Re: Geodetic question

From: Ed Davies (
Date: Fri Aug 17 2001 - 11:04:16 PDT

  • Next message: Kevin Fetter: "Re: Geodetic question"

    Judy May wrote:
    >     This may be affecting me.  Being a backwoods orienteering buff in
    > the U.S., I use a map datum of NAD27 CONUS.  My GPS receiver is set to
    > work in that datum, also.
    >     I have noticed a greater positional error that should occur when
    > using my Meade LX200 telescope.  (Sometimes by a factor of ten minutes
    > of arc)
    >     Kevin commented on converting one datum to another.  Can someone
    > tell me the necessary formulas to go from NAD27 CONUS to the two WGS
    > datums?
    Try the National Imagery and Mapping Agency website at:
    Of particular interest is their document:
      Its Definition and Relationships with Local Geodetic Systems
    However, its all a bit tricky.  You might find the DOS program MADTRAN
    somewhere which does the conversions for various datums (data? :-) to/from
    WGS-84.  I got it from the NIMA site a while back (I think it was still 
    the Defense Mapping Agency then) but it doesn't seem to be publicly 
    available anymore.
    The MADTRAN source code (it was published in some sort of Basic) is a 
    reasonable reference on how to do basic transformations between datums.
    But, if your really need to use a particular datum, why not just switch 
    your GPS to WGS-84 or whatever you want while you survey the location 
    of your telescope?
    >     And lastly, what datum would we expect automated "go-to" telescopes
    > would have been engineered with?  I am not so sure Meade would
    > understand what I was asking if I called them.
    I don't think the telescope would care about the datum.  The lat/long 
    would only be used to give a first approximation to the orientation in 
    space of the telescope's axes.  The effects of alignment errors on the 
    mounting would dominate any difference in datum.
    Except when observing near objects (the Moon or closer?) the actual
    position of the telescope is pretty much irrelevant.  I assume the
    goto function of the telescope doesn't deal with the parallax for 
    close objects anyway.
    Even the best alignment to local vertical will not help much as any
    datum is just an ellipsoid designed to give a reasonable approximation
    to the Earth's shape over a cetain area (the whole world in the case of
    WGS-84).  The local vertical will probably be different anyway.  The
    survey methods used to determine the actual local vertical are pretty 
    much the same as aligning a telescope to reference stars, I think.
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