Puzzled by Light Pollution Maps and Numbers

[MercianDabbler]

I'm planning another short break, on this occasion in sync with the moon phase to give myself a decent chance of some dark skies. I'm not asking for recommendations but I've been looking at the LP numbers for some potential destinations and have so far found them a bit puzzling.

Bortle numbers seem somewhat broad brush - almost everywhere rural that I've looked seems to be Bortle 4. Bortle 3 can be had by jumping in the car and heading out into the wilds for the evening. Bortle 2 in even fewer places.

The CPRE 'Nightblight' interactive map seems a tad broad brush - there is a lot of ground that makes it into the bottom band

Consulting the maps on lightpollutionmap.info gives quite a different picture from the CPRE map. Many places that make the bottom band by the CPRE measure dont seem very dark at all when I look at this one. For example the North York Moors has plenty of 'darkest' areas according to CPRE but is (at best) green on lightpollutionmap.info, unlike, say, Kielder and a donut around Bude which manage blue.

Clear Outside gives four numbers - magnitude (understandable but not exactly intuitive), Bortle (understandable and intuitive but broad brush), 'brightness' and 'artificial brightness'.

Can anyone explain the last two numbers on Clear Outside... and the (different I think) metric used by CPRE and lightpollutionmap.info ('Word Atlas 2015') ?

Should I care more about 'brightness' or 'artificial brightness'?

I've also seen it said that numbers based on IR satellite measurements will underestimate the impact of LED lighting... do any of the above suffer from this?

I'm probably overthinking things as usual... but once I get started I can't resist a good rabbit hole. All of the destinations are a huge improvement on my LP at home.

[Nyctimene]

I can only comment on "lightpollutionmap", as it is the only map I'm using. It will  give you a rough estimate of what can be expected, due to the "Up-Down" measuring method of the satellite (VIIRS). The satellite is "up" and looks "down" at artificial light sources, that contribute to the light pollution. But you, as an observer, are looking "up" and want to know about the amount of light that shines "down" on you. The satellite measurements don't give reliable infos about this. Only the use of a ground based photometer, that looks "up", e.g. the "SQM"-or "SQM-L" device, will help here. But you can get some information about SQM/SQM-L - measured data (mostly by amateurs, as I guess) from lightpollutionmap. Open the "toggle" menu, click on "feature" and select "SQM" and "SQM-L". The map now will show you differently coloured dots (corresponding with the sky brightness; check the legend), each of which can be clicked on for further information. You get a rather dispersed, but more realistic information about sky brightness, and can even enter your own SQM/SQM-L measurements to help other stargazers. Have a look, how the map shows the North York Moors National Park with this feature:

https://www.lightpollutionmap.info/#zoom=7.76&lat=54.2583&lon=-0.9249&layers=B0FFFFFFTFFFFFTTFFF

Hth.

Stephan

 

 

Edited September 21, 2021 by Nyctimene

[PEMS]

Friend looked at them and said they are based on population density and the presumed, assumed or expected light polution associated. He also said they kind of failed to take into account undulations and hills, the general geography. A few villages around here have no lights but with the number of people they come sort of well up the scale. But they are dark, very dark.

Agreed everything comes out as 3 or 4. Think even parts of the North sea are 3 or 4, and some of the 3-4 bits are well off shore.

Use them as an approximate guide to see what improvements may be available to you. But I wouldn't take them as an accurate guide. Well not a 100% accurate guide. Finding somewhere dark is half the work, finding somewhere suitable to observe at or from is the next.

[Nyctimene]

9 minutes ago, PEMS said:

Friend looked at them and said they are based on population density and the presumed, assumed or expected light polution associated.

Not all of them; the maps of French amateur Fréderic  Tapissier  (AVEX maps) seem to be based on such assumptions. The lightpollutionmap data are scientifically based.

A link to the AVEX website (in French only, as it seems):

https://www.avex-asso.org/

[MercianDabbler]

1 hour ago, Nyctimene said:

The map now will show you differently coloured dots (corresponding with the sky brightness; check the legend), each of which can be clicked on for further information. You get a rather dispersed, but more realistic information about sky brightness, and can even enter your own SQM/SQM-L measurements to help other stargazers. Have a look, how the map shows the North York Moors National Park with this feature:

Thanks. I did try toggling that on. It seemed to make a lot of dots on the North York Moors and far fewer elsewhere so I guess that a lot more measurements have been logged by the good people of Yorkshire.

1 hour ago, PEMS said:

Use them as an approximate guide to see what improvements may be available to you. But I wouldn't take them as an accurate guide. Well not a 100% accurate guide. Finding somewhere dark is half the work, finding somewhere suitable to observe at or from is the next.

Yes I'm familiar with part 2 as well. We've now taken the plunge and booked a place in Yorkshire. It wasn't my first choice but it does seem to be somewhat away from built up areas and has open space nearby but is towards the yellow part of the spectrum on lightpollutionmap. The final decision was the product of family negotiation - dark skies are not the only consideration. The North York Moors will be driveable should the need arise but I'm hoping to set up near to the accommodation... it's so much less mucking about.

[Captain Scarlet]

The lightpollutionmap.info website has a drop-down menu where you can select the dataset. Most are VIIRS 20xx , which are “how bright it is” from the viewpoint of the satellite. IE top-down. But one, the Atlas 2015, is a modelled “bottom-up” dataset based on the then most recent VIIRS data. IE it’s what darkness is expected to be like for we, the observers.

Although it’s based on 2014/15 data, I have found it to be pretty good, even perhaps a slight underestimate of how dark it can get. I have approximately 500 points of SQM-L data from the last couple of years from two very differently bright locations. One is near London, which generally gets true best readings of 19.15 from my SQM-L. The website suggests 19.05.

The other location for which I have data is here in SW Ireland, which is 21.80 according to the website. I have recorded 22.0 on occasion.

Cheers, Magnus

[Starslayer]

May I ask - when I first started looking at the Light Pollution map it would show me the Bortle value for wherever I dropped the pin.  It now doers not show any Bortle at all.  I have played with the settings but seem to have lost this function.  What am I doing wrong? 

[gilesco]

Have you set the overlay to "World Atlas 2015"?

[Starslayer]

1 minute ago, gilesco said:

Have you set the overlay to "World Atlas 2015"?

Ah.. haha. I have literally just found that but thank you. Is that the only overlay that has the Bortle info.?  Seems to be. 

[ONIKKINEN]

Dont worry about the bortle number too much, its more of a rough estimate at best. Sky conditions and ground reflectance (snow/water) change every day and so does the value.

 

I have personally noticed that a B6 area can be as bad as B8, or vice versa, depending on the day and time of year. Its best to just look at the general colour of the zone. White=guaranteed bad, red=bad but could be worse (where i mostly go to). Anything better than those 2 are probably safer bets for better skies.

[westmarch]

Hi there,

Having used the websites you mentioned I have visited the more westerly of the N York Moors. The N York Moors dark sky site at Sutton Bank has been blighted by an adjoining horse racing stud farm that has horizontal halogen spotlights to deter horse thieves. 

There is a lay-by at the White Horse Glider Club that has an unobstructed vista and the spotlight from the club house to the North can be obscured by your parked car.

There is low altitude light pollution from York to the South and Thirsk to the West    .

It’s down as Bortle 4 but is my favourite nearby site that offers a decent view of the Milky Way.

NB Sutton Bank is a very steep hill - bear that in mind - a winter descent in the wee small hours,  before the gritters have been out,  is not for the faint hearted.

John