CoSQM - a color hack to the SQM-LE for Light At Night Sensing


Color mapping of the night sky brightness over the Mont Bellevue natural reserve in Canada


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1.  Abstract

We present a methodology to map the night sky brightness in 4 spectral bands on the territory of the Parc du Mont-Bellevue natural reserve (Sherbrooke, Canada). In the long term, we aim to estimate a part of the exposure of living organisms to the indirect urban light as a function of the colors scattered in the atmosphere overhanging the natural reserve. The reserve is located near the Sherbrooke city center inside the city perimeter. If repeated regularly, the mapping technique can become an influential tool to estimate the light pollution transformation as a function of the ongoing Sherbrooke’s light fixture replacement program. In most cases, HPS are replaced by phosphor converted amber LED. Such transformation may shift the color composition of the sky, the major source for the artificial light propagating into the reserve. Our project is part of a new initiative aiming the creation of an “Urban Starry Sky Oasis”, an urban public area where we hope to give back, to the citizens, an easy access to the Milky Way. We performed the mapping with the data acquired with the CoSQM, a new portable device which aims to sample the multispectral properties of the artificial light scattered by the atmosphere.

Poster presented at the Light Pollution Theory Modeling and Measurement - Zselic Valley Leisure Farm, Hungary, June 25-28, 2019

2.  Equipment

The measurements were made using an expedition utility sled on which is fixed a stabilizing support that holds in place the CoSQM.

2.1  Expedition utility sled

Made of high-density polyethylene

Dimensions : 114,3 cm x 50,8 cm x 20 cm

2.2  Stabilizing support

Necessary material to make the stabilizing support :

  • Perforated steel strapping (3/4 IN X 10 FT)
  • Plywood (3/4 IN X 2 FT X 2FT)
  • Wood screw (No. 8, 3/4 IN)
  • Wood screw (No. 8, 1 1/2 IN)
  • 4 T HINGE (3 IN)

3.  Conditions of experimentation

Starting date of measurements 2019-06-03 2019-07-03 2019-08-03
Distance traveled 7,20 km 5,52 km 8,32 km
Positive elevation gain 176 m 172 m 223 m
Max altitude 330 m 359 m 368 m
Elapsed time 4:26:19 3:06:27 3:55:16
Average pace 18:03/km 18:24/km 17:10/km
Fastest intermediate pace 18:08/km 22:14/km 18: 47/km
Average speed 3,3 km/h 3,3 km/h 3,5 km/h
Conditions of visibility of the sky Clear sky

Viewing weather
Clear sky

Viewing weather
Clear sky

Viewing weather

You can find below the work plan of the Mont-Bellevue Natural Reserve Alliance.

4.  Distance covered over the Mont-Bellevue

5.  Maps

The maps represent the artificial light at zenith perceived in the Mont Bellevue natural reserve (Sherbrooke, Canada). The reserve is surrounded by a ski station, thus we compared the sky brightness during and after its operation times.

5.1  Night sky brightness perceived by the clear filter (SQM-LE)

Note that the magnitudes per square arc seconds decrease with increasing brightness, so the lower value is the brightest perceived.

During ski station operationAfter extinction of ski station lights

5.2  Blue index

The blue color index (B-C) indicates the difference between the sky brightness in the blue spectral band and the SQM-LE measurement (clear filter). Note that the sky brightness decrease with increasing radiance, so the smaller B-C is, the bluer is the sky.

During ski station operationAfter extinction of ski station lights

5.3  B-G index

The color index B-G indicates the difference between the sky brightness in the blue spectral band and the green spectral. Note that the sky brightness decrease with increasing radiance, so the smaller B-G is, the bluer is the sky.

During ski station operationAfter extinction of ski station lights

5.4  B-Y index

The color index B-Y indicates the difference between the sky brightness in the blue spectral band and the yellow spectral band. Note that the sky brightness decrease with increasing radiance, so the smaller B-Y is, the bluer is the sky.

During ski station operationAfter extinction of ski station lights

5.5  B-R index

The color index B-R indicates the difference between the sky brightness in the blue spectral band and the red spectral band.

During ski station operationAfter extinction of ski station lights

6.  Mapping methodology

6.1  Download QGIS

We used the 2.18.17 version of the software in this methodology, but you can download a newer version if you prefer.

Complete documentation about the 2.18.17 version

Install GRASS GIS plugin

The Geographic Resources Analysis Support System (GRASS) open source plugin is used as a tool with several GIS functions.

  • In the top toolbar, click on Plugins.
    • Click on Manage and Install Plugins....
      • In the Plugins Window, search for GRASS then click on Install button.

6.2  Comma-separated values (CSV) file

Make sure to convert your data file in CSV format. This is a delimited text file representing tabular data as comma-separated values.

6.3  Adjusting the project Coordinate Reference System (CRS)

  • Adjust the project CRS from the top toolbar and click on Project.
    • Click on Project Properties.
      • Click on the General tab.
        • Under General settings :
          • Project home : Set your project home path.
          • Project title : Set the project name as Before Mont-Bellevue.
        • Under Measurements : Leave default features.
        • Under Coordinate display : Leave default features.
      • Click on the CRS tab.
        • Check Enable 'on the fly' CRS transformation (OTF).
        • Set the Coordinate Reference System to WGS 84 / Pseudo-Mercator [EPSG:4326].

6.4  Adding basemaps to your QGIS project

Install QuickMapServices plugin

The QuickMapServices plugin is used to add basemaps to your QGIS project.

  • In the top toolbar, click on Plugins.
    • Click on Manage and Install Plugins....
      • In the Plugins Window, search for QuickMapServices then click on Install button.

Add a map background

We recommend the use of Wikimedia Commons since it is a multilingual library of content (images, sounds and videos) for educational purposes in the public domain or published under free license.

  • Add a map background from the top toolbar : click on Web.
    • Click on QuickMapServices.
      • Click on OSM.
        • Click on Wikimedia Map.

6.5  Creating a perimeter border of the Parc du Mont-Bellevue territory

Importing geomatics data of the Parc du Mont-Bellevue

Adjusting selected area (Parc du Mont-Bellevue)

  • Adjust the selected area from the toolbar : click on to select features by area or single click and then click on the Parc du Mont-Bellevue natural reserve area (or the selected region to map) and it will become yellow.
  • In the Layers panel (left side of the screen) : right click on Aires_aménagées.
    • Click on Save as.
      • Set the Format to Erpi Shapefile by using the scrolling bar.
      • File name : Set the output shapefile by pressing Browse button. Navigate to the data folder and enter the new filename as Extend Mont-Bellevue.
      • CRS : Selected CRS [EPSG:4326, WGS 84] by default.
      • Check Save only selected features.
      • Check Add saved file to map.
  • In the Layers panel (left side of the screen) : right click on Aires_aménagées and then click on to remove layer/group currently selected.

Reprojecting Extend Mont-Bellevue

To reproject and transform vector datasets to set the appropriate Coordinate Reference Systems (CRS).

  • In the Processing Toolbox search bar(on the right side of the screen) : type Reproject layer.
    • Set the Input layer to Extend Mont-Bellevue [EPSG:4326].
      • Set the Target CRS to NAD83/Quebec Lambert [EPSG:32198] by clicking on .
  • Adjust the style properties from the layers panel : double click on Reprojected Extend Mont-Bellevue.
    • Click on the Style tab.
      • Set the Outline to a black color.
      • Set the Fill style to No Brush.
      • Set the Outline style to Solid Line.
      • Set the Outline width to 0.500000. This will be 0.5 millimeters.

6.6  Creating a layer from your color mapping data

Importing file data

You can find the data used for the map creation below, but the following methodology is performed with the data before closing the lighting of a ski station, which surrounds the Mont Bellevue natural reserve.

Data used for the color mapping - before closing the lighting

Data used for the color mapping - after closing the lighting

  • Create a new layer from the top toolbar and click on Layer.
    • Click on Add Layer.
      • Click on Add Delimited Text Layer.
        • File name : Set the output shapefile by pressing Browse button . Navigate to the data folder and enter the new filename as before.shp.
        • Set the File format to CSV (Comma-separated values).

Reprojecting datasets

To reproject and transform vector datasets to set the appropriate Coordinate Reference Systems (CRS).

  • In the Processing Toolbox search bar type reproject layer.
    • Set the Input layer to before [EPSG:4326].
      • Set the Target CRS to NAD83/Quebec Lambert [EPSG:32198] by clicking on .
  • In the layers panel, double click on Reprojected before.
    • Click on the Style tab.
      • Click on Marker.
        • Click on the city symbol.
      • Click on Simple Marker.
        • Set the Outline to a black color.
        • Set the Size to 1.000000. This will be 1.0 millimeters.
        • Set the Outline style to Solid Line.
        • Set the Outline width to 0.500000. This will be 0.5 millimeters.

Fixing distance buffer

  • Run the buffer process from the top toolbar : click on Vector.
    • Click on Geoprocessing tools.
      • Click on Fixed distance buffer.
        • Set the Input layer to Reprojected before [EPSG:32198].
        • Enter buffer Distance of 200. This will be 200 meters.
        • Enter Segments to 100.
        • Check Dissolve result.
        • Buffer : Set the output shapefile by pressing Browse button . Navigate to the data folder and enter the new filename as Buffer_reprojected_before.shp.
        • Check Open output file after running algorithm and and the resulting shapefile will be added as a layer in your map view.
        • Press Run to run the process.

Vector overlay - Intersection

  • Run the intersection process from the top toolbar : click on Vector.
    • Click on Geoprocessing tools.
      • Click on Intersection.
        • Set the Input layer to Buffer reprojected before [EPSG:32198].
        • Set the Intersect layer to Reprojected Extend Mont-Bellevue [EPSG:32198].
        • Intersection : Set the output shapefile by pressing Browse button . Navigate to the data folder and enter the new filename as Intersection_before.shp.
        • Check Open output file after running algorithm and and the resulting shapefile will be added as a layer in your map view.
        • Press Run to run the process.

Making voronoi polygons

  • Run the voronoi polygons process from the top toolbar : Vector
  • Click on Geometry tools
    • Click on Voronoi polygons
      • Set the Input layer to Reprojected before [EPSG:32198].
      • Enter Buffer region of 100. This will be 100%.
      • Voronoi polygons : Set the output shapefile by pressing Browse button . Navigate to the data folder and enter the new filename as voronoi_polygons_before.shp.
      • Check Open output file after running algorithm and and the resulting shapefile will be added as a layer in your map view.
      • Press Run to run the process.

Clipping vectors

  • Run the clipping process from the top toolbar and click on Vector.
    • Click on Geoprocessing tools.
      • Click on Clip.
        • Set the Input layer to Voronoi polygons_before [EPSG:32198].
        • Set the Clip layer to Intersection_before [EPSG:32198].
        • Clipped : Set the output shapefile by pressing Browse button . Navigate to the data folder and enter the new filename as clipped_before.shp.
        • Press Run to run the process.
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