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#22
posted to rec.boats.cruising,rec.boats
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Navigation question (Projecting a waypoint w/GPS)
On 6/20/2013 1:50 AM, Richard wrote:
On 6/19/2013 10:50 PM, JustWaitAFrekinMinute wrote: On 6/19/2013 10:39 PM, Hank© wrote: On 6/19/2013 10:29 PM, wrote: On Wed, 19 Jun 2013 21:01:14 -0400, " Sir Gregory Hall, Esq·" åke wrote: wrote in message ... Using a Garmin's Waypoint Projection feature, and projecting a waypoint at 90 degrees, shouldn't my projected waypoint end up on the same latitude as the original? And if not, why not? For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. I've tried using the same example on 3 different Garmins with the same result. What am I missing? I suspect it's a matter of choosing the right Datum. http://geography.about.com/od/geogra...n/a/datums.htm The above should explain it well enough. Your direction is only as accurate as the datum chosen. My unit has 110 different Datums to choose from. So far, I've just always used the Garmin default (WGS 84) for navigation and it has always been fine for that. The waypoint projection feature is the only thing that seems a bit squirrely. So if you have a datum recommendation that might cure that, I'll be glad to give it a go. If you correct for deviation your course heading will be 090 true. Is there a static equation for that? Yeah. East is least, west is best. Let me guess. Are you a valley girl/boy? (ref. Calif) |
#23
posted to rec.boats.cruising,rec.boats
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Navigation question (Projecting a waypoint w/GPS)
In article ,
says... On 6/19/2013 10:39 PM, Hank© wrote: On 6/19/2013 10:29 PM, wrote: On Wed, 19 Jun 2013 21:01:14 -0400, " Sir Gregory Hall, Esq·" åke wrote: wrote in message ... Using a Garmin's Waypoint Projection feature, and projecting a waypoint at 90 degrees, shouldn't my projected waypoint end up on the same latitude as the original? And if not, why not? For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. I've tried using the same example on 3 different Garmins with the same result. What am I missing? I suspect it's a matter of choosing the right Datum. http://geography.about.com/od/geogra...n/a/datums.htm The above should explain it well enough. Your direction is only as accurate as the datum chosen. My unit has 110 different Datums to choose from. So far, I've just always used the Garmin default (WGS 84) for navigation and it has always been fine for that. The waypoint projection feature is the only thing that seems a bit squirrely. So if you have a datum recommendation that might cure that, I'll be glad to give it a go. If you correct for deviation your course heading will be 090 true. Is there a static equation for that? No, because it's not static. |
#24
posted to rec.boats.cruising,rec.boats
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Navigation question (Projecting a waypoint w/GPS)
On 6/20/2013 5:03 AM, Hank© wrote:
On 6/20/2013 1:50 AM, Richard wrote: On 6/19/2013 10:50 PM, JustWaitAFrekinMinute wrote: On 6/19/2013 10:39 PM, Hank© wrote: On 6/19/2013 10:29 PM, wrote: On Wed, 19 Jun 2013 21:01:14 -0400, " Sir Gregory Hall, Esq·" åke wrote: wrote in message ... Using a Garmin's Waypoint Projection feature, and projecting a waypoint at 90 degrees, shouldn't my projected waypoint end up on the same latitude as the original? And if not, why not? For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. I've tried using the same example on 3 different Garmins with the same result. What am I missing? I suspect it's a matter of choosing the right Datum. http://geography.about.com/od/geogra...n/a/datums.htm The above should explain it well enough. Your direction is only as accurate as the datum chosen. My unit has 110 different Datums to choose from. So far, I've just always used the Garmin default (WGS 84) for navigation and it has always been fine for that. The waypoint projection feature is the only thing that seems a bit squirrely. So if you have a datum recommendation that might cure that, I'll be glad to give it a go. If you correct for deviation your course heading will be 090 true. Is there a static equation for that? Yeah. East is least, west is best. Let me guess. Are you a valley girl/boy? (ref. Calif) Another of Wilbur's socks? |
#25
posted to rec.boats.cruising,rec.boats
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Navigation question (Projecting a waypoint w/GPS)
wrote in message
... On Wed, 19 Jun 2013 21:01:14 -0400, " Sir Gregory Hall, Esq·" åke wrote: wrote in message . .. Using a Garmin's Waypoint Projection feature, and projecting a waypoint at 90 degrees, shouldn't my projected waypoint end up on the same latitude as the original? And if not, why not? For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. I've tried using the same example on 3 different Garmins with the same result. What am I missing? I suspect it's a matter of choosing the right Datum. http://geography.about.com/od/geogra...n/a/datums.htm The above should explain it well enough. Your direction is only as accurate as the datum chosen. My unit has 110 different Datums to choose from. So far, I've just always used the Garmin default (WGS 84) for navigation and it has always been fine for that. The waypoint projection feature is the only thing that seems a bit squirrely. So if you have a datum recommendation that might cure that, I'll be glad to give it a go. I don't know where you live. But I do know most Garmins default to WGS 84 which the article linked to stated is out of date as of 2010. You need to choose the best available datum for the area of the world where you live. And remember, the world isn't a perfect sphere. Chart datum for a particular part of the globe will take this into account and correct for it even if the charts aren't representing it accurately. -- Sir Gregory |
#26
posted to rec.boats.cruising,rec.boats
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Navigation question (Projecting a waypoint w/GPS)
On Thu, 20 Jun 2013 14:45:50 -0400, " Sir Gregory Hall, Esq·"
åke wrote: wrote in message .. . On Wed, 19 Jun 2013 21:01:14 -0400, " Sir Gregory Hall, Esq·" åke wrote: wrote in message ... Using a Garmin's Waypoint Projection feature, and projecting a waypoint at 90 degrees, shouldn't my projected waypoint end up on the same latitude as the original? And if not, why not? For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. I've tried using the same example on 3 different Garmins with the same result. What am I missing? I suspect it's a matter of choosing the right Datum. http://geography.about.com/od/geogra...n/a/datums.htm The above should explain it well enough. Your direction is only as accurate as the datum chosen. My unit has 110 different Datums to choose from. So far, I've just always used the Garmin default (WGS 84) for navigation and it has always been fine for that. The waypoint projection feature is the only thing that seems a bit squirrely. So if you have a datum recommendation that might cure that, I'll be glad to give it a go. I don't know where you live. But I do know most Garmins default to WGS 84 which the article linked to stated is out of date as of 2010. You need to choose the best available datum for the area of the world where you live. And remember, the world isn't a perfect sphere. Chart datum for a particular part of the globe will take this into account and correct for it even if the charts aren't representing it accurately. I saw that but I'm not sure what they mean by "invalid", especially since a chart isn't even involved in what I'm trying to do. I mean, a "datum" is not a system that can be switched off or abandoned, like Loran for instance. Anyhow, I've tried switching to NAD 83. That's the datum that the article says most units are using now as default. But I still get results as squirrely as before. Basically, no matter what datum I use, I should be able to project a second waypoint. Then, starting with my projected (second) waypoint, I should be able to project a third point, using the same distance but reciprocal bearing, and the third waypoint should be exactly the same, (or reasonably close given rounding errors) as the first. But it's not. It's further off than I would think is reasonable. By the way, I'm on the Gulf Coast between Mobile and New Orleans. Another by the way, I went to the Garmin Suppport site and they want $30 to answer a frickin' question. I guess I just don't want to hear their explanation that bad lol! |
#27
posted to rec.boats.cruising,rec,boats
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Navigation question (Projecting a waypoint w/GPS)
wrote:
Using a Garmin's Waypoint Projection feature, and projecting a waypoint at 90 degrees, shouldn't my projected waypoint end up on the same latitude as the original? And if not, why not? For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. I've tried using the same example on 3 different Garmins with the same result. What am I missing? Don't know, and, sorry, didn't read entire thread, but could you try plotting several additional waypoints, all bearing 90 but each with different distances, say 10, 20, 30, etc miles? You get a "straight line", recognizable great circle, or what? Maybe that additional info would suggest some clue. Some simple (but not simple enough for me to do off the top of my head) spherical trigonometry would tell you if half an extra degree north at 30 north (all true) would keep you on a great circle 10 miles out. Maybe plotting a few points will let you see the answer without "doing the math". -- John Forkosh ( mailto: where j=john and f=forkosh ) |
#28
posted to rec.boats.cruising,rec,boats
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Navigation question (Projecting a waypoint w/GPS)
JohnF wrote:
wrote: For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. Excuse me for following myself up, but either I'm totally confused (a common occurrence) or that example's totally messed up... You say your waypoint's distance is 10nm, but its latitude changes by 23.866'. Heck, even if you headed exactly due north for 10nm, latitude would only change by exactly 10'. -- John Forkosh ( mailto: where j=john and f=forkosh ) |
#29
posted to rec.boats.cruising,rec,boats
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Navigation question (Projecting a waypoint w/GPS)
On Sat, 22 Jun 2013 16:23:37 +0000 (UTC), JohnF
wrote: JohnF wrote: wrote: For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. Excuse me for following myself up, but either I'm totally confused (a common occurrence) or that example's totally messed up... You say your waypoint's distance is 10nm, but its latitude changes by 23.866'. Heck, even if you headed exactly due north for 10nm, latitude would only change by exactly 10'. No, you're not confused. I'm just getting projected waypoints that make no sense. Fortunately, so far, my unit's GPS capability is spot on, so I can still use it to navigate (with backup). I just can't trust the "Projection" feature for accurate results. Never used it much anyway. By the way, I tried projecting 10 miles due north and it changed my latitude by 33' minutes. I'm done fiddling with it at this point. Thanks for the come back tho. |
#30
posted to rec.boats.cruising,rec.boats
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Navigation question (Projecting a waypoint w/GPS)
wrote in message
... On Sat, 22 Jun 2013 16:23:37 +0000 (UTC), JohnF wrote: JohnF wrote: wrote: For example, beginning at 30 degrees N latitude and 89 degrees W longitude, I project a new waypoint at a distance of 10 miles with a bearing of 90 degrees. As expected, my projected longitude changes to a point further east. But for some reason, the projected latitude ends up at a higher latitude than 30 degrees. In my example, the projected latitude ends up as 30 degrees, 23.866 Minutes North. Excuse me for following myself up, but either I'm totally confused (a common occurrence) or that example's totally messed up... You say your waypoint's distance is 10nm, but its latitude changes by 23.866'. Heck, even if you headed exactly due north for 10nm, latitude would only change by exactly 10'. No, you're not confused. I'm just getting projected waypoints that make no sense. Fortunately, so far, my unit's GPS capability is spot on, so I can still use it to navigate (with backup). I just can't trust the "Projection" feature for accurate results. Never used it much anyway. By the way, I tried projecting 10 miles due north and it changed my latitude by 33' minutes. I'm done fiddling with it at this point. Thanks for the come back tho. Then you've got it set to magnetic north or it's stuck on magnetic north no matter how you set it. Set it to true north and try it again. -- Sir Gregory |
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