North on compass-oriented map does not coincide with Google North - ClubTread Community

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post #1 of (permalink) Old 06-09-2016, 04:38 PM Thread Starter
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Default North on compass-oriented map does not coincide with Google North

Hi
I have oriented a Google Earth Map printout for a certain location with a compass, but the compass North ( supposedly the magnetic North) on the oriented map points to a direction which is about 40 degr West of the North as shown on the Google Earth Map.

In other words, the direction of the North on the Google Earth map is exactly perpendicular to the upper edge of the map, whereas when I orient the map with a compass, the North appears shifted to the left ( westward by about 40 degr). How come? Perhaps the Google North on the map is not the Magnetic North? But even so, the magnetic declination is only about 13 degr. !

The Google Earth subject area on the search box is “ Anse a l’Orme Bay, Montreal, Quebec, Canada “.



Thank you for the elucidations

Ittiandro

Last edited by ittiandro; 06-09-2016 at 04:56 PM.
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post #2 of (permalink) Old 06-09-2016, 05:01 PM
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Weirdest first post ever!
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post #3 of (permalink) Old 06-09-2016, 05:56 PM Thread Starter
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Weirdest first post ever!
Weird answer, too ! I posted my question to have an answer, not to be rated in terms of weirdness !

I may have made a mistake in my calculations, all right, but hoped this Forum would be here to listen to questions and provide an answer. I am sure that somebody else will!
Besides, if an orienteering question like this is, as it seems according to your comment, beyond the scope of this Forum, I wonder what its real purview is ( in your mind, of course! ). Perhaps giving recipes for deodorizing smelly boots of hikers after a week’s hike in the Nahanni?


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post #4 of (permalink) Old 06-10-2016, 12:41 AM
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Quote:
Originally Posted by ittiandro View Post
Hi
I have oriented a Google Earth Map printout for a certain location with a compass, but the compass North ( supposedly the magnetic North) on the oriented map points to a direction which is about 40 degr West of the North as shown on the Google Earth Map.

In other words, the direction of the North on the Google Earth map is exactly perpendicular to the upper edge of the map, whereas when I orient the map with a compass, the North appears shifted to the left ( westward by about 40 degr). How come? Perhaps the Google North on the map is not the Magnetic North? But even so, the magnetic declination is only about 13 degr. !

The Google Earth subject area on the search box is “ Anse a l’Orme Bay, Montreal, Quebec, Canada “.



Thank you for the elucidations

Ittiandro
North on Google Maps is approximately true north (there can be discrepancies due to the projection Google uses). The most common error is to ensure you've adjusted your declination in the right direction (otherwise you'll be 30 degrees off)

Last edited by kellymcdonald78; 06-10-2016 at 10:52 AM. Reason: spelling
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post #5 of (permalink) Old 06-10-2016, 10:52 AM Thread Starter
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North on Google Maps is approximately true north (there can be discrepancies due to the projection Google uses). The most common error is the ensure you've adjusted your declination in the right direction (otherwise you'll be 30 degrees off)
Thank you so much

I probably made a mistake: my orienteering skills must have become a bit rusty with time !

I am a windsurfer and the reason for my question was to visualize on the map the wind direction on a lake in my area, in order to see if there is a correlation between wind direction and choppy waters. If there is one, I’d like to avoid a long drive, if the local weather forecast calls for a risky (=choppy waters) wind direction.

Unlike topographic maps, the Google maps have no grid and don’t explicitly show the true North direction, so I thought the True North may not necessarily coincide with the upper edge of the map (or be aligned with the vertical sides ) or even be substantially different. This is why I tried to orient the maps with a compass, but I came to realize that a 35-40 degrees variation is a bit too much and I retried..


In fact, on a 2nd attempt, I aligned the red pointer of the compass ( the one imprinted in the plastic) with the vertical side of the map and then I turned map and compass until the North red needle was aligned with the vertical side.

By doing this, I realized that I had the correct North : it coincides with the “ North “ ( the upper edge) of the map and with Google Earth : now the North is no longer at a 40 degrees angle. True, this is the Magnetic North and still not the True North, but 13 degrees of magnetic declination is not too much for my purposes.


I thank you for your answer . If you have any comments or something incorrect in my reasoning, please let me know.

Thanks

Last edited by ittiandro; 06-10-2016 at 10:53 AM. Reason: eliminate bold font
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post #6 of (permalink) Old 06-10-2016, 03:26 PM
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Just remember that magnetic declination near Montreal is -14 vs Vancouver +13, or a 27 degree difference
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post #7 of (permalink) Old 06-10-2016, 07:04 PM Thread Starter
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Originally Posted by kellymcdonald78 View Post
Just remember that magnetic declination near Montreal is -14 vs Vancouver +13, or a 27 degree difference
The topographic map I have for a local area here in Quebec shows the Magnetic North shifted to the West ( left) in relation to the True North, but the angle is 16°35', not 14°. Any comments on this?
Also, if you are familiar with Wind Guru, I wonder if you know if their wind direction and the degrees they give is in relation to the Magnetic North, as I think, or the True North.

Thank you again

Franco V.
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post #8 of (permalink) Old 06-10-2016, 07:19 PM
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post #9 of (permalink) Old 06-11-2016, 01:07 AM
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I'd check the date on your map as magnetic declination can change surprisingly quickly depending on where you are. As of today Montreal is -14.31 degrees, but I'd need the exact long/lat of your map to comment.

Sorry I am not familiar with Wind Guru
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post #10 of (permalink) Old 06-11-2016, 11:34 AM
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Local micro-climate patterns, including wind and wave conditions can often be different than the larger scale the forecast gives. Local conditions are created by the larger patterns, part of good local knowledge is correlating weather forecasts with what happens on your lake, but I wouldn't expect lake-level wind to match up with the large-scale trend. I'm just not so sure it's enough to draw a line on a map, even if orientation is precise. You'll just have to go to your lake over and over, observing conditions, seeing what forecast pattern creates the conditions you like. Then you will be able to look like a mystical seer in your ability to nail the conditions, just like a local!

I'm heading up to a lake I know well today and the wind there has only two patterns, it'll fall into one or the other depending on the overall flow, and on warm days creates it's own breeze (always one way) even with a dead calm forecast.
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post #11 of (permalink) Old 06-11-2016, 04:16 PM Thread Starter
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Quote:
Originally Posted by kellymcdonald78 View Post
I'd check the date on your map as magnetic declination can change surprisingly quickly depending on where you are. As of today Montreal is -14.31 degrees, but I'd need the exact long/lat of your map to comment.

Sorry I am not familiar with Wind Guru
You are absolutely right. I overlooked the yearly variation in the magnetic declination. The area is 46°40' N Lat and 73°35' W Long.The map showing a magnetic declination of 16°35'dates back to 2006. Using the magnetic declination calculator at:

http://geomag.nrcan.gc.ca/calc/mdcal-en.php


I get a magnetic declination for the area of 14° 57.24' West for 2016, which agrees with what you said

Thank you again

Last edited by ittiandro; 06-11-2016 at 04:18 PM.
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