Dark-sky maps can disagree because they show different quantities, use different data periods or models, or describe a different part of the sky from the one you are inspecting. Start by matching the layer, units, date, and direction. Then check the conditions and local lights at the actual observing position. Do not average unlike numbers or assume a color is a measurement of tonight's sky.
Identify the layer before ranking the site
Open each map's legend and data information, not just its location popup. Write down the exact layer name and provider. A satellite basemap underneath a colored overlay does not establish where that overlay's values came from.
Lightpollutionmap.info's current help distinguishes VIIRS radiance from modeled zenith Sky Brightness and measurement overlays. It documents newer Black Marble 2.0 products alongside older World Atlas layers. Record the selected product and year rather than assigning one legacy date to the whole website.
| Evidence | Question to ask before using it |
|---|---|
| Satellite radiance layer | What upward light does this satellite product describe, over which area and period? |
| Modeled sky-brightness layer | Which part of the sky is estimated, and what conditions or reference assumptions are used? |
| Ground observation | When, where, how, and in which direction was it taken? Does it include natural and artificial light? |
On that provider's Sky Brightness and World Atlas overlays, displayed SQM and Bortle values are derived from model output. An “SQM” popup label alone therefore does not establish that a meter was used at the pin. Keep a model-derived label separate from an instrument reading or visual assessment.
Separate the data period from the website date
Record the data coverage period, the product or model version, and when you accessed the page. A recently updated interface can still display an older layer. A new data release can also use a different method, making a simple before-and-after comparison misleading.
NASA's Black Marble Collection 2.0 guide describes monthly and yearly nighttime-light composites made from corrected daily observations, with filtering and quality information. A yearly composite summarizes a period; it is not a live view of the ground or the sky on your observing night.
When two maps disagree, first try the same source product, period, and location. If one is a model and the other is an observation, keep both records and name that difference. If the documentation does not reveal the data period, mark it unknown rather than borrowing the website's copyright year.
Match the metric and direction
The National Park Service's night-sky metrics guide distinguishes zenith brightness from mean all-sky brightness. Zenith means overhead; an all-sky average summarizes a wider view. Neither alone describes every direction along the horizon. The guide also explains that, in magnitudes per square arcsecond, higher numbers mean darker skies.
For the comparison, write out what the number means instead of copying only the value. Keep its units, sky region, and natural-light treatment together. A Bortle class is based on visible sky features, while a brightness value is a different kind of record; do not average two classes to create a supposedly more accurate site rating.
The NPS measurement method uses calibrated V-band images covering the sky from horizon to horizon. Its observed images contain natural and artificial light; a modeled natural component is subtracted to estimate artificial skyglow. A total observed brightness and an artificial-only estimate are not interchangeable.
If a meter reading is involved, identify the instrument and pointing. Unihedron explains that SQM and SQM-L models have different fields of view. Keep the model and direction with the reading instead of treating all “SQM” entries as the same measurement.
For your photograph, note the direction and altitude range of the intended frame. Inspect that region from the exact setup position. A reassuring overhead value does not settle a problem with glow or a direct lamp in the composition. Use the horizon-recording guide for the separate task of documenting obstructions.
Compare the map with the actual night
Keep the map record unchanged and add a separate visit record. Note the time, position, target direction, Moon, clouds or haze, direct lights, and any meter method. This lets you investigate a mismatch without quietly replacing a long-term estimate with one night's impression.
The NPS guide to natural night light describes contributions from the Moon, Milky Way, and airglow. Moonlight reduces faint-object visibility, and natural sky light varies. A brighter total sky does not by itself demonstrate increased artificial light.
The World Atlas research paper by Falchi and colleagues also describes changes with atmospheric conditions: aerosols and clouds can alter skyglow differently depending on the setting. Do not apply a universal “clouds make it brighter” correction to reconcile two visits.
Inspect direct lamps and light domes in the direction you need. Record what is visible and when, without assigning a source or cause you have not checked. If you can make another visit, compare similar directions and conditions and note what remains different. The Moon guide and transparency guide help with those separate planning checks.
Without a meter, a dated description of the view is still useful. Label it as a visual note. Keep reference photographs to remember the scene, but label them as photographs rather than calibrated brightness measurements unless you used a validated measurement method.
Choose an action from the evidence
| What disagrees | Next useful action |
|---|---|
| Colors or numbers from different layers | Copy the layer names, units, and definitions. Compare the same quantity and period where possible; retain unlike evidence separately. |
| A recent-looking map and an older field report | Check data coverage dates and product versions, then check the report's actual visit date. Do not use page freshness as a substitute. |
| An overhead estimate and a bright target-direction horizon | Inspect the intended frame and record its direction. Decide whether another composition or position addresses the problem. |
| A modeled value and a ground reading | Check natural-light treatment, conditions, instrument, and pointing before interpreting the difference. |
| Different results on two visits | Keep both dated records. Identify changed conditions and local lighting, then repeat a comparable check if the decision warrants it. |
| A blank, gray, or unavailable layer | Read the legend and coverage notes. Record “unknown” until verified; absence of a displayed value is not evidence of zero light. |
Copy the comparison worksheet
Create one record per source and one per visit. Leave a field unknown when the source does not supply it.
| Field | What to keep |
|---|---|
| Source and location | Link, selected coordinates, exact layer or report name, and date accessed. |
| Metric and units | Full label, value, units, and whether it is measured, modeled, or assigned as a class. |
| Data period and version | Observation date or coverage period; product or model version; any unknowns. |
| Sky region and light included | Pointing or region described, field of view where known, and total or artificial-only treatment. |
| Method and limits | Instrument or model documentation, stated assumptions, and missing metadata that prevents a fair comparison. |
For the visit, record:
- Setup: exact position, local date and time with time zone, and the target or composition direction.
- Conditions: Moon position and phase, clouds or haze, and visible direct lights or glow. Separate what you observed from a forecast.
- Evidence: visual notes, reference-photo identifiers, or readings with instrument, pointing, units, and the manufacturer's method.
- Mismatch: the specific claim that did not match the visit, rather than simply “the map is wrong.”
- Decision: use this position for the intended frame, change the frame or time, compare another spot, or gather a more comparable record.
Finish with one sentence: “For this target and time, I am using this source because it describes ___; I still need to verify ___.” That gives the comparison a practical outcome without pretending every source has been reduced to one score.
Use DarkScout's estimate to build a shortlist
DarkScout's darkness layer uses nearby city populations and distances to estimate darkness for initial location comparison. Treat it as a shortlist tool, rather than a satellite, Bortle, or meter measurement of the sky at your setup. Check actual lights and the target-direction view before relying on a candidate location.
Save useful observing spots in DarkScout and compare Moon, cloud, and planning windows for the night. Keep the comparison worksheet and field evidence alongside that plan. The current App Store listing describes the free planning features; the app requires iOS 26 or later.
Turn a shortlist into a night plan.
Compare saved observing spots with Moon and cloud conditions, then check the view for your intended photograph.
Get DarkScout for iPhone All features free · iOS 26 or laterSources and further reading
- Lightpollutionmap.info: help and layer documentation — current layer distinctions and derived popup values.
- NASA: Black Marble User Guide, Collection 2.0 — corrected observations and monthly or yearly composites.
- NPS: Night Skies Report metrics guide — units, visual classes, and the sky region described.
- NPS: Measuring Night Sky Brightness — calibrated observations and separation of natural and artificial components.
- Unihedron: Sky Quality Meter FAQ — instrument field-of-view differences.
- NPS: Natural Light in the Night Sky — Moon and other natural contributions.
- Falchi et al.: The New World Atlas of Artificial Night Sky Brightness — atmospheric effects and model context.