Mounted search and rescue

[Una Smith <una.smith@att.net>]

Chuck's problem probably is not a datum error.  At least, not involving
NADxx vs WGSxx;  30 seconds is too much.  On the other hand, the consistent
magnitude of the error suggests either datum or coordinate format.  But
the small sample size (2 points) isn't much to go on.

The following is by Bob Cowan, New Mexico SAR navigational guru and a
frequent instructor at the annual ESCAPE conference here in New Mexico
and the annual NASAR conference (which travels).

>    The maps that USGS are now publishing use the WGS83/84 datum.  Older
>maps use NAD27.   What is the practical difference for SAR use?   It turns
>out to be quite simple.
>    To convert a UTM location in NAD27 to WGS83, just add 200 meters (.200
>km) to the UTM north coordinate.  No correction in the east-west direction
>is needed.
>    This 200 meter correction is fairly constant in New Mexico.  Using it
>will always be accurate to within about 10 meters.  This is more than enough
>accuracy for SAR work.
>    Conversely, to go from WGS83 to NAD27, subtract 200 meters.
>    Two hundred meters is about 7 seconds of latitude.

        Una Smith
        New Mexico