Active Routing

[hoolie]

I have a GPS 152 as well, and as you say it has no knowledge of heading.

I'm sure with mine I do not follow a dogleg (interesting source of that expression!) since the GPS 152 is directing COG not heading. When the arrow is dead ahead it is NOT pointing the boat heading as in your diagram but pointing the COG, drift included. Of course if you're way off course this will change, but if you keep to what it directs you will follow a perfectly straight track to the waypoint. My OpenCPN recorded tracks confirm this

[HappySeagull]

I'm saying the same thing again -that'll make three of us. My old Garmin(and O too I guess) just points at the next mark from where I am and that's it. It figures your POS and says with it's arrow or Hwy or whatever, " the mark is over there a bit".
"Dogleg" is true IF you are constantly reacting to the arrow and steering it's Direction to WPT as a course. It is always the wrong CTS unless you are walking a survey line in the prairie.... But I'm betting, in practise, most people steer from a compass or sails or stars to keep that arrow pointing "straight ahead" when it's important.

But there's more:
It's actually a problem I recall from my first gps. How to steer the last mile as a straight course in the dark, because variations get wilder as you get closer. The answer I came up with, is to PROJECT a waypoint further away so the Route passes through your actual destination without impinging on dangers. Now, the last mile, is directional and you will see your actual destination as an incidental mark ON the route....


COG and SOG are just histories on my gps. They are never true in the present moment except as a fluke.

[Jman]

Any GPS only (no heading information) can only show COG and SOG. If it has an arrow showing your direction (which is what this Garmin, that I am not familiar with, apparently does) and lines up when you are pointed to the WP, then it means that COG is pointed at the WP, not the bow of the boat. Now if current, speed vs. current, leeway, etc changes as you progress and you only occasionally make adjustments then you will end up making a "dogleg" course. What is handy and hardly seen is a CTS (Course-to-Steer), this could take into account difference between COG and SOG vs. HDG and BTW, calculating a new HDG which you could steer to on your compass (again if you just follow that HDG and something changes to cause the math to change, and you don't react, then you are back to a dogleg).

[tom_BigSpeedy]

Originally Posted by hoolie

... but if you keep to what it directs you will follow a perfectly straight track to the waypoint....

Hi Hoolie,
but then you use an active route, don't you ?
In this case of course, the GPS has a wp1 where you come from and a wp2 where you go to. With this information, the GPS "knows" when and how far you're off the direct line ...
That's the same calculation than used in the "highway" display.

But if you don't use a route, and just "Goto" to a waypoint (that's what I normally do, as I'm lazy), it's simply pointing towards the waypoint.

HappySeagull and Jman are talking about this too (I hope )

What I was thinking about is a simple "Goto" (with no route active), but overlay the arrow pointing towards the wp with drift information, which is (may be) available in O as "ownship Heading".
In this scenario, you may be at any position, off route or not, the program simply reads your COG + the drift (= difference from COG to ownship heading) and constantly calculates both against the wp.
The arrow will point not directly to wp (as it would without drift) but by some degrees (drift) to either side ...

I sailed a narrow channel this year in Greece (entrance to Preveza), with about 1.5 knots of sideways current. I had only the inner end set as wp and activated in the GPS with a simple Goto.
Although steering/pointing the GPS arrow to the wp, we drifted out of the buoys. (this would not have happend with a route, but it was force 7 winds and a big wave, and I had other things to do in my 31footer than programming routes ...)
In the end I simply kept my COG indicator in O directly on wp, meaning that the Heading indicator went off by some 10 Degrees.
But this angle would actually be compensation for the drift ...


Thomas

[Sandero]

I don't use routes... seems rather odd to me.

I punch in a waypoint and my GPS gives me a course to steer and COG and I set the AP to steer to the course required. I don't use cross track, but compare the COG with CTW and adjust as required. When sailing one often cannot fetch the waypoint and you have to tack or gybe.... Then you need to do some mental or actual plotting depending on your boats polars etc.

I have a few stored waypoints which I bothered to name. But the simply way is to move the cursor to the desired waypoint and set navigate to cursor and it becomes a waypoint. This takes all of 2 seconds. Much less time that scrolling through libraries and setting waypoints on a route. That is goofy feature and a waste of time... but it seems very cool. I've never understood the purpose of or obsession with waypoints and routes and libraries and planning journeys which can be done in a few minutes if you know your boat's performance and general/average sea and weather conditions.

[HappySeagull]

defjef, that's fine. Plenty do that. But, if fiddling with a gps is dangerous in a moving car, then maybe in a boat, dropping gotos on the fly everytime might be seen as a little risky to me. Especially using an autopilot. Cripes. Not at night- not me if I can help it...Better to have something down ahead of time than rush headlong down a channel. Where do you find the time if you are alone on watch and the ***** begins to go down? You don't think it might be an advantage to have a thought-out route clear of all dangers and with the next leg laid down beforehand? I don't know how big your screen is, but mine is too small to see much in detail except where I am. It's a btch flogging sail in the night, panning a chart around to see what comes next...but even then, a route guides me as I pan .. It's the only substitute for getting a real chart on the huge chart-table( that I don't have)

Real life intrudes, but courses drawn on charts are handy. Always have been. Always will be.
and some boats have crew.
How else to be sure the crew on watch will have a hardcopy guide as to the Master's intent? Sure, you can tell them but they can get it wrong or forget. I need some sleep. Surely the crew can make a course change without calling me up at every instance.