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Author Topic: map shift  (Read 2527 times)


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map shift
« on: August 12, 2013, 04:56:17 AM »
new to this site.. looks great
i have a working dinosaur  garmin gpsmap 76s which i use for hunting and fishing.  i originally had a map 76 and mapsource topo maps.  unfortunately left the 76 on bumper of truck and it was lost.  bought the 76s on ebay.
i loaded the topos for nw penna and heres the problem.  when i am at a pretty exact position, like the middle of a crossroad, the cursor indicates i am about 75 feet east and just a little north.  i have tried conus 27 and wg84 settings. also tried lock roads on and off.
is there a way to 'shift' the map to match up .  what i'm not sure of is whether readings are correct on gps and its just the map shift.  accuracy reading is about 10'


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Re: map shift
« Reply #1 on: August 12, 2013, 05:14:02 AM »
Most likely it's just bad map data. Most of the maps on this site use data from the US Census Bureau known as "TIGER". They have improved this somewhat recently, but there are still plenty of errors. If the map used the older TIGER data, then an error of 75 feet would not be unusual at all.

I also wouldn't put much faith in the "accuracy" reading you see on the GPS. Consumer level GPS devices are typically only accurate within +/- 5 meters (16 feet). If you combine this error with the map error, your position can be offset from a road by quite a bit.

Garmin's old 100k topo maps (Topo US 2008, etc) also use the old TIGER data and have some pretty major errors, often in the hundreds of feet.


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Re: map shift
« Reply #2 on: August 12, 2013, 06:51:08 AM »
Old TIGER was definitely a problem in eastern PA.  Roads in Valley Forge were set 150' east of their actual location.  Regarding "lock on road",  That won't work unless you are using a routable map.  Not many of those at GPSfileDepot, but you can find some at garmin.openstreetmap.nl .