Posted: 12/2/2014 11:25:01 PM EDT
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Grid coordinates moving over the years.
A little back story, I purchased a used Garmin ETrex GPS unit and used it on a hunting trip recently. According to a topographic map dated 1983 my GPS coordinate taken from the ETrex was roughly 1/2 a grid square too far North. Is it possible that with the extra precision with today's satellites that my ETrex could have been that far off or has the grid moved over the years to reflect the fact that the topo map I had was outdated?? I saved the way-points and entered them into Google maps and it appears that the ETrex way-points are spot on! This led me to a conclusion that the 1983 survey for the topo map either moved or was off to begin with. Discuss AKASL LIVE FREE OR DIE |
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The 1983 map was a USGS Topographic Map that uses Grid North with a Magnetic North indicator. AKASL Quoted:
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Was the old grid based off of true north? Or magnetic North? The 1983 map was a USGS Topographic Map that uses Grid North with a Magnetic North indicator. AKASL Declination angle. |
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Probably related to the WGS84 vs NAD83 differences, or similar.
http://en.wikipedia.org/wiki/World_Geodetic_System |
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Was your GPS set in the same projection/coordinate system as the map? If the map is a USGS topo from 1983, I'd assume it was in NAD27, while your GPS was likely in WGS84. I don't have the map in front of me so I can't comment on the NAD27, but I do know the GPS was set for WGS84 AKASL |
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I don't have the map in front of me so I can't comment on the NAD27, but I do know the GPS was set for WGS84 AKASL Quoted:
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Was your GPS set in the same projection/coordinate system as the map? If the map is a USGS topo from 1983, I'd assume it was in NAD27, while your GPS was likely in WGS84. I don't have the map in front of me so I can't comment on the NAD27, but I do know the GPS was set for WGS84 AKASL Just looked it up and verified that most USGS topos printed prior to 2009 use NAD27 datum. That was likely the source of the error. http://education.usgs.gov/lessons/mapmargin.pdf |
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Probably related to the WGS84 vs NAD83 differences, or similar. http://en.wikipedia.org/wiki/World_Geodetic_System For all intents and purposes there's no discernible difference between nad83 and wgs84. Huge difference between nad27 and nad83 |
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For all intents and purposes there's no discernible difference between nad83 and wgs84. Huge difference between nad27 and nad83 Quoted:
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Probably related to the WGS84 vs NAD83 differences, or similar. http://en.wikipedia.org/wiki/World_Geodetic_System For all intents and purposes there's no discernible difference between nad83 and wgs84. Huge difference between nad27 and nad83 Yeah I couldn't remember which ones were similar, vs different. So I tagged on "or similar" to cover myself 
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