Legend

A map legend, sometimes called a map key, helps to explain the symbology on the map. Not all maps need a map legend, as some simple features such as water bodies and roads may be self-explanatory. However, maps that contain thematic data, such as this earthquake map, will need a map legend.

From the main menu, select Insert, then Legend. The Legend Wizard appears. By default, ArcMap will try to add all your layers to the map legend. In this instance, you only need the earthquake data. Click the button that has the double arrow pointing left to remove all of the layers.

An image of the legend wizard default layers
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Then, under Map Layers, select the earthquake layer and click the button with a single arrow pointing right. When you are ready, click Next.

An image of the Legend wizard add single layer
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Never use the word “Legend,” or a similar word, as a legend title. Your map readers already know that they are looking at a legend. Using this word is redundant, and it will also make you look unprofessional. When choosing a legend title, utilize a descriptive title or leave it out altogether. In this instance, a descriptive title would be useful for explaining the meaning of the different sized circles. Change the title to Earthquake Magnitude. Change the font to Garamond to match the style of the map title. When you are ready, click Next.

An image of the Legend wizard legend title
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Add a 1.0 point border, a gray 10% background, and a gray 50% drop shadow. Change the gap to 5. When you are ready, click Next.

An image of the Legend wizard border and background
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The next window on the legend does not apply to our data. Go ahead and click Next.

An image of the Legend wizard symbol patch
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The last window on the Legend Wizard allows you to change the space between the legend title and the legend items. In most cases, the default settings here work fine. When you are ready, click Finish.

An image of the Legend wizard spacing options
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Your legend will appear near the center of the map. As a general rule, anything placed near the top of the map will rise in the visual hierarchy. Secondary map elements, such as north arrows, scale bars, and legends should be placed somewhere in the lower half of the map if possible. In this instance, you may consider placing the legend in the Pacific Ocean, just to the right of the inset map.

The legend still needs a little more work. The magnitude values have too many decimal places, and you should remove the layer name. Right-click on the legend and select Properties. On the Legend Properties window, navigate to the Items tab. On the lower left, click the button that says Style.

An image of the Legend properties style button
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There are many different legend styles from which to choose. However, the second style down on the left, Horizontal Single Symbol Label Only, will work to remove the layer name from the legend. When you are ready, click OK, Then click OK again.

An image of the Legend Style Horizonal Single Symbol Label Only
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The legend style updates, but the legend labels still have too many decimal places. To correct this issue, you will have to open the layer properties for the earthquake data in the Table of Contents under the California Earthquakes data frame. Then, navigate to the Symbology tab. Near the center of the Symbology tab, right-click on the word Label and select Format Label.

An image of the symbology tab format label option
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When the Number Format window opens, enter 1 for the number of decimal places. When you are ready, click OK, then click OK again.

An image of the Number format window
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You may need to adjust the size of the legend labels. Return to the Items tab on the Legend Properties and change the font size to 12. Take a moment to save your map document before moving on to the next step.

An image of the font adjustment on the Item tab
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