[NLRS] GPS Map Datum

Thu, 25 Dec 2003 19:03:57 -0700

I use CorpsCon for datum transformations. It's available here:
http://crunch.tec.army.mil/software/corpscon/corpscon.html
73,
mark WB�OAJ

On Mon, 22 Dec 2003 07:57:43 -0600 "John (JK) Kalenowsky, K9JK"
<[email protected]> writes:
> FWIW...I remember seeing a 'translator' for NAD27 to/from 
> NAD83/WGS84. Can't
> recall if it was a web site or a little app that could be downloaded 
> (maybe
> it was both?). If there is interest, I can try to find it again 
> (though,
> given that "I" found it on the web..."you" probably can too).
> 
> JK
> ----- Original Message ----- 
> From: "Donn Baker" <[email protected]>
> To: "Scott Blixt" <[email protected]>; "NLRS" 
> <[email protected]>
> Sent: Sunday, December 21, 2003 22:56
> Subject: Re: [NLRS] GPS Map Datum
> 
> 
> Hi Scott,
> Yes, WGS84 is the current standard for mapping in the United States. 
>  There
> ARE "differences" in the two datums... obviously, the locations 
> haven't
> moved, but the mathematical description (of the earth spheroid) used 
> is
> different between 1927 and 1984.  The differences between the two 
> shouldn't
> be more than a few tenths of a second of arc; maybe up to a second 
> in
> extream cases. Even then its NOT uniform.  Some places it will be 
> zero (0).
> 
> One minute of latitude is 1 nautical mile, or ~6,076 feet (OK, 
> 6,075.8').
> Sixty seconds of arc per minute of arc, so one second of arc is 
> ~101.26
> feet.  The differences fall to maybe 10-30 feet.  Does that come 
> close to
> what you have noticed ?
> 
> 73 Donn
> WA2VOI/0
> 
> 
> At 16:57 21-12-03 -0600, Scott Blixt wrote:
> >Just curious. To those that use GPS. Which map datum are you all 
> using?
> >I believe mine came defaulted to WGS 84.
> >
> >Ive been using WGS84 because that is supposed to be the most recent 
> map
> >datum. The numbers relating to the year when they were done. 27 as 
> in
> >1927 and 84 1984. I have found though that there is some 
> differences in
> >actual locations between the 2.
> >
> >
> >
> >
> >
> >
> >Here is something I found during a search:
> >
> >Map Datums
> >
> >A datum describes the model that was used to match the location of
> >features on the ground to coordinates and locations on the map. 
> Maps
> >all start with some form of survey. Early maps and surveys were 
> carried
> >out by teams of surveyors on the ground using transits and distance
> >measuring "chains". Surveyors start with a handful of locations in
> >"known" positions and use them to locate other features. These 
> methods
> >did not span continents well. Frequently they also did not cross
> >political borders either. The "known points" and their positions 
> are
> >the information that the map datum is based. As space based 
> surveying
> >came into use, a standardized datum based on the center of the 
> earth
> >was developed.
> >
> >Every map that shows a geographic coordinate system such as UTM or
> >Latitude and Longitude with any precision will also list the datum 
> used
> >on the map.
> >
> >The Global Positioning System uses an earth centered datum called 
> the
> >World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world
> >standard from a datum called the North American Datum of 1983 or 
> NAD
> >83. For all practical purposes there is no difference between WGS 
> 84
> >and NAD 83.
> >
> >Most USGS topographic maps are based on an earlier datum called the
> >North American Datum of 1927 or NAD 27. (Some GPS units subdivide 
> this
> >datum into several datums spread over the continent. In the 
> Continental
> >United States use NAD27 CONUS.)
> >
> >In the Continental United States the difference between WGS 84 and 
> NAD
> >27 can be as much as 200 meters.
> >
> >You should always set your GPS unit's datum to match the datum of 
> the
> >map you are using.
> >
> >On a USGS topographic map the datum information is in the fine 
> print at
> >the bottom left of the map. The datum will always be NAD 27. There 
> may
> >be information on how many meters to shift a position to convert it 
> to
> >NAD 83. Think of this as the error that will be introduced if you 
> leave
> >your GPS unit set to WGS 84. A dashed cross in the SW and NE 
> corners of
> >the map gives a visual indication of the difference between the two
> >datums.
> >
> >If you have somehow set your GPS to use the Borneo Datum of 1818, 
> it's
> >hard to say how far off you position may be. Let's just sat that 
> this
> >"datum thing" is something you need to pay attention to.
> >
> >If you are coordinating with aircraft, they will likely have their
> >datum set to WGS 84, as most aviation charts now use WGS 84. Should 
> you
> >worry about the difference in datums? Typically a pilot will not 
> have
> >any difficulty locating you on the ground if you can get them 
> within
> >several hundred meters of your location. If you are engaged in a
> >mission that requires more precision, then your datums should 
> match.
> >
> >
> >
> >
> >Scott Blixt
> >ka0jwc
> >[email protected]
> >
> >_______________________________________________
> >NLRS mailing list
> >[email protected]
> >http://mailman.qth.net/mailman/listinfo/nlrs
> >
> >
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