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

Adding XY Data

As you learned in a previous step, a CSV is one of the simplest forms of geospatial data. As mentioned before, each column of data is separated by a comma, and the first row in the table contains the field names. When adding XY data, it is the two columns, Latitude and Longitude, that are necessary to place this information on a map.  When I talk about XY data, the Longitude in decimal degrees is the X coordinate, and the Latitude in decimal degrees is the Y coordinate.

You will learn more about latitude and longitude in a later chapter. For now, just understand that ArcMap will read these numbers, the latitude, and longitude values in decimal degrees, to figure out where to place the points on the map. In ArcMap, from the File menu, select Add XY Data.

An image of the add XY data option for earthquake map
XY Data refers to data with coordinates, such as latitude and longitude. Click to view a larger sized image.

When the Add XY Data window opens, click the yellow file folder icon and browse to the Original folder.

An image of the Add XY data window browse button
The yellow file folder icon allows you to browse to a specific folder.

Choose earthquake2017.csv and click Add.

An image of the earthquake2017 csv file selected in the add xy data dialog window
Be sure to save the CSV file to your Original folder.

ArcMap reads the CSV table and tries to find the XY data automatically. In this instance, it assigns Longitude as the X field and Latitude as the Y field.

An image of the Add XY data window X field and Y field
The most common XY data is latitude and longitude.

The next step is the most important one. Most students make mistakes here. Many people assume that latitude and longitude coordinates are universal. They are not. There are many different spatial reference systems available when you make maps. Different spatial reference systems use different latitude and longitude values. You will learn more about spatial reference systems in a later chapter. For now, just understand that ArcMap does not know from which spatial reference system the latitude and longitude values come. Instead, it tries to guess. The mistake students make is that they allow ArcMap to guess incorrectly. Check to make sure that the Geographic Coordinate System is GCS WGS 1984.

If not, then follow the next few steps to manually set the spatial reference information for the earthquake data. On the Add XY Data window, click the Edit button under Coordinate System of Input Coordinates.

An image of the edit button on the Add XY data window
When adding XY data, always click the edit button. Never accept the default coordinate system.

The Spatial Reference Properties window opens up. Scroll to the top and expand the Geographic Coordinate Systems folder.

An image of the Spatial Reference Properties window
The Spatial Reference Properties window allows you to choose a spatial reference system from two main folders, Geographic Coordinate Systems and Projected Coordinate Systems.

Then scroll down until you see the World subfolder. Expand the World folder.

An image of the World subfolder in the Spatial Reference Properties window
Be sure you are selecting the World folder from the Geographic Coordinate Systems folder.

Scroll down until you see WGS 1984. There are several versions. Select the simple one that only says “WGS 1984.” When you are ready, click OK.

An image of the WGS 1984 in the Spatial Reference Properties window
According to the NCEDC website, WGS 1984 is the correct spatial reference for the latitude and longitude values they provide.

Check to make sure everything matches the settings below. When you are ready, click OK.

An image of the Add XY data window with options for earthquake data selected.
Your Add XY Data window should look like the image above.

You will get an error message warning you that the table does not have an object id field. Take a moment to read the warning. When you Add XY data, ArcMap creates a temporary representation of the data. It might look like a regular shapefile, but it does not have a database. To make it into a permanent shapefile with a database, you will have to export the layer. Go ahead and click OK to close the warning.

Click to view a larger sized image.

All the earthquakes above a magnitude of 3.0 get added to the map. In the Table of Contents, right click on earthquake2017.csv Events and select Zoom to Layer.

An image of the earthquake data added as an events layer
XY data is initially added as a temporary events layer. Click to view a larger sized image.

Make the earthquake layer a permanent shapefile by exporting it. The process is the same as when you exported the California selection and the Humboldt County selection. However, in this instance, with nothing selected, the entire layer gets exported. In the Table of Contents, right-click on earthquake2017.csv Events and select Data, then Export Data. Save the file to your Working folder and name it “CA_earthquakes_2017.” Add the exported data as a map layer. When done, right-click on the Events layer and select Remove.

You should now have four layers in the Table of Contents, the California earthquakes of 2017, Humboldt County, California, and the United States. Take a moment to save your map document before continuing to the next step.