GPS coordinates?

Have any of you had any problems with trying to go to spots marked as
good fishing locations on a store-bought map, and it is waaaay off of where
you are supposed to be? I bought one of the first maps of Pickwick I ever
saw with GPS locations back years ago, and think it is worthless - I considered the possibility that I am running into confusion over whether the
locations are in decimal degrees, or degree/minutes/seconds format.
The map appears to be d/m/s, and I can put either into my unit, but even
playing with this hasn't helped. Maybe some of the early maps were just
somethin' to get my money? Has happened before! :rolleyes: :)

Quote:

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Originally Posted by J White

Have any of you had any problems with trying to go to spots marked as
good fishing locations on a store-bought map, and it is waaaay off of where
you are supposed to be? I bought one of the first maps of Pickwick I ever
saw with GPS locations back years ago, and think it is worthless - I considered the possibility that I am running into confusion over whether the
locations are in decimal degrees, or degree/minutes/seconds format.
The map appears to be d/m/s, and I can put either into my unit, but even
playing with this hasn't helped. Maybe some of the early maps were just
somethin' to get my money? Has happened before! :rolleyes: :)

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A WAAS correcting GPS receiver is much more accurate on reaching specified coordinates on a map. If your GPS unit does not have this correction then you could be greater than 75 feet from the actual coordinate. Finding underwater structure this much distance could be the difference in locating the fish or not.

Somewhere on the map will be the "Map Datum" that is used. Your GPS needs to be set for the same one. WGS84 and NAD27 are two common ones, but there are others.

I'm still using a differential set-up, should be plenty accurate though.
I haven't had any trouble getting back to stuff that I saved myself,
just stuff off that map. 75 feet? I mean some of those points, guestimating
by my screen size/zoom range, are off like close to or maybe even over a mile! The reason I say guestimating, is that I'd have to get out and walk
overland to get to them! :) Now the datums, that is interesting, will have
to check into that. Could it make that much difference? Anyway, thanks
guys.

I wish my memory wasn't so bad. Back in the old days one of my jobs was to plot recon courses for our pilots to fly photographic missions. So I became quite familiar with maps and charts. The most common used was GeoRef or Geographic Reference coordinates. These were expressed in degrees/minutes and seconds. Another tpye was UTM or Universal Transverse Mercator projection. This was a combination of letters and numbers. If you used enough numbers you could reference a place on the earth 1 inch square. I never seen it used to that accuracy but it could be done. Of couse there were no electronics back then to tell you where you were, but they were great for telling other commands where to look on their maps. Now enter the GPS. It appears that all they use is degrees and thousands of a minute (I think). All the current maps I've seen still are marked with GeoRef coordinates. So unless you know how to convert the minutes and seconds to thousanths of a minute, you can't enter it in your GPS. At least not accurately. I hope I made some sense and haven't got my terminolgy turned around.

I may have confused myself, I am used to in my line of work, converting
angles back and forth between decimal degrees and dms - lots of old
parts prints still use dms, and I have to swap it over to plug it into the
machine controller - can do a lot of it in my head, like 7deg 30 min =
7.5 deg, but I may be taking liberties with this GPS stuff I shouldn't.
Have to check into it further, esp. the datum thing. I remember seeing
a page on my setup screen with a whole bunch to choose from.
Oh, and it may be cause my unit is an older one, but when you want to
type in a position manually, it gives you a choice to put in degree, minute,
second, if you want.

Quote:

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Originally Posted by J White

I may have confused myself, I am used to in my line of work, converting
angles back and forth between decimal degrees and dms - lots of old
parts prints still use dms, and I have to swap it over to plug it into the
machine controller - can do a lot of it in my head, like 7deg 30 min =
7.5 deg, but I may be taking liberties with this GPS stuff I shouldn't.
Have to check into it further, esp. the datum thing. I remember seeing
a page on my setup screen with a whole bunch to choose from.
Oh, and it may be cause my unit is an older one, but when you want to
type in a position manually, it gives you a choice to put in degree, minute,
second, if you want.

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I haven't checked mine, maybe I can change it to something I recognize.