Estimating Algae using Water Surface Reflectivity
One way to measure the amount of algae present in a body of water, such as a lake, is to measure the reflectivity of the water surface. This measurement can be much easier to take than actually trying to count the number of algae present in a water sample. To use the reflectivity to predict the amount of algae present we need to have an equation that relates the amount of chlorophyll present in the water and the reflectivity measurement. The data points in the table below were collected experimentally. The NIR/RED column is the reflectivity measurement.
Using a statistical package or a graphing calculator, make a scatterplot of this data set1 with the NIR/RED values as the independent variable and the Chlorophyll-a values as the dependent values.
Does it look linear or quadratic?
Fit a line and a quadratic to this data set and discuss which equation best fits the data set. For a reminder of how to choose which equation is best go here.
For the quadratic equation, find the vertex and the x-intercepts.
Explain what the x-intercepts are describing in this physical situation.
|
Chlorophyll-a |
NIR/RED |
Chlorophyll-a |
NIR/RED |
|
3 |
1.04 |
12 |
1.31 |
|
2 |
1.05 |
15 |
1.38 |
|
1 |
1.05 |
14 |
1.37 |
|
4 |
1.06 |
13 |
1.35 |
|
5 |
1.18 |
20 |
1.48 |
|
6 |
1.19 |
18 |
1.4 |
|
9 |
1.2 |
17 |
1.55 |
|
7 |
1.2 |
16 |
1.43 |
|
8 |
1.25 |
19 |
1.4 |
|
10 |
1.3 |
24 |
1.75 |
|
11 |
1.3 |
25 |
1.7 |
|
25 |
1.75 |
26 |
1.78 |
|
27 |
1.78 |
28 |
1.77 |
|
40 |
1.8 |
42 |
1.81 |
|
46 |
1.86 |
49 |
1.87 |
|
50 |
1.9 |
52 |
1.92 |
|
55 |
1.9 |
56 |
1.95 |
|
56 |
1.75 |
58 |
1.68 |
|
58 |
1.97 |
59 |
2.11 |
|
60 |
1.79 |
61 |
2.11 |
|
62 |
2.1 |
62 |
2.1 |
Ratio of NIR to RED versus Chlorophyll-a
1Schalles, et al., 1997. Estimation of algal and suspended sediment loads using hyperspectral sensors and integrated mesocosm experiments. Proceedings of the 4th International Conference on Remote Sensing for Marine and Coastal Environments, Orlando, FL.
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