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Mike73

LP maps - Is it that bad?

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I used to use Needless-org for to get a rough idea how good or bad possible dark sites would be but since I've been taking my own sky measurements I've discovered that Needless is way out on its estimations.

I know Needless is just a simulation but I used to use it a lot and travel up to an hours drive just to get somewhere that it said was good when I could of stayed a lot closer to home and had just as dark skies.

My local dark site Needless says its LM 5.5 but its actual readings from NELM and SQM are more like 6.3!

This winter I'II be saving a lot of money in fuel and staying closer to home!

Anyone else compared actual on the ground readings to a LP map and had a pleasant surprise? :)

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Can't say that I have Mike but would have thought you were fairly spoilt for dark skies down there in Cornwall. I'm at our caravan in Newquay in Ceredigion and the difference is chalk and cheese here to back home. Not sure what scale is relevant to the actual seeing conditions just know that if I look up the milky way is there looking back at me. Sorry if I've gone a bit off topic still marvelling at the skies down here. Just pictured yours on the coast down there being the same. :-)

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I wouldn't worry about sites like this. At best they are there to make a point, rather than to give precise readings to specific locations. It's unclear where they get their basic data from, to make a LP model - or when it was last updated. Also, they don't take into account natural variation (up to ¾ of a Magnitude) in VLM, due to solar activity. The best way to get an idea of what LP will be like is simply to look at where local population centres are, and find a place well away from them.

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We've experimented with overlaying the excellent Avex light pollution map, which is basically a population map with a point spread function applied, onto our own (MyDarkSky) map, which is based on both naked eye limiting magnitude and sky quality meter data. We've found that the correlation is quite impressive. Not 100% but it certainly gives the gist - kudos to Avex.

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I was looking at the Needless Sky Simulator the other day and was thinking the same. Areas of Snowdonia looked quite poor because there was a tiny village nearby (perhaps even in the next valley).

I think most of the images of light pollution are in Infra Red, small towns will be warmer than the surrounding countryside but may be quite dark.

One mile from where I live is a very small nature reserve/park surrounded by bright city lights, the sky is much darker there.

The inverse square law works in our favour regarding LP. If I were one mile from a light source (town) then travelled a further mile away the direct light will be one quarter the brightness not half. The light hitting the sky directly above even less. edit: A bit of a simplistic statement/illustration but I'm a simpleton :)

Alex

Edited by alexog

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I don't think it's consistent, light pollution levels change dependent on skies condition's, factors such as humidity, general pollution, reflected light, due to wet surfaces, even living 4.5 miles from Manchester city centre I have had some cracking nights. I must confess I start AP because of light pollution, but generally it's not to bad if the conditions are right, saying that, that’s not very often, but hay ho, that’s what you get from living on the outskirts of a large city.

Edited by Si W

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