Purpose:
To provide exposure to: (1) acquisition of spectral radiometric data, (2) calibration to radiance values, (3) spectral signature interpretation, and (4) visualization of results in graphic and tabular formats. This will enable you to gain an awareness of several fundamental aspects of reflected radiation.
Materials:
Email your completed word document and excel spreadsheet to: kwarkentin@sdsu.edu with your names and lab 4 in the subject line.
A user may be able to choose the proper bands and filters for a given remote sensing task by collecting and analyzing reflectance data for various targets, which have been or are to be imaged in a given scene, and then by identifying spectral bands which maximize class separability. Theoretically, the greater contrast in reflectance between two imaged objects, the easier it should be to distinguish between them. The easier objects are to distinguish, the greater is the potential for fast, accurate interpretation.
Band Spectral Bands (mm) Exotech bands in bold
1 0.45 – 0.52 blue
2 0.52 -0.60 green
3 0.63 -0 .69 red
4 0.76 -0 .90 near IR
5 1.55 – 1.75 mid IR
7 2.08 – 2.35 mid IR
6 10.4 – 12.5 thermal
A four channel Exotech radiometer was used several years ago to collect raw spectral radiometric measurements for general surface material types around campus. The radiometer measures the radiation reflected from the surface as a voltage. You will be provided with radiant exitance values using the calibration data in Table 2.
The voltages recorded on the data sheets for each band of each target were multiplied by the calibration for the band, field-of-view (FOV) and sensor gains used in the conversion program.
Calculate radiant exitance and enter below for each target. A 15 degree FOV was used for all measurements. Average the beginning and ending panel radiant exitance.
You may choose to do all table-based calculations in the Excel spreadsheet which is provided as part of this exercise, following the general workflow described here.
Table 2: Radiant Exitance (Wm-2)
Target | Band 1 | Band 2 | Band 3 | Band 4 |
average panel | 140 | 152 | 95 | 154 |
asphalt | 22 | 35 | 12 | 45 |
concrete | 93 | 86 | 43 | 35 |
grass | 6 | 11 | 5 | 103 |
shrub | 1 | 3 | 1 | 72 |
soil | 3 | 7 | 65 | 39 |
shaded grass | 2 | 5 | 2 | 26 |
shaded concrete | 49 | 21 | 13 | 8 |
The halon coated panel used to measure the incident radiation at the surface is not a perfect, i.e., neither 100% nor an isotropic (Lambertian) reflector. Therefore, an adjustment must be applied to the panel measurements.
The average panel radiant exitance that you calculated above must be divided by a factor taken from Table 3 below for the time that most closely matches when your observations were taken (around 2:30 PM).
The magnitude of the adjustment is dependent on the solar zenith angle which is a function of the time of day, day of the year and latitude. For your calculations, use the 2:30 PM adjustment factors.
Time (PDT) Band 1 Band 2 Band 3 Band 4
2:00 .935458 .939679 .934222 .919215
2:15 .932881 .937191 .931803 .916819
2:30 .92994 .934368 .929077 .914118
2:45 .926623 .931202 .926039 .91111
3:00 .922896 .927663 .922662 .907772
3:15 .918706 .923701 .918895 .90406
3:30 .913986 .919248 .91467 .899914
3:45 .908651 .914222 .909903 .895261
4:00 .902605 .90853 .904497 .890013
Enter the adjusted panel values below (you can use the excel spreadsheet for the calculations) :
Avg. Adjusted Panel Values = Average panel radiant exitance ¸ panel adjustment
(Divide the average panel radiant exitance by the correct panel adjustment factors)
Band 1 | Band 2 | Band 3 | Band 4 | |
Average Adjusted Panel Values |
Now, calculate the spectral reflectance factor for each band of the seven targets (numbers and calculations can be stored and submitted in the spreadsheet). The spectral reflectance factor is the target radiant exitance divided by the adjusted average panel radiant exitance. Express the reflectance as a decimal fraction out to four significant digits beyond the decimal point.
Reflectance = Target exitance (Wm-2) ¸ Adj. avg. panel exitance (Wm-2)
Reflectance
Material | Band 1 | Band 2 | Band 3 | Band 4 |
Asphalt | ||||
Concrete | ||||
Grass | ||||
Shrub | ||||
Soil | ||||
Shaded Grass | ||||
Shaded Concrete |
ANALYSIS
Save the graphs in the excel spreadsheet. You do not need to copy them to the word doc.
Asphalt | Concrete | Grass | Shrub | Soil | |
Asphalt | |||||
Concrete | |||||
Grass | |||||
Shrub | |||||
Soil |
Vegetation | Asphalt | Concrete/Soil | |
Vegetation | |||
Asphalt | |||
Concrete/Soil |
Explanation: