1. Why is Canada lit up like a christmas tree?
Two words. Aurora borealis
2. Why is the map missing data near the poles?
All due to continuous sunshine in summer/winter months for northern/southern hemisphere.
3. Why is on a 2015 map less light pollution than on 2014? Isn't it supposed to get worse each year?
There are numerous reasons for this. One could be lack of good data. For example some areas have always some clouds and as VIIRS overlay is composited monthly, it is impossible to get good quality data coverage for that month. It could simply be that the radiance actually decreased or that HPS lamps
have been replaced by high color temperature
LEDs to which VIIRS sensor isn't too sensitive
4. Why are swath trails visible on some VIIRS maps?
Maybe because of not perfect onboard calibration? Best to direct the question to NOAA.
5. What's an SQM?
SQM stands for Sky Quality Meter. It's a small device that can measure sky quality ie. darkness. It's widely used by amateur astronomers because it's fairly cheap and gives you fast results which can be easily compared to other SQM users. Be aware that two versions exist (SQM and SQM-L) and their readings are not comparable More about SQM product visit the manufacturer website
6. How can I add my own SQM data?
To add your SQM measurements turn on the "User SQM" overlay in "SQM overlay & filter" menu and click on the pencil icon in the toolbar. Please note that the time entered should be your local time the measurement was taken.
7. I have hundreds of measurements. How can I submit them all easily?
If they have at least coordinates, date/time, then contact me.
8. What is the map projection?
The original data is in a WGS84 geographic coordinate system. To have it display as overlays on top of Bing maps the data needs to be projected to a Spherical Mercator projection
. When you export to raw GeoTIFF the data is in the original WGS84 coordinate system.
9. What is the resolution of the overlays?
The original VIIRS data has a resolution of 15 arc seconds (30 for DMSP) for each pixel. When projected the resolution is around 500m for each pixel.
10. How do you convert your data to MPSAS (magnitudes per square arc second) or Bortle scale? What does W/cm2 * sr mean anyway?
* sr is a SI radiometry unit for radiance
. Radiance is radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. It sounds complicated right? Well that's because it is. Things get a bit more (actually a lot more) complicated if you want to do a simple conversion to MPSAS. I'm not even going to touch the Bortle scale issue because it is highly subjective scale. Anyway if you are still interested in a "conversion" I can try to explain the problem. Imagine you have a small light source aimed at the sky and this light source gets picked up by the VIIRS detector
detector. The VIIRS detector is monochromatic and has its own spectral response curve. It has no idea of the spectral curve of the light source. MPSAS or more specific magnitude is a measure that is measured in a “specific wavelength or passband”. See the problem? That's one major issue. The other major issue is that light from the light source passes through the atmosphere and while doing this it scatters due to air molecules and aerosols. So you need to create a model of light propagation for the entire Earth taking into the account local air conditions, earth curvature, light absorption, Earth terrain and what not. You can read more about it here
. If you got skills to do this and willing share the result I'll be glad to include it!
11. Where can I get the original data?
12. How can I view the data? The files are too big to be opened by Photoshop or similar software!
To work with this kind of data you need to have GIS software installed (QGIS, uDig, ArcGIS, etc...)
13. How should I credit the use of www.lightpollutionmap.info?
The use of data from www.lightpollutionmap.info should be credited to "Jurij Stare, www.lightpollutionmap.info", but because the original data is sourced from Earth Observation Group, NOAA National Geophysical Data Center it should also be credited as such.
14. I see the coordinate display is in a decimal format, but can you display them as degrees/minutes/seconds?
Click on the coordinates and it should toggle between those two display modes.
15. What is the precise date when satellite data was collected?
"layer VIIRS 2016" data was taken between dates 20160301-20160331
"layer VIIRS 2015" data was taken between dates 20150301-20150331
"layer VIIRS 2014" data was taken between dates 20140301-20140331
"layer DMSP 2010" data was taken during the whole 2010
16. VIIRS version 1 series is run globally using two different configurations. Which one are you using?
17. Exported GeoTIFF is all black!
Exported GeoTIFF is the raw VIIRS/DMSP data. VIIRS is in a 32bit floating point while DMSP is in a 8bit unsigned integer pixel type GeoTIFF. Use GIS software (QGIS, uDig, ArcGIS, etc...) to stretch the histogram, reclassify etc...
18. How are the country statistics made?
19. We want to use your WMS/WMTS service
Contact me and we'll work something out.
20. How can i search by coordintes?
Just enter coordinates into the "Search" field located top left in a "latitude, longitude" format.