[Alberg30] Electronic Mapping

[Robert Kirk]

Greg... Can't help you much on the various varieties of chartplotters. I 
suspect it's like religion and everyone has his own favorites. You'll find 
many varieties of PC based map plotters at  http://joe.mehaffey.com  where 
you'll probably find more than you want to know. I just use a simple Garmin 
III+ handheld myself.

But for the GPS itself: now that the SA dithering has been removed you'll 
find that you can know your boat's position to the order of a boat length 
with any modern GPS. WAAS is just coming into being, now and will improve 
that knowledge to less than a boat length.  The are  still some problems in 
the WAAS algorithm acquisition and maintaining track which could degrade 
the  position accuracy  in poor acquisition conditions like forest cover or 
building multipath. Neither seem much of a problem at sea, though.  Those 
problems should diminish as they get WAAS on more satellites and it becomes 
"operational" from its current "pre-operational" status. Remember that we 
used regular GPS for years before it was declared operational.  There are a 
couple of GPS receivers just entering the market now with WAAS capability, 
and eventually they all will probably have it. The current scheme is to 
drop 2 of the 12 usual GPS channels on a receiver and use them for two WAAS 
channels. Thus, the added WAAS accuracy comes at the expense of some very 
minor loss of redundant basic capability. But that is a much better 
arrangement than the normal differential GPS correction where you have to 
have a separate radio for receiving both.  WAAS, by the way, is just a 
subcategory or different method of implementing differential corrections.

So, WAAS (or regular differential) is a nice-to-have which doesn't make 
much difference - to me, anyway. If you're within a boat length of the buoy 
you'll probably see it.

Bob Kirk
Isobar #181


At 10:26 AM 5/11/01 -0400, greg vandenberg wrote:
>To the Group:
>
>I have a (few) question for the group regarding maps for GPS /
>Chartplotters. I can not seam to find a store that can let me compare
>map styles. They might have one
>and not the other and there seams to be numerous sizes so one won't fit
>in the other ect ect ect. Any opinions from the group on which
>manufacture offers the best
>presentation of information (map detail ect.)? Also is there now any
>reason that GPS with  differential is better than not and is WAAS going
>to make more than a few
>feet of difference. I am leaning toward a Raytheon unit model # 425 w/
>WAAS which accepts Navionics micro-charts. BUT  I can have a good deal
>on an older model #420 which is
>GPS/ Chartplotter only (no D & no WAAS). AND... O' yes the cost of the
>chips is $200 plus. And do you believe they  split Lake Michigan into
>two
>maps... Ugh!

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