53 replies to this topic

[Matt_Lily]

Does anyone have any idea of what is considered the most accurate map/app for predicting light pollution? 

 

I've perused quite a few and there is some variance between them. On most light pollution maps, it seems there is some guess work in rural areas between small towns.  For example, one map might have such an area as a Bortle 3, while another map will say Bortle 4. 

[mg07]

No idea but I tend to use lightpollutionmap.info for its easy to use interface. One issue with that is that latest data is from 2015 and therefore many places are probably worse than they were 7 years ago. It's also worth noting that Bortle ratings were supposed to be particular not only to a place but also to a time. For example a location might be a 3 on a perfect night in summer and a 4 in winter after a snow comes in and causes more light to be reflected off the ground. There might be some clouds off in the distance over a light polluted city that change conditions at your location. Or to use a very extreme example, a location directly under a solitary streetlight might be a 9 when it's on, but a 1 if you manage to get it turned off. So I wouldn't worry about which maps are most accurate. Unless a giant factory or fracking rig has recently been installed nearby they are likely to be roughly in the ballpark, and on the margins what will really make a difference are surface level phenomenon (getting away from street lights, busy roads with car lights, etc) that a light pollution map won't show.

[Matt_Lily]

No idea but I tend to use lightpollutionmap.info for its easy to use interface. One issue with that is that latest data is from 2015 and therefore many places are probably worse than they were 7 years ago. It's also worth noting that Bortle ratings were supposed to be particular not only to a place but also to a time. For example a location might be a 3 on a perfect night in summer and a 4 in winter after a snow comes in and causes more light to be reflected off the ground. There might be some clouds off in the distance over a light polluted city that change conditions at your location. Or to use a very extreme example, a location directly under a solitary streetlight might be a 9 when it's on, but a 1 if you manage to get it turned off. So I wouldn't worry about which maps are most accurate. Unless a giant factory or fracking rig has recently been installed nearby they are likely to be roughly in the ballpark, and on the margins what will really make a difference are surface level phenomenon (getting away from street lights, busy roads with car lights, etc) that a light pollution map won't show.

Very good points. In my backyard, I have an estimated Bortle 5.6, but no annoying streetlights to contend with. This spring and summer I will have access to an estimated 2.9 Bortle sky in a rural area, but may have to deal with some bright (hopefully distant) farm lights.

[rgk901]

This one is updated to 2020

 

https://djlorenz.git...erlay/dark.html

[Matt_Lily]

This one is updated to 2020

 

https://djlorenz.git...erlay/dark.html

Thanks!

[DaveL]

The National Park Service collected a bunch of data on night sky brightness around the US (almost 400 sites). I've compared the different light pollution atlases across these sites. lightpollutionmap.info is systematically biased too optimistic by 0.34 mag/arcsec^2. The bias in my maps is 0.08 mag/arcsec^2. This is using the darkest point in the sky rather than zenith because sometimes the Milky Way is overhead. And also this is for sites brighter than 21.5 (darker than this, variability in airglow and extinction make comparisons of measurements with light pollution maps less meaningful). 

 

lightpollutionmap.info is used the most often it seems, but the authors of that map changed the assumptions about the light sources: they assume more light goes up instead of nearly horizontal. This unrealistically darkens the rural areas and leads to the bias (note: the satellite data on which these maps are based only gives the amount of light, not the direction). In other respects the model in lightpollutionmap.info is more advanced, but they "undid" the other improvements with this change.

 

I also do not like lightpollutionmap.info because it conflates the Bortle scale with zenith sky brightness. Bortle is a full sky metric that considers brightness away from zenith, and also transparency. Zenith sky brightness is just zenith brightness and that's it.

 

-Dave

Edited by DaveL, 18 February 2022 - 09:47 PM.

[PEterW]

The annual VIIRS data is quite helpful in seeing trends, for instance a block of housing beware me darkening as LED were installed. Satellite data is always hard to compare as much of the bad light is the stuff going out sideways near the horizontal where it brightens the skies for miles around. Light going straight up is not really a problem as almost none comes back down again.
Different spectra from different lamps, especially LED can give systematic differences. Not sure how well calibrated phone apps are, have to compare to my SQM sometime.

Peter

[DropsOfJupiter]

lightpollutionmap.info has VIIRS data up to 2021.

 

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[Tony Flanders]

Satellite data is always hard to compare as much of the bad light is the stuff going out sideways near the horizontal where it brightens the skies for miles around. Light going straight up is not really a problem as almost none comes back down again.

Exactly. This is a fundamental problem with attempting to estimate skyglow based on satellite measurements. Take a searchlight and point it straight up and it appears super-bright to a satellite, but yields very little skyglow, because the light takes the shortest possible path through the atmosphere. Take the exact same light and point it horizontally, and it will be invisible to the satellite, yet fully 50% of its light ends up scattered back to the ground as skyglow.

That's obviously a degenerate case, but it does point out the limitations of the method. Moral: all satellite-based light-pollution maps are at best crude approximations, and should be treated as such.

[Jon Isaacs]

In addition to what Tony wrote, I have measured the Sky Brightness with an SQM at a number of sites around the west. 

 

Sites like the Navajo National Monument that Light pollution maps 2015 suggest at 21.97 measure 21.8-21.9 mpsas. I've been visiting the National Monument for more than 30, the area has grown very little.

 

Sites that are supposed to measure 21.8 will measure 21.5 or brighter. My place in the high desert is supposed to be 21.64. I've seen anywhere from 21.0 to 21.5… 

 

21.5 mpsas are the result of clouds covering the populated coastal regions, not uncommon, plus cloud cover over the low desert.

 

Jon

[Tony Flanders]

Just to clarify, in case it's not obvious to everyone: Sky brightness often varies greatly at any given site from one night to another, and frequently from one hour to another.

 

At sites where artificial light pollution averages stronger than natural skyglow, the variation is primarily due to atmospheric conditions. At my country home, the major light source is Albany, NY. Not infrequently, the Hudson Valley (including Albany) fills up with fog, which traps much of the light and makes my sky darker than usual. Conversely, if there are high clouds above Albany on an otherwise clear night, those clouds shine like crazy and make my sky brighter than usual. In fact even New York City, well over 100 miles away as the crow flies, can be a serious problem when there are high clouds over it.

 

At sites where artificial light pollution averages weaker than natural skyglow there's a substantial variation in skyglow due to the position of the Milky Way and the zodiacal light, as well as the semi-random variation in airglow, which at its worst can be quite obtrusive. And then, of course, there's auroras, which can make the sky really, really bright.

[Ron359]

In addition to what Tony wrote, I have measured the Sky Brightness with an SQM at a number of sites around the west. 

 

Sites like the Navajo National Monument that Light pollution maps 2015 suggest at 21.97 measure 21.8-21.9 mpsas. I've been visiting the National Monument for more than 30, the area has grown very little.

 

Sites that are supposed to measure 21.8 will measure 21.5 or brighter. My place in the high desert is supposed to be 21.64. I've seen anywhere from 21.0 to 21.5… 

 

21.5 mpsas are the result of clouds covering the populated coastal regions, not uncommon, plus cloud cover over the low desert.

 

Jon

just to mention - all the variations of SQM readings you cite and comparison with LP map values are within the  Unihedron stated accuracy of any reading with the SQM.   Which says the LP maps in your comparison are at least as accurate as your SQM readings.   

[Tony Flanders]

just to mention - all the variations of SQM readings you cite and comparison with LP map values are within the  Unihedron stated accuracy of any reading with the SQM.   Which says the LP maps in your comparison are at least as accurate as your SQM readings.   

Any given SQM unit is consistent with its own readings +-.05 at worst, unless it's defective. Variation among different units can be as high as 0.3, but if any given unit is 0.1 off the baseline, Unihedron will likely replace it free of charge. (They did that for me once.)

 

By contrast, my SQM readings at my country home (with one device) routinely vary from about 20.5 early on a snowy evening to about 21.4 in the small hours of a summer morning. That means that no map can hope to be within 0.3 of reality on more than 50% of all nights, given that reality varies +-.45.

[Jon Isaacs]

just to mention - all the variations of SQM readings you cite and comparison with LP map values are within the  Unihedron stated accuracy of any reading with the SQM.   Which says the LP maps in your comparison are at least as accurate as your SQM readings.   

 

I am confident in my SQM.

 

- I have compared my SQM to others and they are in reasonable agreement.l, less than 0.1 mpsas under skies darker than 21.0 mpsas.

 

- Under pristine skies, I have measured 21.9 mpsas. Darker skies are more difficult to measure because of the small signal. Clearly my SQM does not overestimate the brightness to be more than 0.1mpsas greater than it is.

 

Under brighter skies where the SQM variation is less because of the greater signal, the charts differ even more.

 

I am measuring sky brightness. The charts are trying to estimate/guess it from other data.

 

Jon

[Ron359]

I am confident in my SQM.

 

- I have compared my SQM to others and they are in reasonable agreement.l, less than 0.1 mpsas under skies darker than 21.0 mpsas.

 

- Under pristine skies, I have measured 21.9 mpsas. Darker skies are more difficult to measure because of the small signal. Clearly my SQM does not overestimate the brightness to be more than 0.1mpsas greater than it is.

 

Under brighter skies where the SQM variation is less because of the greater signal, the charts differ even more.

 

I am measuring sky brightness. The charts are trying to estimate/guess it from other data.

 

Jon

I didn't say your SQM is inaccurate.  You can believe whatever you want when you don't understand the difference between accuracy and precision. You might as well make up the numbers then.  Your claims just wont' have any validity other than one person's  non-science based opinion, i.e., hot air.    Math, and science of data validation doesn't care.   The range of readings you give in comparisons are all within the accuracy of the instrument as stated by the manufacturer for ALL the instruments they make.  I've quoted that standard many times in these threads and not going to bother repeating again.  

Edited by Ron359, 25 February 2022 - 10:53 AM.

[Ron359]

Any given SQM unit is consistent with its own readings +-.05 at worst, unless it's defective. Variation among different units can be as high as 0.3, but if any given unit is 0.1 off the baseline, Unihedron will likely replace it free of charge. (They did that for me once.)

 

By contrast, my SQM readings at my country home (with one device) routinely vary from about 20.5 early on a snowy evening to about 21.4 in the small hours of a summer morning. That means that no map can hope to be within 0.3 of reality on more than 50% of all nights, given that reality varies +-.45.

 

 

Like many others, you continue to ignore the given accuracy of all units is +- 10% of the reading.

[csa/montana]

Let's please let everyone have the opportunity to voice their experience & opinions; there is not one person on this site that knows it all.  

 

From the Terms of Service:  Be respectful in your replies!

[GeorgeLiv]

I find that far too many amateurs nit-pick over numbers. You don't become a professional astronomer when you assign a number on such a thing as "brightness". Given this, I added my 2 cents worth here: https://www.cloudyni...aps-vs-reality/

[Jon Isaacs]

I didn't say your SQM is inaccurate.  You can believe whatever you want when you don't understand the difference between accuracy and precision. You might as well make up the numbers then.  Your claims just wont' have any validity other than one person's  non-science based opinion, i.e., hot air.    Math, and science of data validation doesn't care.   The range of readings you give in comparisons are all within the accuracy of the instrument as stated by the manufacturer for ALL the instruments they make.  I've quoted that standard many times in these threads and not going to bother repeating again.  

 

There is precision, there is accuracy and then there is calibration.

 

I don't know how much research you've done, I was involved in the experimental side of materials science for most of my working life.  Obviously one does not use Nist traceable calibration schemes but the fact that my SQM measured 21.9 mpsas under pristine skies at 7300 ft indicates that it's reasonably close at the 22.00 mpsas end and isn't more than 0.1mpsas bright. 

 

That provides and endpoint calibration and since I'm working with in 1.0 mpsas of that end point, I think there's good reason to consider the readings valid.  I'm definitely with 0.1 magnitude of base line.

 

 

Like many others, you continue to ignore the given accuracy of all units is +- 10% of the reading.

 

10% is 0.1 magnitudes. It seem Tony is well aware of this fact.

 

Jon

[blind daniel]

when I stack the last 12 years in lightpollutionmap.info ,

I dont see an increase in urban sprawl , any thoughts ?

 

https://imgur.com/a/JUBgZVZ

 

(it seems most intense in 2013)

Edited by blind daniel, 26 August 2024 - 05:01 PM.

[astrokeith]

Any info based at least in part on VIIRS will be problematic.

 

The change to LEDs and their higher colour temperature is a problem for the (now quite old) VIIRS sensors and their spectral response.

 

I know of an area in the UK that is visibly getting brighter due to new LED street-lighting and business, yet VIIRS shows it as declining.

[PEterW]

I have an area near me that was full cut off led lit and shows getting darker as there is likely less spill… but I agree that it might not be totally correct. With it hard to determine an accurate trend, I guess all we can push for is full cutoff lights only when/where necessary with lower colour temperature and hope that that this delivers better skies. Has anyone estimated the likely % error between led colour and SQM/VIIRS measurements… approximately….

Peter

[mr.otswons]

https://lighttrends....lutionmap.info/

 

Radiance trends

[mr.otswons]

when I stack the last 12 years in lightpollutionmap.info ,

I dont see an increase in urban sprawl , any thoughts ?

 

https://imgur.com/a/JUBgZVZ

 

(it seems most intense in 2013)

There is something about LED lights not being detected as well by the satellites.

[Ron359]

There is something about LED lights not being detected as well by the satellites.

Most commonly used LED lights have huge spectral peak emissions in near UV and blue.  Like the Unihendron Sky Quality Meters, many 'normal' imaging satellite sensors are not sensitive (basically blind)  to the bluer, highly scattering, wavelengths of LED light pollution, so the increases go 'un-detected.' 

Edited by Ron359, 29 October 2024 - 12:40 PM.