[R-sig-Geo] newbie qx on differences between WGS84 and NAD83

[Ben Madin]

David

You need to look at the underlying definition - you can find it at http://spatialreference.org/ - this is the Human readable OGC WKT

WGS84 = EPSG: 4326

GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.01745329251994328,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]]

NAD83 = EPSG: 4269

GEOGCS["NAD83",
    DATUM["North_American_Datum_1983",
        SPHEROID["GRS 1980",6378137,298.257222101,
            AUTHORITY["EPSG","7019"]],
        AUTHORITY["EPSG","6269"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.01745329251994328,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4269"]]

It would seem that the different datums use the different spheroid, but the same prime meridian and units. I confess to not understanding the Authority fully, nor why the NAD83 shown at http://spatialreference.org/ref/epsg/4269/ seems to cover almost all of the northern hemisphere except North America...

cheers

Ben





On 19/07/2011, at 8:46 AM, David Nelson wrote:

> In my first foray into spatial data, I'm dealing with both NAD83 and WGS84 coordinates.
> 
> Now, I understand that (1) they use slightly different reference ellipses and (2) the centers of the two ellipses track different things (apparently, the North American Plate vs. earth's center of mass).
> 
> But I don't understand exactly HOW they both define zero degrees longitude. Do they both use the IERS reference median, or does NAD83 use something else, like Airy's prime meridian?
> 
> Seems simple, but I can't find the answer. Hints and pointers would be greatly appreciated.
> 
> Thanks,
> 
> David Nelson
> 
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