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    Re: GPS selective availability
    From: Paul Hirose
    Date: 2018 Sep 25, 15:10 -0700

    On 2018-09-22 23:01, Geoffrey Kolbe wrote:
    > There would appear to be an ad-hoc program in the US to accurately determine 
    the position of existing benchmarks, but this would appear to be on a 
    volunteer basis with members of the public being invited to send in the 
    latitude and longitude of benchmarks determined using whatever GPS device 
    they have. This program has the advantage that it costs the US Government 
    essentially nothing, but it is not the same as the UK passive GNSS network, 
    set up to a unified standard by a government agency.
    Laymen often call any survey mark a "bench mark," but the US National
    Geodetic Survey takes pains to explain:
    "In a geodetic context, the terms 'survey mark' and 'bench mark' are not
    the same. A bench mark is a specific type of survey mark that has a
    known elevation above or below an adopted surface or datum. Survey mark
    refers to any permanent marks or disks placed in the ground or attached
    to a permanent structure with known latitude, longitude or height
    information. Other terms used for survey mark are 'survey point' or
    'control point.' Colloquially (and incorrectly) bench mark is used
    interchangeably with survey mark."
    That's still not very definite, since all the NGS station descriptions
    seem to include elevation. However, a true bench mark is primarily about
    height. Its horizontal coordinates are secondary, and often of low accuracy.
    The US Coast & Geodetic Survey used different disks to indicate the type
    of mark. They said BENCH MARK or TRIANGULATION STATION. Nowadays the
    latter is more often called a horizontal control station, triangulation
    having gone out of fashion. Professionals in the US often use "monument"
    as a general term for any survey mark.
    As for the NGS soliciting reports from the public on bench mark
    locations, that applies to bona fide bench marks only. Back in the day,
    their horizontal positions were recorded only approximately, height
    determination by spirit level being the item of interest. Thus many old
    BMs are hard to find today, if indeed they still exist. It saves
    surveyors much labor if the NGS online database shows that a mark has
    survived and includes coordinates, even from a consumer grade GPS.
    By no means is the UK unique to provide accurately surveyed marks where
    a base station antenna may be set up for differential GPS operations.
    The US has thousands. For instance, the old historically significant
    station "Meades Ranch" in Kansas has been surveyed within a few
    millimeters by GPS.
    It has been said that "NAD 83 was the first civilian horizontal datum
    anywhere to have the origin of the coordinate system defined as close to
    Earth geocenter as the technology would allow, rather than as a specific
    survey mark in the middle of a field somewhere." (Doyle, 2015)
    Well, from ca. 1900 to the release of NAD83, Meades Ranch was the "mark
    in a field" for the United States. Such is its celebrity that the most
    recent entry in the datasheet mentions a tour by a group of 30.
    Another old monument with a story is Pasadena East Base in California.
    It was instrumental in the famous speed of light determination by A.A.
    Michelson in the 1920s, where he measured the round trip time from Mount
    Wilson to a mirror on a peak 22 miles away. Of course time is useless
    unless distance is known, so the Coast & Geodetic Survey measured a
    parallel baseline down in the Los Angeles basin with extraordinary care.
    Theodolite observations were taken from the baseline end points to fix
    the locations of Michelson's stations on the mountain peaks. The story
    is told here:
    Pasadena East Base survives, with modern precise coordinates and still
    in use by the California Department of Transportation:
    There is concern in the surveying community that the data on many marks
    will not be updated to the next edition of the North American Datum in 2022:

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