Lesson 3 of 4 · Plan for useful observations, not just passes
Plan for useful observations, not just passes
6 minute lesson · 3 checks along the way
Your goal for this lessonAccount for cloud cover, acquisition timing and delivery delay when evaluating an optical information service.
In this lesson 3 sections · 3 checks
Read & understand
A pass is an opportunity rather than a usable observation
An optical spacecraft may pass over the intended region while clouds obscure the ground. ESA describes cloud masking as a way to exclude obscured pixels from analysis. A cloud-covered location is unknown in that image, not necessarily dry land, open water or an unchanged field.
The Landsat view in this course shows Lake Baringo in May 2020, years before Taifa-1 launched. NASA’s article explains that persistent cloudiness limited the available clear views. This is a real example of the observation problem, not evidence that Taifa captured the scene or used Landsat’s camera. The acquisition date must remain attached to any decision based on the image.
A natural-color Landsat 8 OLI view of Lake Baringo, Kenya, acquired on 1 May 2020 and published by NASA Earth Observatory. NASA’s complete detail view, labels and scale bar are retained. This earlier observation illustrates a Kenyan information need; it is not Taifa-1 imagery or evidence of its camera performance.
Look closely
What does this view show, and what cannot be determined from a single scene?
See what to notice
The lake, islands and surrounding land are visible, with sediment-colored water. A single dated view does not measure a change in water level, establish the cause of flooding or show Taifa camera performance. A suitable comparison and other observations are needed.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Read & understand
Use the correct denominator
Consider an invented study region of 100 square kilometres. Clouds obscure 40 square kilometres, leaving 60 observed. The algorithm identifies 12 square kilometres of water within that clear area. Water covers 20 percent of the observed area, because 12 divided by 60 is 0.20. This is not enough to report that 20 percent of the entire region is water.
The cloudy 40 square kilometres could contain additional water. Reporting 12 percent as the total water fraction would also treat unobserved ground as non-water. A more honest delivery reports the observed water area, the usable coverage and the missing region. A later clear acquisition or other suitable evidence can reduce the uncertainty.
Use the observed area as the denominator
Original teaching diagram. Invented coverage map; no tiles describe real Taifa or Landsat classifications.
Open full-size diagram in a new tab: Use the observed area as the denominatorCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Read & understand
An old clear image may miss a new event
For an invented event-response request, the first usable acquisition arrives 18 hours after the request. Downlink takes another 3 hours and processing and review take 2 hours. The product is available 23 hours after the request, even if the image transfer itself was quick. This is a teaching schedule, not Taifa’s measured response time.
A task needing information within 12 hours would not be satisfied. Options might include another suitable spacecraft, ground observations or a revised decision that explicitly accounts for missing information. National ownership adds choices about tasking, but dependable service still requires coverage, quality, timing and interpretation appropriate to the actual need.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Explore further · Optional
Go to the source
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