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Author Topic: Newbie question about map projections  (Read 2361 times)

gnorthern

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Newbie question about map projections
« on: June 16, 2009, 12:15:11 PM »
Can somebody explain to me how map projections work in GPS units?  It seems to me you need to do something, like set the GPS unit to the correct map projection before you load the map?  If you do, how do you know the projection of the map you loaded, such as the Ibycus data on this web site.  Or does Mapsource magically handle everything so you do not need to worry about that? 

One more question.  I just got my GPS and tried it yesterday, recorded a route, and changed the projection to the two most common I see (NAD27 and 83).  The position of the route relative to the map seemed to stay the same.  Are GPS units so smart they adjust everything realted to the map projection on the fly or was this a coincidence?

Thanks for the answer.

leszekp

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Re: Newbie question about map projections
« Reply #1 on: June 16, 2009, 03:45:01 PM »
The native coordinate system for all GPS is geographic (latitude/longitude), WGS84 datum. If you set a different coordinate system for display, the unit does the coordinate display conversion on the fly, but all data you display and save (waypoints, tracks, etc.) is still in geographic/WGS84. MapSource sends all data to the GPS in that coordinate system, and also receives any data from a GPS in that coordinate system. As you say, "MapSource magically handles everything".

NAD27 and NAD83 aren't projections, they're datums, different shapes of the world that flat maps are projected onto; NAD83 is virtually the same as WGS84, within a meter across the entire US. It's not that the GPS units are so smart that they correct for different datums, it's that changing the datum only changes the coordinate displays; the data displayed stays in geographic/WGS84.

-Oz-

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Re: Newbie question about map projections
« Reply #2 on: June 16, 2009, 05:07:23 PM »
With the technical answer done... you don't need to change anything when you load the maps; the GPS is smart enough to know.
Dan Blomberg
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gnorthern

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Re: Newbie question about map projections
« Reply #3 on: June 16, 2009, 09:06:32 PM »
Thanks for both answers.  The technical answer explained a lot of things, but the magic answer made it 100% certain.  I also needed to get my terms straight since it has been so long since I learned the terms.  I do not need the terms to read a map and compass, but I do need them to communicate.


 

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