Author Topic: New Light Pollution Atlas w/o Snow Cover  (Read 291 times)

ringnasingsimb

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #15 on: January 21, 2018, 04:11:29 AM »
I've just realized that my post directly above is not really the right way to look at the problem. It does correctly give the relative roles of reflected versus direct light (when there's no snow), but how this translates to the snow problem and the satellite bias is not clear from the above graph. I'll have the revised figure soon, but I have to go.

The short answer is:
when there is snow on the ground the satellite over-estimates the light pollution by a factor of 1.6 near the source, a factor of 3 at 50km, a factor of 4 at 170km etc. (This is if you simply apply the light pollution model without changing the parameter related to the reflectivity of the surface.)

-Dave

Jason Simpson

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #16 on: January 23, 2018, 05:25:21 AM »
I now have results for how snow cover affects the satellite compared to how it affects "reality" (i.e. the light pollution model but properly taking into account the reflectivity of snow). The original light polution model has the albedo of the ground as a parameter, so changing this from the default setting to a snow-like setting will give the effect of snow.The albedo of snow is:Fresh snow, dry = 0.85Fresh snow, wet = 0.80Old snow, dry, clean = 0.70Old snow, wet, clean = 0.60Old snow, wet, medium dirty = 0.50Old snow, wet, heavily dirty = 0.40I used the new wet snow (0.80) and the old, wet, clean snow (0.60). The effect of snow in "reality" with these parameter settings is given by the purple and blue curves. (In my previous post I used an albedo of 0.85.) The snow effect is largest closest to the source, and decreases to about 25% larger at a distance of 300km.If the satellite predominantly measures light directed nearly straight up, then it is measuring mostly reflected light. To make the light pollution map from a satellite, you use this "straight up" light as a proxy for the light in all directions. If you assume the wrong surface albedo (which is what you do if you apply an asphalt/concrete albedo to snow covered ground (as in the original atlas)), then you essentially multiply all the light by a certain factor independent of direction. This is unlike reality, where snow only changes the reflected component. The factor that you wrongly apply to all light is simply the albedo of snow divided by default, asphalt/concrete albedo (=0.15). The effect of snow wrongly estimated from the satellite is shown by the red and orange lines. So as Tony said, the original light pollution atlas is not very useful <em class="bbc">even when there <em class="bbc">is snow on the ground. For example, if the snow albedo is 0.6, the orignal light pollution atlas increases the light pollution by a factor of 4 everywhere (= 1.3 light zones). In reality, the effect of snow is more like the blue curve, which is a factor of 1.7 at 30km (= 0.5 light zones) and smaller for greater distnces. If, on the other hand, you do take into account the snow albedo, then the satellite data shoud correctly recover the blue and purple curves.-Dave(For a point source, the blue and purple curves would go up to the orange and red at very close distances. The source here has a finite size.)

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steviselath

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #17 on: January 23, 2018, 05:31:40 PM »
Here I compare Tony's Sky Quality Meter data in the Boston area with the light pollution atlas using the 1996/1997 data and the 2001 data. Tony's data is from his blog entry entitled the Ground Truth for the Light Pollution Atlas.

The x-axis in the figure is the distance from Cambridge Common and the y-axis is the brightness in magnitudes per square acrsec (bigger means darker). The black line is the actual measurements taken by Tony with a Sky Quality Meter (SQM-L). The red and the blue lines are from my calculations of the atlas using the 1996/1997 data (which is contaminated with snow cover) and the new 2001 data.

As Tony mentioned above, the new atlas based on the 2001 data is in better agreement with real observations. Nevertheless, the 2001 atlas still has a tendency to over-estimate the brightness (i.e. the blue line is too low compared to the black line). Also note the jumps and plateaus in the real data which are not in the atlases (see Tony's blog).

The discrepency could be caused by a variety of different things: 1) the day Tony took measurements could have been especially clear of aerosols. Therefore, if we change the aerosol parameter in the model, we could reproduce the data, 2) other parameters in the model just need to be "tuned", 3) the assumed relationship between the satellite brightness and the light sources needs to be modified or 4) something is more fundamentally wrong with the model.

-Dave

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cieledrore

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #18 on: January 25, 2018, 01:54:23 PM »
Quote
The discrepancy could be caused by a variety of different things: 1) the day Tony took measurements could have been especially clear of aerosols ... 2) other parameters in the model just need to be "tuned", 3) the assumed relationship between the satellite brightness and the light sources needs to be modified or 4) something is more fundamentally wrong with the model.

Yet another possibility -- something is fundamentally wrong with the SQM-L. Remember, this is not a sophisticated scientific instrument. It's a clever, cheap gizmo that has been tuned to some semblance of scientific accuracy by seat-of-the-pants methods. I think that two different SQM-L measurements are comparable to each other, but you can't necessarily expect them to agree with results from scientific istruments. Among other issues, the SQM-L is well-known not to have the same spectral response as a CCD fitted with a V-band filter.

Steven Autio

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #19 on: January 25, 2018, 03:29:06 PM »
Dave,

First off,
That's a lot of hard work that went into this.  This is a long-overdue refinement of the light pollution map.  Regardless of where your new model still has lingering issues, one thing's certain, at least IMO--it's more accurate than the previous one.

I frequent many sites in Ohio and noticed that a lot them in the "yellow" zones were actually quite dark.  The areas around Mohican State Park and, in NW Ohio, the area around Harrison Lake always seemed like they were darker than their "yellow zone" would indicate based on observations I'd had there.

Many sites in SE Ohio ("green" on the old map) were good enough to rival some "blue" zone sites in western PA that I'd previously used.  According to the new data, some of these sites, notably the Zaleski State Forest / Lake Hope State Park area are borderline blue/green, and I've found this matches previous observations of sky darkness.

Further refinement, as you've already pointed out, is necessary.  But a job well done, nonetheless.  The 2001 data and accompanying image much more closely resemble sky conditions I've seen in the areas that I've frequented.

One thing I did note was that Cherry Springs, according to the 2001 data, has gone from "blue" to "dark gray".  I'd have to heartily concur.  I use a "dark gray" site frequently, Calhoun County Park, WV, and on nights of similar transparency, they're very close.

Clear Skies,
Phil

manreistilles

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #20 on: January 25, 2018, 07:59:36 PM »
The new map looks more accurate to what I’ve experienced living in rural NY, with a camp in the central Adirondacks, plus frequent trips to Cherry Springs and once-a-year to Stellafane.Snow is bad news for observational astronomy! I’ve been getting about .3 to .5 brighter SQM readings with snow cover, and qualitatively the sky just looks brighter – not to mention the annoying light from the ground while trying to observe.

artufanchess

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #21 on: January 26, 2018, 01:43:14 AM »
I'm posting because I have discovered a relatively minor problem with the 2001 DSMP satellite data used to make the new atlas. Apparently, in some locations the light sources in the 2001 data are displaced from where they should be. The maximum displacement is about 4 to 5 km in the east/west direction and 2 to 3 km in the north/south direction. I have determined the origin of this problem is the DSMP satellite data online because there are also independent population and land-use data in the same location as the DSMP satellite data. These are on the same projection/grid and there is no horizontal displacement in these datasets. I dicovered this problem by overlaying the maps in google earth and zooming in on small towns.

I think I might be able to come up with an algorithm to fix the problem.

This doesn't affect the what we have talking about above, and you would never notice it if you didn't zoom in on google maps. Nevertheless I'm sorry I didn't check things out more closely before I posted.

-Dave

Aaron Maggot

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #22 on: January 26, 2018, 02:14:34 AM »
I've come up with an algorithm to fix the displacement errors in the 2001 light data. The new data is now available on the webpage. The fix is not perfect but it's definitely better than before. Most people will not notice, unless you overlay the maps in google earth and zoom way in.

The adjustment is not uniform across the map and some area have no adjustment. The biggest adjustment is about 5 pixels (the size of a pixel is 1/120 degrees). I'll add more detail on the adjustment on the web site soon.

-Dave

telschronexic

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #23 on: January 30, 2018, 06:38:08 AM »
You can read about the small position errors in the 2001 light data and how I corrected for them here. None of this effects the brightness of the lights, etc.The corrections are small (I thought about reposting the figures in this thread, but I can't see a difference after I apply the corrections). You will only notice the improvement when you compare the light pollution atlas to small towns when zoomed in in Google Earth or something similar.-Dave

Greg Fleming

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #24 on: January 31, 2018, 01:33:50 AM »
Dave, thanks for all the work you have put into this!  It is greatly appreciated!

John Abreu

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #25 on: January 31, 2018, 03:02:38 AM »
Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. The image should be semi-transparent so that you can see the info from google maps underneath (I'm hoping all web browsers show it the same way).

-Dave

Artavius Murphy

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #26 on: January 31, 2018, 10:32:22 AM »
Quote
Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. ...-Dave

   It works great Dave, and at least in my area of NY/PA it makes more sense than the old map.

niososerso

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #27 on: February 02, 2018, 07:02:21 PM »
Thank you Dave for this valuable tool!  I used to observe around Wildcat Mountain S.P. in Wisconsin and the skies seemed much darker than a green zone.  Indeed, it's actually in the GRAY!  Likewise, my dark observing sites in Maryland just got darker.

carewemi

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #28 on: February 02, 2018, 10:36:23 PM »
Your welcome! I usually observe in southwest Wisconsin (although not at Wildcat Mountain unless I'm camping there too). It turns out that all my green dark sky sites are actually blue and my blue dark sky sites are gray. These new colors seem to make more sense to me when I compare my sites to ones further south.-Dave

David Williams

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #29 on: February 04, 2018, 09:45:03 AM »
Thanks for all great work. Glad to see that my neck of the woods--NW Wyoming/Jackson Hole is getting darker!Dark skies.mm