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Posting a reply to: RE: Garmin Recreational Lakes w/ Fishing Hotspots

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[QUOTE]lambeau - 4/22/2005  1:34 PM

here is a good primer:
http://www.cmtinc.com/gpsbook/index.htm
chapters 6-9 discuss some of the issues with accuracy and errors (note that SA was turned off the the DoD).

better units can provide better accuracy to a small degree.

most all modern GPS units have at least 12 parallel channels so are getting plenty enough data to be accurate, but still may suffer from the inherent error sources. computing power is also a factor...units with more computational ability will figure out your location more quickly and more frequently - increasing your accuracy when you're moving.

one way to overcome some of the inherent errors is by getting a DGPS enabled unit - these correct for the differential errors in the standard GPS signals and can provide sub-meter positioning accuracy. (for example, all Lowrance units are DGPS ready through the use of a whip antenna and add-on receiver.)

since DGPS may be impractical or expensive, getting a WAAS-enabled unit will increase accuracy under certain conditions when the fixed-orbit WAAS satellites are visible on the horizons.

another error source is the accuracy of the maps you are using. this will impact the image you see on the screen. ie., your GPS is placing you in the right spot (lat/long) but the map is not drawn in the right spot.
i've encountered this problem on occasion in northern WI using my eTrex Legend and the Mapsource v5 FHS maps. it resulted in my boat appearing to be driving on dry land approximately 30m to the west of the lake on the map. my GPS accuracy was good (reading within 3m) but my apparent location on the map image was clearly off by some distance. this is the kind of error that would make it very difficult to troll down a breakline while watching your GPS...or could run you into a reef at night. thankfully, this is VERY rare.

i trust my Legend and Mapsource maps almost all of the time, but it's not a substitute for common sense and relying on collateral information such as your depthfinder and your eyes.

i know that many people report amazing success/accuracy while using marine GPS/sounders and Navionics or Lakemaster map chips. i'd like to hear from some of them as i'm considering something like that for my next boat upgrade![/QUOTE]

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lambeau

Posted 4/22/2005 1:34 PM (#144164 - in reply to #144101)
Subject: RE: Garmin Recreational Lakes w/ Fishing Hotspots

here is a good primer:
http://www.cmtinc.com/gpsbook/index.htm
chapters 6-9 discuss some of the issues with accuracy and errors (note that SA was turned off the the DoD).

better units can provide better accuracy to a small degree.

most all modern GPS units have at least 12 parallel channels so are getting plenty enough data to be accurate, but still may suffer from the inherent error sources. computing power is also a factor...units with more computational ability will figure out your location more quickly and more frequently - increasing your accuracy when you're moving.

one way to overcome some of the inherent errors is by getting a DGPS enabled unit - these correct for the differential errors in the standard GPS signals and can provide sub-meter positioning accuracy. (for example, all Lowrance units are DGPS ready through the use of a whip antenna and add-on receiver.)

since DGPS may be impractical or expensive, getting a WAAS-enabled unit will increase accuracy under certain conditions when the fixed-orbit WAAS satellites are visible on the horizons.

another error source is the accuracy of the maps you are using. this will impact the image you see on the screen. ie., your GPS is placing you in the right spot (lat/long) but the map is not drawn in the right spot.
i've encountered this problem on occasion in northern WI using my eTrex Legend and the Mapsource v5 FHS maps. it resulted in my boat appearing to be driving on dry land approximately 30m to the west of the lake on the map. my GPS accuracy was good (reading within 3m) but my apparent location on the map image was clearly off by some distance. this is the kind of error that would make it very difficult to troll down a breakline while watching your GPS...or could run you into a reef at night. thankfully, this is VERY rare.

i trust my Legend and Mapsource maps almost all of the time, but it's not a substitute for common sense and relying on collateral information such as your depthfinder and your eyes.

i know that many people report amazing success/accuracy while using marine GPS/sounders and Navionics or Lakemaster map chips. i'd like to hear from some of them as i'm considering something like that for my next boat upgrade!

Edited by lambeau 4/22/2005 1:37 PM