INDIRECT MEASUREMENTS AND CALIBRATION
Determining a relationship between instrument readings and gravimetric soil
moisture content (the CALIBRATION procedure) is a major task for the soil
moisture
protocol but the concept of an indirect measurement is easy to understand.
Here is something each class can try in oreder to help illustrate this concept,
even if you have not started making soil measurements yet. Anthropologists
often estimate the AGE of Humans based on skull SIZE. Let's
conduct an experiment to test how effective this is.
- You will need a cloth tape measure and data sheet,
- Make a table of each persons's age and skull circumference ("size"),
- Measure carefully, just above the eyebrows and approximately parallel
(level) with the floor as your subject sits or stands normally,
- Collect at least 30 pairs of measurements from fellow students,
teachers and family members,
- Plot each pair on graph paper as skull "size" (x) vs. age (y),
- Draw a best-fit (smooth, linear?) line through your points,
- This will be your CALIBRATION curve - use it to PREDICT
or ESTIMATE age based on skull "size" by finding where new skull
measurements intersect your best-fit line,
- Collect a second set of verification data from a different group
of people.
- For this second group, make a table of: skull circumference, age,
predicted age, age difference (predicted-actual), % difference.
- Use this data to test the effectiveness of your calibration curve.
Now think about and discuss the following questions:
- How well were you able to estimate this second set of ages?
- Will this calibration work for people in a different state?
- What are some possible sources of experimental (measurement) error?
(Consider accuracy of tape measure, hair thickness, etc.)
- How important is the numerical size of your sampling population?
(eg. how many calibration data pairs you collected?)
- How important is the age diversity of your sampling population?
(What would be different between a calibration curve based on sixth
graders versus one based on whole families of sixth graders?)
What you can do in the classroom with skull and age data is similar to
what we would like GLOBE students to do in the field where they
collect both gravimetric soil moisture and gypsum block meter readings
for a limited calibration period. In this case, the goal is to estimate
soil moisture based on an INDIRECT measurement of the
moisture-related electrical properties of buried gypsum blocks.
This is a time-consuming step but
necessary because of the unique way soils and gypsum blocks interact. Your
soil moisture data is only meaningful once this calibration is done and
is the ONLY format acceptable to the data archive.
Back to GLOBE home page
Last updated: 12/1/95
Comments? globe@hwr.arizona.edu