Part 3—Generate Time Series Plots

The rate of station motion is a measure of its change in position over time. Given that your spreadsheet describes a series of positions at different times, it seems that it would be simple to calculate the rate of station motion using the standard distance formula (Distance = Rate X Time). However, as daily positions of the station show substantial variation, the long term trend of the station's change in position is a much more valid measurement of a station's motion.

To calculate the velocity of a station, the GPS data are plotted into graphs comparing position to time. This type of graph is called a Time Series Plot (TSP). The slope of the trend line of data in a TSP reveals the velocity of the station in a specific direction.

In this activity, you will construct a North TSP and an East TSP to calculate the station's velocity in each direction. You'll combine these two velocity vectors to calculate the total horizontal motion of the station. The vertical motion of the GPS station will not be explored in this activity; however, it is a useful parameter for analyzing crustal movement near glaciers, volcanoes, calderas, and normal and reverse faults.

1. Save your spreadsheet from Part 1 as an Excel Worksheet (.xls)
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   Choose File > Save As... In the dialog box, choose a location to save the file on your computer and use the pull-down menu to change the format to Excel Workbook.
   
   
2. Choose Edit > Move or Copy Sheet... and copy your current worksheet onto a new worksheet for making graphs.
   
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3. On your second worksheet, delete the top 8 rows of information to make it easier for you to generate graphs.
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   Click and drag your cursor in the number column at the far left to select rows 1 through 8, then select Edit > Delete.
   
   
4. Apply appropriate formats to the first four columns of data.
   • Click in a column header to select the column.
   • Choose Format > Cells...
   • Select Date for the first column with Type: mm/dd/yy, and Number, with 2 decimal places for the next three columns.
5. Save your file again.

1. Click the column A header to select the Date column.
2. Control-click (Command-click on a Macintosh) the column B header to add the North column to your selection. NOTE: When you come back to these instructions for the East Time Series Plot, use this same method to select two columns of data that are not directly next to one another.
3. Click the Chart Wizard icon or select Insert > Chart...
4. Make a graph that shows Dates on the X axis and North Positions on the Y axis. Choose the XY (Scatter) under Chart Type and the first example under Chart Sub-type (scatter). Click Next. NOTE: Depending upon your version of Excel, the directions may look slightly different for selecting Scatter Charts.
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Based on your data selection, Excel's Chart Wizard tries to interpret what kind of graph you'd like to create. Sometimes it is right, but it can also be wrong. If you don't end up with the graph you expect, you'll need to come back to this point and inspect the cell addresses of the Series that will be plotted on the X and Y axes.
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In Step 2 of 4 of the Chart Wizard, the Data Range tab will show the cell addresses of all the cells you selected. For the North example, this includes addresses in columns A and B. As your data are in columns, the Columns radio button should be selected.
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Still in Step 2 of 4, click the Series tab to check or modify which data series will be plotted on each axis. You want to plot Date values (cell addresses from column A) on the X axis, and North positions on the Y axis. NOTE: In some versions of Excel, the data range and cell addresses can be checked or adjusted by selecting Chart > Source Data...
Show me a sample of the data series (cell addresses) for each axis
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5. In Step 3 of 4 in the Chart Wizard,
   • click the Titles tab and give descriptive names to your graph and both axes.
   • under the Legend tab, uncheck the Show Legend box as legends have no meaning for scatter plots.
   • choose your own preferences for options under the other tabs.
6. In step 4 of 4 in the Chart Wizard, click the radio button to place the chart in a new sheet and click Finish.
7. Take a look at your graph... The X axis may appear in the middle of the graph, but if you have Dates along the X axis and North positions on the Y axis, choose File > Save to save your work.
   Show me a sample graph to check my work
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   If your graph shows something very different from the one below, back up to the troubleshooting tips under the blue box above. Enter the data series manually to generate an appropriate graph.
   

   

   
   
   
   

1. Change the placement where the X axis crosses the Y axis.
   • Double-click any number on the Yaxis.
   • Click the Scale tab.
   • For the Value (x) axis crosses at, type in the minimum Y value from your graph, for example -10, then click OK.
2. If necessary, adjust the number format and grid units of the X axis.
   
   
   • Double-click any date on the X axis of your graph.
   • If your dates appear in an odd format, click the Number tab and select Date. Under Type select the mm/dd/yy option.
   • If your data are 'squished' in the middle of the graph, click the Scale tab and change the values in the X axis scale so the data fill the graph.
   • Make any other adjustments as desired to change the appearance of your graph then click OK.
3. Save your file.