NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: George Huxtable
Date: 2006 Jul 23, 18:50 -0500
I had written-
| "So what's to be gained by invoking GPS, I ask? Certainly, you are
| likely to get into trouble if you compare compass heading with the
| course shown by a GPS receiver, under way. What you need, for the
| deviation, is to know the heading of the bow. If there's any
leeway,
| your course through the water will differ from that. If there's any
| current, your course over the ground will differ, again, from that,
| and that's what the GPS will tell you."
And Frank commented-
| Unless I have misunderstood, they're not talking about using the
GPS
| receiver's calculation of course based on motion. Instead, you use
its built-in
| ability to calculate bearings of known objects from your current
position. You
| enter those known objects as way-points, by sailing up to them and
pressing a
| button, as Lu Abel suggested, or by keying in the coordinates taken
from a
| chart. Then wherever you happen to be floating when you decide to
check compass
| deviation, the GPS will give an extremely accurate true bearing to
any of
| those way-point objects. It's very simple. And note that your boat
can be moving
| with a current and it won't do any harm since the bearings are
updated
| continuously based on present true position.
No, I don't think Frank misunderstood that point, and I hadn't either.
I was aware of the method being proposed, which has its own
weaknesses. It would be very sensitive to any discrepancies between
charted positions and GPS coordinates, and perhaps I should have
pointed that out.
Only when an object is firmly fixed in deep water, such as a dolphin
or a jetty, can one sail up to it, and record a GPS position.. A
tethered floating buoy is usually too mobile to be used for such a
task. So one has to rely on taking positions of landmarks from the
chart.
Not all charts are to WGS84. One has to be very careful, in such a
sensitive application, to apply any offsets on the chart, to bring it
into line with WGS84, or to set a GPS receiver to use the coordinate
system of the chart (OSGB, for many of my charts). But not both, of
course!
However, if the observer uses that same chart to define his own
position, rather than GPS, any such errors are eliminated.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 Jul 23, 18:50 -0500
I had written-
| "So what's to be gained by invoking GPS, I ask? Certainly, you are
| likely to get into trouble if you compare compass heading with the
| course shown by a GPS receiver, under way. What you need, for the
| deviation, is to know the heading of the bow. If there's any
leeway,
| your course through the water will differ from that. If there's any
| current, your course over the ground will differ, again, from that,
| and that's what the GPS will tell you."
And Frank commented-
| Unless I have misunderstood, they're not talking about using the
GPS
| receiver's calculation of course based on motion. Instead, you use
its built-in
| ability to calculate bearings of known objects from your current
position. You
| enter those known objects as way-points, by sailing up to them and
pressing a
| button, as Lu Abel suggested, or by keying in the coordinates taken
from a
| chart. Then wherever you happen to be floating when you decide to
check compass
| deviation, the GPS will give an extremely accurate true bearing to
any of
| those way-point objects. It's very simple. And note that your boat
can be moving
| with a current and it won't do any harm since the bearings are
updated
| continuously based on present true position.
No, I don't think Frank misunderstood that point, and I hadn't either.
I was aware of the method being proposed, which has its own
weaknesses. It would be very sensitive to any discrepancies between
charted positions and GPS coordinates, and perhaps I should have
pointed that out.
Only when an object is firmly fixed in deep water, such as a dolphin
or a jetty, can one sail up to it, and record a GPS position.. A
tethered floating buoy is usually too mobile to be used for such a
task. So one has to rely on taking positions of landmarks from the
chart.
Not all charts are to WGS84. One has to be very careful, in such a
sensitive application, to apply any offsets on the chart, to bring it
into line with WGS84, or to set a GPS receiver to use the coordinate
system of the chart (OSGB, for many of my charts). But not both, of
course!
However, if the observer uses that same chart to define his own
position, rather than GPS, any such errors are eliminated.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
