Analyst guide · Historical Intelligence
Direct answer: A before-and-after comparison needs compatible dates, measurements and observation masks. Calculate change within the area valid in both images rather than using an attractive full-frame denominator.
A practical verification workflow
- Record the acquisition date, pixel size and product type for both images. A mission launch anniversary or archive upload date cannot replace image acquisition metadata.
- Define the shared study boundary and remove cells invalid in either observation. State the remaining comparable area before calculating an area or percentage change.
- Check that classifications and preprocessing are comparable. A difference in class labels or resolution can create an apparent change that is not directly a land-cover observation.
- Separate measured class change from cause, legality and responsibility. A consistent image comparison does not automatically identify who caused it.
A separate worked illustration
A fictional study square covers twelve square kilometres, but only nine are visible in both acquisitions. A change percentage should identify whether its denominator is those nine comparable square kilometres or a particular baseline class. Using the full twelve without explanation changes the question being answered.
The common reasoning error
Do not hide the excluded region. Readers need to see where the comparison is unknown as well as where a classified change was observed.
Keep an auditable analyst note
Record the question, source identifier, original observation, conversion or calculation, and the narrow conclusion it supports. Separate a confirmed fact from an inference and from information that is still unknown. Keep the original timestamp or unit alongside any normalized value so another reader can reproduce the reasoning.
Practise with the related case
Operation Archive Orbit: Compare a Long Satellite Record applies this skill in a fictional evidence packet with five graded objectives. This guide explains the method, not the hidden answer key. Case evidence and entities are invented; the separate illustration above is not a substitute for reading the dossier. Sign-in is required to use the case terminal.
Questions readers ask
Does a recent scan mean a recent satellite capture?
No. Acquisition and later publication are separate events.
Can a forest-class decline establish criminal deforestation by itself?
No. Cause, authorization and attribution require additional evidence.
Reporting context and further reading
NASA Landsat 9 anniversary report · 2026-09-29. This primary-source reporting provides recent topic context only. It is not evidence that the invented case happened, and its publication date may differ from the incident dates described.
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