Danny, Rick -
Thanks for the explanation about why the mapped CBC circles using Audubon's
coordinates are not necessarily where they are supposed to be. Unknowing
that Audubon had or was producing an online map of all the circles to work
in Google Earth, I created my own map using the same program to match the
"MD and Nearby Area" CBC list that gets posted on MOS website and in the
Yellowthroat. Actually, I should say "recreate" a map of CBC circles
because when I first joined MOS back around 2004 there used to be a map of
MD CBC circles on the MOS website that I learned this past fall had been
done by Phil Davis.
Shortly after posting the link to my CBC map of MD area circles on MOS
Website, I got some emails from folks (including Phil) around the state
that some circles seemed off from the orignal paper maps they had. Seneca
seems shifted east by a 100 yards or so. Phil Davis felt Annapolis circle
might be off base by possibly 1/2 mile. Even the mapped center point of the
newly minted Loch Raven count was off by about 25 yards according to what
was expected by Kevin Graff. I had no explanation for them until now.
Guess I will need to add a disclaimer to the CBC page at MOS site, at least
until I can get some more accurate center points and re-map it.
Andy Martin
Gaithersburg
On Wed, Dec 19, 2012 at 8:29 AM, Ross Geredien <goodmigrations...>wrote:
> FYI, I have made full color GISaps of the Annapolis count circle if anyone
> wants a copy. I can send digitally. Also, let me know if you want a
> close-up of one of the Area/Sector numbers.
>
> Ross Geredien
> Edgewater
>
> Sent from my iPhone
>
> On Dec 19, 2012, at 7:29 AM, danny bystrak <dbystrak...> wrote:
>
> Rick and all,
>
> Actually there is an issue with probably all of the CBC count centers.
> For many years I was the "official" (volunteer) approver of all new CBC
> circles for Audubon. This was back in the days when maps were on paper and
> determining center coordinates was challanging in most cases. I had access
> to all the 7 1/2 minute topographic maps at work, so I would do my best to
> get them as accurate as possible. But the problem was not one of accuracy,
> it was one of precision. Audubon only wanted the coordinates to the
> nearest minute of latitude and longitude, so in most cases they were off by
> as much as a half mile or so even if they were completely accurate. When
> they put the data on the web, they used those 'nearest minute' coordinates
> to calculate the decimal degrees. The decimal degrees were calculated to as
> many as 4 decimal places (remember 'significant digits?'), giving the
> impression of great precision. But because they were based on the original
> rounded minutes, they rarely pinpoint you at the actual center. This is
> very obvious when the Annapolis, Bowie and Triadelphia circles are
> plotted. We positioned the Bowie count to exactly touch the other two
> circles, but not overlap. When you plot the three, there is overlap.
> Also, the center of the Annapolis count is the south end of the rte 50
> bidge over the South River. The coordinates at Audubon are .58 mi. WSW of
> there. So looking at the edge of a circle for what is in or out can be
> tricky. Of course, it can be argued that a half mile is not a big deal,
> but many counts were positioned to capture specific areas at the edges.
>
> Now that we have Google Earth and other such resources, it would probably
> be wise for count compilers to re-figure center coordinates to an accurate
> 4-decimal place figure and inform Audubon to correct them. Audubon should
> also have a disclaimer about this and somehow differentiate corrected
> centers from the "incorrect" ones.
>
> Danny Bystrak
> Bristol A A Co.
>
> *From:* Rick Hollis <xiboia...>
> *To:* MdDC Birds <mdbirding...>
> *Sent:* Tuesday, December 18, 2012 6:44 PM
> *Subject:* [MDBirding] KLZ files [google earth] for count circles
>
>
> The NAS has count circle files which are downloadable at
> http://birds.audubon.org/faq/where-can-i-see-cbc-circles-map
>
> http://www.nemesisbird.com/2012/12/mapping-the-christmas-bird-count/ has
> a discussion
>
> BUT there is a issue with some of the centers. The Iowa City Christmas
> Count Circle was off by 1.4 mile in the data release in 2011. The files
> they released this year is even farther off, 1.67 miles.
>
> I am not sure what the NAS chart options descried in the nemisisbird post
> above, gives the wrong center for our count.
>
> Having said that, the Google Earth views are extraordinary -- far better
> than any old fashion maps. If you locate your real center and draw your
> circle, you can find out whether bushes are in count circles -- the photos
> for our circle are within the past couple months. My Mom's house in Balto
> has 2010, where as my Sis [Surgarloaf Circle] has 2011 images.
>
> Rick Hollis
> North Liberty IA & Arbutus MD
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