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Thread: GPS/Nautical chart inaccurate -Seward harbor

  1. #1

    Question GPS/Nautical chart inaccurate -Seward harbor

    Anyone have this problem? My Raymarine chart plotter shows a nice rectangled area, basicly the area of the harbor and right in front of the harbor entrance, to be off by several hundred feet, easily the width of the breakwater. It is an annoyance at this point, but will become more annoying on the day I am trying to get back in with heavy fog! Everywhere else, in Seward, Whittier, Homer, etc, its right on the money. Seems strange???

  2. #2
    Moderator Daveinthebush's Avatar
    Join Date
    Jul 2003
    Valdez, Alaska

    Default Maps

    Most maps of Alaska have not been updated in many years. So the manufactures are taking their maps off of the originals. For example: The Richardson Hwy. near 46-50 mile is on the wrong side of the Tiekel River. The road was built several years after the area was mapped and no one ever moved the road on the map to reflect the change. Also depths in PWS can be off by 11' or so, henceforth, no changed since the 64 quake.

    Best bet: On a clear day, lay a route out of the harbor going to a certain point that you frequently pass. Then save the route. It is now your back up. Same thing on your handheld. Plot a route out of the harbor. Save it and now you have two backups of a safe route back to port.
    "Never again shall one generation of Veterans abandon another".
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  3. #3


    Dave, Good idea! I have the track lines running over and over across the rocks, that is what I will do in an emergency. I can't recall if it has always been off and I just didn't notice, or if it mysteriously could have happened at a later time??? An investigation of any track lines that are not "in-line" with the others could be interesting.

  4. #4
    Join Date
    Oct 2007

    Default GPS inaccurate


    Update your GPS's software also. This is not the software on the chip plugged in for showing the established contours but the software used by the GPS for interpreting the chip's information. Hope that makes sense. Two times while running through Culross Pass last year my GPS suddenly showed me boating through the mountains to the side of the water I was actually running in??!! I called Garmin to inquire how I was "4 wheeling" with my boat and they said to go to their website and download an upgrade onto the appropiate chip and then plugging that chip into my GPS and updating that software. Fixed it like a charm. I would imagine your GPS has the same capabilities.

  5. #5


    Make sure the your chart and GPS are on the same datum.

    A point having a given latitude and longitude in NAD27 may be displaced on the order of many tens of meters from another point having the identical latitude and longitude in NAD83 [WGS84 - Ish]. So it is important to specify the datum along with the coordinates. The North American Datum of 1927 is defined by the latitude and longitude of an initial point (Meades Ranch in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The North American Datum of 1983 is based on a newer defined spheroid (GRS80); it is an Earth-centered datum having no initial point or initial direction.

    The "native language" of GPS is WGS84. Your unit has to convert from WGS84 to display a position in any other earthshape.

    Killin' it!

  6. #6
    Join Date
    Aug 2006

    Default Chart inaccuracies

    Check this out; and click on "C-Map" (upper RH corner tabs) and look at the c-map notice. It might apply to inaccuracies you've noted....also, if there are inaccuracies on the actual charts, you can contact the CG or NOAA to get them fixed.

  7. #7
    Join Date
    Sep 2006


    the seward harbor was rebuilt with a new entrance in 2004-2005.
    maybe your chart isn't up to date with the new entrance

  8. #8


    Thank you guys for this info, what you all said makes good sense, I will go forth and do my homework and fix this. I like the idea of ALL malfunctions happening IN the harbor!


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