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Sparavigna, A. (2016). The Charity Shoal Crater of Lake Ontario in Google Earth Images. PHILICA.COM Article number 597.

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The Charity Shoal Crater of Lake Ontario in Google Earth Images

Amelia Carolina Sparavignaunconfirmed user (Department of Applied Science and Technology, Politecnico di Torino)

Published in geo.philica.com

Abstract
In the Lake Ontario it exists a circular depression being a kilometer in diameter, having a continuous encircling rim that in the nautical charts is referred to as the Charity Shoal. Since this circular structure could be an impact crater, it is also known as the Charity Shoal Crater. The rim rises to depths of 2-6 meters below the level of the lake. In this manner, besides that in bathymetric maps, the crater can be seen in satellite images too, as we will shown here, using a Google Earth image and a suitable image processing of it.

Article body

 

 

The Charity Shoal Crater of Lake Ontario in Google Earth Images

Amelia Carolina Sparavigna,

Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy 

     

Abstract

In the Lake Ontario it exists a circular depression being a kilometer in diameter, having a continuous encircling rim that in the nautical charts is referred to as the Charity Shoal. Since this circular structure could be an impact crater, it is also known as the Charity Shoal Crater. The rim rises to depths of 2-6 meters below the level of the lake. In this manner, besides that in bathymetric maps, the crater can be seen in satellite images too, as we will shown here, using a Google Earth image and a suitable image processing of it.

 

Keywords: Google Earth, Image Processing, Fractional Gradient, Astrofractool.

         
 

As told in a study of the National Oceanic and Atmospheric Administration (NOAA) [1], made for the Great Lakes Data Rescue Project, in the Lake Ontario it exists a circular depression being a kilometre in diameter, having a continuous encircling rim. The circular basin is slightly deeper than 18 meters and the rim rises to depths of 2-6 meters, below the level of the lake. The rim coincides with the feature that in the nautical charts is referred to as the Charity Shoal. At southwest, to the circular rim it is connected an elongated ridge.

According to NOAA, the origin of the Charity Shoal could be that of a meteor crater. In fact, besides the circular rim, the aeromagnetic mapping by the Geological Survey of Canada had shown that the Charity Shoal has a negative magnetic anomaly, which is a characteristic of simple impact craters [1,2]. About the age of this crater, it is hypothesized to be of Middle Ordovician period [3,4].

The Charity Shoal ring was first observed in 1990 [5], then it was mapped in detail as part of a bathymetric map of Lake Ontario by NOAA. In 2010 and 2011, high-resolution bathymetric and acoustic backscatter data were collected, followed in 2012, by more detailed magnetic and bathymetric surveys [6]. As we will show in the following discussion, the crater is visible also in the satellite images, after being subjected to some specific image processing.

In the Google Earth, the crater is visible in the bathymetric map (Figure 1, left panel), whereas in a satellite image of 2004, the same area appears of deep blue with a very fain circle in the middle (Figure 1, right panel).  We can apply to the image on the right of Figure 1 some processing in order to see the appearance of this crater. To improve the visibility of this structure, several approaches are possible, the more immediate are those we can obtain using the GIMP, that is, the GNU Image Manipulation Program. In the Figure 2 we can see the results (for a detailed discussion and examples of the use of GIMP in enhancing images, see please References 7 and 8): in the panel A it is given the original image, B it is showing the result of a black and white thresholding, C is the result obtained by an equalization of the histogram and D is the result of a GIMP Retinex filtering, converted into grey tones. However, we can also apply to the image an approach previously used in [9]. Before adjusting with GIMP the brightness and contrast of the image, it is pre-processed using AstroFracTool, a software we developed, based on the fractional gradient [10]. In this case, we use AstroFracTool as a denoising filter. In fact, with a negative fractional index, the fractional differentiation becomes a fractional integral, and then the fractional approach can be used for denoising [11].

From Figures 2 and 3, comparing the southwest tail we can observe in them and the bathymetric map of Ref.1, we can argue that in the satellite image it is possible to distinguish some features till to a depth of about 15 meters. Then, the method proposed in this paper could be interesting for other investigations, such as archaeological researches, based on the satellite images of the shallow waters of coastal areas of lakes and seas.  

 

References

[1] Vv. Aa. NOAA, Great Lakes Data Rescue Project, Lake Ontario, retrieved on May 9, 2016, at the web page  Bathymetryhttps://www.ngdc.noaa.gov/mgg/greatlakes/lakeontario_cdrom/html/gmorph.htm

[2] Pilkington, M., & Grieve, R. A. F. (1992). The Geophysical Signature of Terrestrial Impact Craters, Reviews of Geophysics, 30(2), 161-181.

[3] Holcombe, T.L., Youngblut, S., & Slowey, N. (2013) Geological structure of Charity Shoal crater, Lake Ontario, revealed by multi beam bathymetry. Geo-Marine Letters, 33(4), 245-252.

[4] Suttak, P.A. (2013). High-resolution lake-based magnetic mapping and modeling of basement structures, with examples from Küçükçekmece Lagoon, Turkey and Charity Shoal, Lake Ontario, MS thesis, School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario.

[5] Edsall, T.A. (1990) Surficial substrates and bathymetry of five historical lake trout spawning reefs in nearshore waters of the Great Lakes. Great Lakes Fishery Commission. Ann Arbor, Michigan.

[6] Vv. Aa. (2016), Wikipedia, Charity Shoal Crater, and references therein.

[7] Sparavigna, A. C. (2015). An image processing approach based on Gnu Image Manipulation Program Gimp to the panoramic radiography. International Journal of Sciences, 4(5), 57-67.

[8] Sparavigna, A. C. (2015). Gimp Retinex for enhancing images from microscopes. International Journal of Sciences, 4(6), 72-79.

[9] Sparavigna, A. C. (2010). Crater-like landform in Bayuda desert (a processing of satellite images). arXiv preprint arXiv:1008.0500.

[10] Marazzato, R., & Sparavigna, A. C. (2009). Astronomical image processing based on fractional calculus: the AstroFracTool. arXiv preprint arXiv:0910.4637.

[11] Jalab, H. A., & Ibrahim, R. W. (2012). Denoising algorithm based on generalized fractional integral operator with two parameters. Discrete Dynamics in Nature and Society, 2012.

 

Figure 1 - In Google Earth, the Charity Shoal Crater is visible in the bathymetric map (left panel), whereas in the satellite image of 2004, the same area appears of deep blue with a very fain circle in the middle (right panel). 

 

Figure 2 – The images are showing results of the use of GIMP in enhancing the original image given in the panel A. B it is showing the result of a black and white thresholding, C is the result obtained by an equalization of the histogram and D is the result of the GIMP Retinex filtering, converted into grey tones.

 

Figure 3 –  The images are showing results of enhancing the original image given in the panel A. B it is showing the result of adjusting brightness and contrast with GIMP. C is the result of the filtering with AstroFracTool (fractional order of gradient (nu) = – 0.8, contrast enhancement (alpha) = 0.5). D is the result of a further adjustment of contrast and brightness of C using GIMP. Note the denoising effect obtained by the AstroFracTool filtering.   



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The full citation for this Article is:
Sparavigna, A. (2016). The Charity Shoal Crater of Lake Ontario in Google Earth Images. PHILICA.COM Article number 597.


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1 Author comment added 10th May, 2016 at 11:11:17

In the discussion and in the caption of the Figure 1, ” fain ” is a misprint for ” faint ” . Let me stress that, without a processing of the given image, the crater is too faint for being visible.




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