[NLRS] GPS Map Datum

Mon, 22 Dec 2003 07:57:43 -0600

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]
>
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