Measuring Distance From Photographs

Motion and the measurement of motion

Measuring complex motion:

  • The motion of animals is more complex than the motion of objects like cars and airplanes.  Unlike cars and airplanes, animals do not move as a single rigid body, rather they have many moving parts whose movements are often not simple.
  • Watch this video of a lizard jumping.
    • How is the motion of the lizard changing?
    • The footage was recorded at a rate of 500 frames per second. How much time passes between every frame?
  • How do we measure the movement of the lizard using the video?
  • Measuring motion from a photograph:
    • In order to measure the speed of the jumping lizard need to be able to measure both time and distance traveled.  Can we do this from the video images?
    • Measurement of time - video was shot at 500 frames per second.  Determine the time passed with each frame from this information.
    • Measurement of distance - Can we measure distance directly from the photos? Having something in the photograph of a known size, or a reference dimension, allows us measure other distances in the same photograph by comparison.
      • Manual method: In our example, we known that the distance between the tip of lizard's snout to the third dot (counting from the snout) is 60 mm. Using this dimension we can now create a grid on the photograph as is shown in this form (Lizard_Jump.pdf). In the Lizard Jump worksheet each of the images shows the time information. From this information we should be able to graph the motion of the lizard.
      • Computerized method: Using a computer and specialized software the task of measuring motion can be automated. As long as there is a clearly identifiable mark for the software to “lock” on to, programs such as Tracker can automate much of the data collection required.
      • Demonstration: Using Tracker (more information) open the Lizard_Jump.trk file. This file contains the motion tracking information along with the video of the lizard jumping.
    • This graph is a distance versus time graph of the lizard video created with a spreadsheet program using data from Tracker.
      • What can we tell from the graph?
      • What parts of the graph correspond to events in the video?