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Light polution map

bomberbaz

By bomberbaz
in Getting Started With Observing

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Vox45

Vox45

Posted
Posted

I use the AVEX one, it was published in 'Ciel & Espace magazine' number #530 and you can hear a podcast (in french) about how it came about

www.cieletespaceradio.fr/une_carte_de_la_qualite_du_ciel_nocturne.952.TERR_001

I also stumbled upon this overlay on googlemap (a bit date tough)

http://djlorenz.github.io/astronomy/lp2006/overlay/dark.html

Clear skies !

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acey

acey

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As far as I'm aware, all the currently available maps are simulations based on 15 year old data and an inadequate visiblity model. Recent satellite data is available for light that's going up into space, but that doesn't tell you what things will look like from the ground, where climate and topography are crucial. So I'd say the only reliable way of judging the quality of any site is to go there and look.

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jnb

jnb

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Posted

As far as I'm aware, all the currently available maps are simulations based on 15 year old data and an inadequate visiblity model. Recent satellite data is available for light that's going up into space, but that doesn't tell you what things will look like from the ground, where climate and topography are crucial. So I'd say the only reliable way of judging the quality of any site is to go there and look.

They are useful as a vague guide but they can not possibly hope to indicate local issues. You might be in the middle of the darkest area of the country but that's no use if your neighbour takes it into their mid to leave a floodlight on in their backgarden.

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acey

acey

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They are useful as a vague guide but they can not possibly hope to indicate local issues. You might be in the middle of the darkest area of the country but that's no use if your neighbour takes it into their mid to leave a floodlight on in their backgarden.

And if you're in the brightest area then surrounding your garden with high screens to block streetlights will do nothing to alter the skyglow. So these maps can't show you where to find dark places, but can show where to find places that are certainly too light polluted (which is most of the country).

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acey

acey

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Interesting, thanks. I've not seen that map before, it would be nice to know where they got their data from.

From looking around the website I gather the Avex maps were made by Frederic Tapissier in 2010. He makes the following remark: 

"Following recent advances in data management, maps in Google Earth have significantly improved their positioning precision.

Anytime, urbanization datas I have are relatively incomplete, missing some small villages that have _after checking_ urban lighting! So, in rural areas, some sources of light pollution are not on maps. I’m sorry but

right now I have no way to do otherwise."

From which it seems that the author used data on population density to estimate light output from individual areas. I couldn't find acknowledgement of this data source anywhere, except in his reply to a comment by a site visitor, from which I gather that Tapissier used Corine Land Cover 2006, a dataset of land usage by the European Environment Agency.

http://www.eea.europa.eu/publications/COR0-landcover

I had a very quick look at the Corine technical documentation, from which *I think* it used modelling of samples of satellite images in order to ascertain usage of land (e.g. agricultural, urban etc) in Europe. I believe the data covers 2000 to 2006.

So *I think* what has happened here is the following:

1. Daytime satellite images from 2000 to 2006 were sampled and modelled (by the European Environment Agency) to produce estimates of population density across Europe

2. Those estimates were used (by Tapissier) to estimate the light that would be sent up into space at night from those areas. (No indication of how - maybe just a simple estimate of light output per person?).

3. This is shown as a map, with colour coding to indicate intensity of light.

That's what I think, anyway. Does all this modelling accurately reflect what we would actually see on the ground? To answer that, we would need to have ground-based night-time sky measurements from sufficiently many locations. In which case we could just make the map from those.

Moral of the story:

Just because it looks like an image from space, doesn't mean it's an image from space.

And just because it shows light going into the sky, doesn't mean it tells you what things would look like on the ground.

But congratulations to F. Tapissier for a lot of hard work.

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acey

acey

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That image is based on 2012 data from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the SUOMI Satellite

http://www.nasa.gov/mission_pages/NPP/news/earth-at-night.html

Suitable clear-sky night-time images were selected and processed, then pieced together. A great deal of processing work went into it - very valuable data. Thouh again, as a way of finding a dark site anywhere near your house, it's a lot poorer than getting in a car and driving. 

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Liquid360

Liquid360

Posted
Posted

There's apps, dark sky finder, which in addition to using government maps, allows users to pin any site using GPS and then share that info among all users of the app. It's a great app!!

Maybe we could start a SGL LP map where we all pin out locations and take a 30 second ISO 800 f 5.6 50mm shot straight up at midnight attached ...

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DaveS

DaveS

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Maybe we could start a SGL LP map where we all pin out locations and take a 30 second ISO 800 f 5.6 50mm shot straight up at midnight attached ...

Good idea. Who will organise it? Are we well enough spaced to make something reliable?

What people need to do is set their DSLRs to ISO 800 and f/4 and take exposures of increasing length until the histogram peaks at 40%

Then use the scale from this site

http://www.pbase.com/samirkharusi/image/37608572

To estimate your NELM

Of course you *could* use a SQM but I think most people would have to buy one while most people here I think have DSLRs.

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knobby

knobby

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Posted

Good idea. Who will organise it? Are we well enough spaced to make something reliable?

What people need to do is set their DSLRs to ISO 800 and f/4 and take exposures of increasing length until the histogram peaks at 40%

Then use the scale from this site

http://www.pbase.com/samirkharusi/image/37608572

To estimate your NELM

Of course you *could* use a SQM but I think most people would have to buy one while most people here I think have DSLRs.

I guess organising it would be a nightmare as publicising it for a certain date / time , we'd be at the mercy of the weather (as usual)

Maybe if we all tried over a week in beginning of November once the clocks go back ?

Anyone interested ... Should we start a new thread ?

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RichM63

RichM63

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Good idea, but I don't think that a certain time and date are all that important.

If taken in Autumn/Winter then I would have thought that any time between say 10pm and 1am would suffice on a clear night. ie the time when humans and artificial light is likely to be most active.

After 1am I would expect to see a decrease in LP as folk turn out the lights and maybe cities or councils have automatic switch off policies.

Local LP is likely to be fairly constant on a normal night.

I'm up for it, and I'm generally well enough spaced. ;)

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