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    Re: NOW what?
    From: Derrick Young
    Date: 2015 Aug 14, 23:34 -0400

    First thing first - I read this board a lot and don't chime in very often.  
    Like Frank, I have taught piloting and celestial navigation for more years 
    than I want to sit down and count.  Lot's more than I have fingers and toes.  
    But that does not matter here.
    In my possession, I have charts that go back some 65 years, as well as 
    electronic charts of the latest generation.  I have navigation books that are 
    between 150 and 250 years old on up to the most current texts.  All of them 
    are well marked and noted.  An unread book is worthless, and an unmarked 
    chart is nothing but modern art.
    I have records that show that in the early days of the French Navy they used 
    degrees west of Paris, just as the Russian navy (in the 1800'sra's) used 
    degrees west (or east) of their harbor of departure.
    In those days, they assumed the earth was pretty close to a sphere, and for most sailing it worked.
    As our understanding of the physical model of the earth changed we have gone 
    through many changes in the model used mathematically represent all of the 
    differences in gravity readings, as well as sea level anomaly's and 
    everything else that we have been measuring around our planet.  We also 
    adopted the Prime Meridian going through Greenwich Observatory.  There was a 
    very short time in the post-colonial period when some in the US wanted a new 
    Prime Meridian that went through the nation's capital (no, NOT DC, it was New 
    York City).  But nothing ever came of that.
    At various times, new GPS receivers come out with optional GPS models on them. 
     I believe that the unit that I have provides me a choice of 2 or 3 different 
    earth models.
    If I chose on that is newer (or older) than the model used for the chart that 
    is loaded into my GPS for the local area, then my GPS unit will give me a 
    correct location (remember what GPS really reports!!) BUT when that location 
    is displayed on the chart that is based on a different model, it may appear 
    to be some 300 feet away from where I really located.
    How do I fix the apparent problem?
    First - check the world map option on my GPS and see if there is one that more 
    closely aligns with my true location.  Does the Chart's World Model match 
    what I have selected in the GPS?
    If the University had checked - I believe they would have found their problem. 
     It is a simple error made by many beginning navigators.  They don't verify 
    the world model in their GPS units with the world model where they were 
    located (or on the legend of the chart).  
    If they are not the same then you are not where you think that you are.
    Derrick Young
    Phd, Mathematics, Computer Systems Design and Development, Education
    Humble Navigation Instructor for 30+ years.

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