How do I read a point from a curve or line?
Sometimes, in introductory geoscience courses, you are asked to read data from a graph (the graph may have a line or a curve on it). You may have constructed the line or curve yourself (for more information see constructing a best fit line) or it may be a graph that was constructed for you. Either way, the procedure is the same for finding a data point. A quick review of the basics of graphing might help - particularly the tutorial on plotting points since the basic idea comes from this concept.
Reading data from a graph
When should I read a point from a graph?If you are given a value for one of the parameters (or variables) on the graph and asked to determine the value of the other, the question is asking you to read a point from the plot. Remember that variables will be labeled on the axes of the graph (e.g., Epicenter distance and travel time on the graph at the right). In the plot to the right - showing P- and S- wave travel times through the Earth, the x-axis is labeled 'EPICENTER DISTANCE' and the y-axis is labeled 'TRAVEL TIME'. Thus, if you are given a travel time for the P-wave, you should be able to determine the distance to the epicenter. For example, if I tell you that it took 7 minutes for the P-wave (the red curve) to arrive, by the time you finish reading through this page, you should be able to tell me that the epicenter of the earthquake was 4000 km away.
How do I read a point from a graph with a curve or line?Let's use the plot to the right as an example for reading data from a graph. This plot shows travel times as related to distance for P-waves (red curve) and S-waves (blue curve) from the epicenter of an earthquake. P- and S-waves are two types of waves that are generated by earthquakes; P-waves travel faster than S-waves. If you would like a PDF to print out so that you can look at the full sized image, you can download one here (Acrobat (PDF) 149kB Jun19 09).
Some simple steps for reading data from a graphThere are a few simple steps for reading a point from a line (or curve). We will use the following example to illustrate our steps:
The S-wave takes 13 minutes and 20 seconds to arrive at our measuring station. How far away is the epicenter?
- Begin by making sure you know the scale and units on each of the axes.
- Look at the scale of each of the axes.
- Next, determine which axis shows the data you have been given in the question. Find the given value on that axis.
- Draw a line (horizontally if you know y or vertically if you know x) from the point you marked on the axis until it hits the curve or line you are interested in.
- Now, you can find the answer by drawing a straight line from where your line from step 4 intersects the curve to the other axis.
Where is graph reading used in the geosciences?Graph reading can be found throughout the geosciences in a variety of contexts. There are very few topics in introductory geoscience that do not include graph reading of some sort. Some of the more common uses of generating data from a graph, or reading points from a graph include:
- geothermal gradients
- geologic time (absolute time)
- flood frequency
- glacial retreat
- climate change
- stream discharge
- and many more!