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Forums —» General Caching —» How To? —» Round earth vs Flat earth
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Round earth vs Flat earth
Have question about how to use your GPS, hide a cache or solve a problem, let the local experts try to help.
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Derigol



Joined: Oct 06, 2006
Posts: 3
Location: Victoria

PostPosted: Mon Feb 12, 2007 6:11 pm    Post subject: Round earth vs Flat earth Reply with quote

A couple of caches that I have been to tell me to assume round earth. Does this mean that my GPSr calculates waypoints in a straight line?
If so how to I convert to round earth measurements.

Derigol
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MrGigabyte



Joined: Dec 01, 2003
Posts: 113

PostPosted: Mon Feb 12, 2007 7:57 pm    Post subject: Re: Round earth vs Flat earth Reply with quote

I assume you are referring to a Team KFWB caches (or caches based on their caches), since you are from the Island.

In geocaching speak, round earth refers to the default coordinate system that your GPS is normally set to. This is latitude, longitude based on a specific datum and commonly just referred to as WGS84 or NAD83. The units usually are expressed in degrees, minutes and decimal minutes and is generally referred to a geographic coordinate system.

Flat earth on the other hand, refers to the Universal Transverse Mercator (UTM) projection. This is a plane coordinate system as opposed to an geographic coord system and over short distances compensates for the curvature of the earth. The coordinates are referenced in metres north of the equator relative to 123rd meridian for most of BC.

You can easily switch coordinate systems in your GPS or mapping software to enter values in either system, then switch back to complete the conversion. There are many online geographic to UTM convertors as well, but doing it in you GPS is just as easy. Cache pages on geocachhing.com show the cache coordinates in both systems, right at the top of the page.

So, if the cache page says assume round earth, just use what ever tools you would use to project a waypoint (like the handy widget built right into the left sidebar right here at at the BCGA website) or right in your GPS, and you should be set to go.

BTW: This is an overly simplified explanation... please do not get me started. Smile
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Derigol



Joined: Oct 06, 2006
Posts: 3
Location: Victoria

PostPosted: Mon Feb 12, 2007 8:22 pm    Post subject: Re: Round earth vs Flat earth Reply with quote

Actually it was one of V-I cacher's caches but why state assume round earth if it is that easy? I have also seen this reference in Extreme GPS teams caches and others. Maybe just to make us use our brains?
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MrGigabyte



Joined: Dec 01, 2003
Posts: 113

PostPosted: Mon Feb 12, 2007 8:52 pm    Post subject: Re: Round earth vs Flat earth Reply with quote

I have never encountered the terms flat earth / round earth, in reference to geocaching or for that matter, in geomatics in general, outside of the Lower Mainland and Vancouver Island.

For my next cache hide I think I will specify that you must must use round batteries in your GPS. Smile

Hope this helps.
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Derigol



Joined: Oct 06, 2006
Posts: 3
Location: Victoria

PostPosted: Mon Feb 12, 2007 8:53 pm    Post subject: Re: Round earth vs Flat earth Reply with quote

lol
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V-I-cacher



Joined: Nov 18, 2004
Posts: 56
Location: Vancouver & Island

PostPosted: Fri Feb 16, 2007 8:28 am    Post subject: Re: Round earth vs Flat earth Reply with quote

interesting!

well, goes to show how island-folk has the time to figure out ways of throwing stones into the way of novice cachers Twisted Evil

I agree, once identified how to do it, this is a simple matter,
but it threw me thru a loop when I came across it in my first couple finds,
thus I decided to include it as well in my first hide.

you might want to to check out www.vigps.com
and look at their contests. these "simple" variations between
flat, round, meters, miles, nautical miles, true, magnetic, etc.
once mixed together can pose all kinds of margins of error.

Cache on!
And good luck with my cache Wink
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Red90



Joined: Dec 21, 2006
Posts: 1
Location: Calgary

PostPosted: Tue Sep 25, 2007 11:16 am    Post subject: Re: Round earth vs Flat earth Reply with quote

So..... the representation of the earth surface such as WGS84 is an ellipsoid.

The projections the occur onboard of a GPS use spheroidal calculations. In other words they treted th earth as a a sphere.

Some computer programs (such as Oziexplorer and Cachemate) use the more accurate ellipsoid calculations.

It is important that if a cache page utilises a projection, then they MUST specify which method was used. If the prhjection distance is over around 2 km, then the difference in the results can be greater than the accuracy of a GPS.
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