Dan Hagen <dan_at_hagen.net> wrote: <snip> >>>>>Note that you do not have to buy electronic charts to do this (although that is an option). If you already have paper charts, you can scan them into your computer, and use a program such as OziExplorer (the best!) to convert the scanned images into charts. <<<<< Why do you consider OziExplorer the best? (Just curious). I started out using Ozi, but found GPSTool to be a little more intuitive, plus it's available in a freeware (rather than shareware) version with relatively few features disabled (at least that I would be very concerned about). Also, even the freeware version allows you to use 3 points for calibrating scanned charts, while Ozi (I believe) only allows 2 in the shareware version. (Granted, probably not a key difference for non-tightwads). <another snip> >>>>>(4) The standard eTrex does not have differential (DGPS) capability built in. Newer Garmin receivers (including the 76) can provide differentially corrected (DGPS) fixes without the use of a separate DGPS receiver/antenna. This improves the accuracy of the fix, and more importantly the accuracy of the compass function and speed readout. Even when zoomed into the "20-foot" setting on my newest receiver (where each pixel is less than one foot), I see no significant drift when standing still. DGPS really works! I do not see why anyone would buy a handheld receiver without built in differential capability.<<<<< With the discontinuation of selective availability, do you still find this to be a significant advantage? I've found that on open water, I can often get position fixes that my GPS "thinks" are good to within 15 to 20 feet. I'll agree that I've been disappointed with the accuracy of the GPS compass readings, though speed readings seem very accurate (at least compared to the speedometer in my car!). >>>>>(5) The eTrex lacks a wide range of other useful features, including tide tables. My newest handheld displays tide tables (and graphs) for a wide range of primary and secondary tide stations in the U.S. and Canada.<<<<< That sounds really cool. What GPS's have this feature? By the way, thanks for the very informative post (especially interesting for us Etrex users!) Evan Woodinville, Washington *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Dan Hagen wrote: <snip> <<< The standard eTrex does not have differential (DGPS) capability built in. Newer Garmin receivers (including the 76) can provide differentially corrected (DGPS) fixes without the use of a separate DGPS receiver/antenna. This improves the accuracy of the fix, and more importantly the accuracy of the compass function and speed readout. Even when zoomed into the "20-foot" setting on my newest receiver (where each pixel is less than one foot), I see no significant drift when standing still. DGPS really works! I do not see why anyone would buy a handheld receiver without built in differential capability. >>> As far as I understand, DGPS is a way of correcting the Selective Availability, the deliberate inaccuracy that was put in the data from the satellites. Since Selective Availability is OFF since May last year, DGPS is not needed anymore. My Etrex does not show any drift while standing still. I agree with most of the other disadvantages that Dan Hagen pointed out for the Garmin Etrex, but I got one thing to add: Although the Etrex shows a minimun of data on each display, at least that data is shown clearly and as large as possible. Inside a damp drybag under your bungies you have a chance to be able to read the screen, which was almost impossible with my previous GPS, a 12XL. Niels. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Niels Blaauw wrote: > As far as I understand, DGPS is a way of correcting the Selective > Availability, the deliberate inaccuracy that was put in the data from the > satellites. Since Selective Availability is OFF since May last year, DGPS is > not needed anymore. Actually, Selective Availability is only part of the "error budget". The removal of SA has eliminated MOST of the benefit from DGPS, but not all of it. There are still other sources of error, including atmospheric refraction. Here is the error budget: GPS Error Budget Source Uncorrected Error Level Ionosphere 0-30 meters Troposphere 0-30 meters Measurement Noise 0-10 meters Ephemeris Data Error 1-5 meters Clock Drift 0-1.5 meters Multipath 0-1 meter (in rare cases much more) Selective Availability 0-70 meters Note that these are not always "additive" (one component can partially cancel another component), but this gives you an idea of what is possible. Nonetheless, I certainly agree that DGPS is MUCH less useful in the absence of SA. Still, it can be of some help even with SA off. (Note that DGPS does not correct for multipath error, which is highly localized.) >My Etrex does not show any drift while standing still. That's because it cannot zoom in very far. :-) The lowest zoom setting is "200 feet". If you had a receiver that could zoom in to ten times this scale ("20 feet"), you would see drift. Again, I agree with your basic point, which is that DGPS has become much less useful since SA was turned off. Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
-----Original Message----- From: Blaauw, Niels [mailto:nblaauw_at_foxboro.com] Although the Etrex shows a minimun of data on each display, at least that data is shown clearly and as large as possible. Niels. >> I wonder if I have an older model Etrek as Niels statement is true except for one important reading - the UTM/LongLat readings. These seem to be in very small writing making it difficult to read through a bag. I am hoping that the later Etrek software will make these readings the same size writing as the Bearings/speed etc. David *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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