Find out what the map is measuring.
A change map helps you find places to review before preparing a report or sending a field team. Its colors may represent a difference in a measured quantity, a change in classification or an area selected for closer inspection.
Look for the name of the measured quantity, its units, the comparison dates and the rule used to identify change. Check whether the map shows every small difference or only differences above a threshold.
Keep missing data visible. “No usable observation” and “no change detected” describe different situations. A legend should let you tell them apart without relying on color alone.
Use the site table alongside the map. Site names, results and coverage let you compare locations and create a review list without interpreting every coloured patch individually.
A visible difference needs an explanation.
Move the divider to compare the same imaginary site. Then change the example to see how season or missing coverage changes what the comparison supports.
Observed surface difference
The marked parcel looks different under comparable illustrative conditions. The comparison points to a review location; it does not establish the cause or legal status of the change.
- Area
- Marked parcel
- What the comparison supports
- Visible difference
- Useful next action
- Review site records or visit
Season can change the signal
Vegetation and soil look different across seasons even when parcel boundaries remain the same. A similar seasonal window or local records can help test whether the difference is relevant to your question.
- Area
- Marked parcel
- What the comparison supports
- Season differs
- Useful next action
- Choose comparable dates
No observation is not no change
The hatched area cannot be evaluated from this pair. Keep it out of the calculated comparison and retain it on the review list until another observation is available.
- Area
- Hatched area
- What the comparison supports
- Insufficient coverage
- Useful next action
- Find another observation
Prepared teaching illustration. Landscapes, dates, and results are fictional; no live imagery search, analysis, or app generation is running.
Separate three kinds of statement.
Start with what the sensor recorded. Then identify what the method calculated. Finally, consider what the result might mean for your work. Keeping these steps distinct makes an explanation easier to question and improve.
Practice the decision
What kind of claim is this?
Explore three statements from a fictional change review. Each needs a different kind of supporting evidence.
“These observations cover the selected dates.”
This statement describes the inputs. Check the source, acquisition dates, usable area, and quality information. It does not yet say what changed or why.
“This area passed the change threshold.”
This statement describes a calculation under a chosen method and threshold. Inspect the measured quantity, units, parameters, and validation. A threshold may be a review preference rather than a scientific boundary.
“This site may need a field visit.”
This connects the result to a decision. It can be a useful next step while the cause remains uncertain. Local records, another observation, or a field check may distinguish possible explanations.
Fictional examples for learning. No live analysis is running.
A result can justify a closer look while leaving the cause unresolved. For example, a vegetation-related difference might help prioritise a visit; confirming a particular disease or treatment need calls for additional evidence.
Check what makes the dates comparable.
Before explaining a difference, ask whether the observations describe comparable conditions. Season, illumination, sensor characteristics, and processing choices can affect a comparison. NASA’s data guidance recommends checking calibration, time sampling, and coverage when combining sources. Read NASA’s explanation of data comparison.
- Dates: Are these individual captures or date windows? Are they appropriate for the event or season?
- Coverage: Which parts were evaluated on both dates? Which parts were excluded?
- Alignment: Do both images put the same features in the same place? Misaligned boundaries can create apparent change.
- Method: Were inputs made comparable, and were parameters recorded?
- Validation: What was checked, against which reference, and for what type of place?
- Context: Do site records or local observations support the proposed explanation?
Software completing its calculation only establishes that it ran. The method and its application still need evidence that supports the scientific conclusion.
Choose what happens after the review.
For each site, name the next useful action. Add it to a visit list, check another date, compare a field note or include the supported result in a report. Ask for specialist review when the method or interpretation needs it.
Save the result with its sources, dates, method and limitations. When new information arrives, you can revisit the interpretation without losing the original basis for it.
For a client or colleague, write a short explanation in this order: what was observed, what was calculated, what remains uncertain and what to do next. In a recurring monitoring app, those same fields can help your team complete the next review consistently.
Explore site change reporting or recurring site monitoring to see how this fits the work.