Say you have a couple large scale charts with a course that’s going to go through them all. Does anyone recall the formula for finding the Lat &/or Long of the edge of a chart you’re putting a course line on when you have a given starting position, end position and course? I’m without Bowditch or formula for mariner.
Well, I guess I should ask does anyone know what I’m talking about?
My answer depends in just how large scale chart are you talking about, I mean beyond a certain scale the formula will more be more accurate then your pencil lead… ergo a waste of 2nd mate’s Facebook time.
Let’s say I don’t have the course. Charts between 1:50k & 1:175k.
Your query is not making sense. What are you trying to do?
From what I get here, you are trying to DR a course line across several charts. I have always laid out the original course, picked off the LyL at the other end, transposed that position to another chart and ran another DR line on the same heading.
Or are you talking about finding initial GC Course?
Points along a great circle.
If one had used it before they know what I’m talking of.
There’s a formula that eliminates all of that extra work. It allows one who has the start & end Lat/Long of a passage, to just pick up any larger scale chart, and it [the formula] then gives the Lat or Long in which to begin the course at on that chart.
Easier to show on a chart than explain. But I was just looking for the formula. It’s not particularly a GC formula, works for RL. I just forgot the equation & don’t have a Bowditch nearby to search in.
If I find it I’ll post here.
Must be from The Sailings; I’ve used it but only for exams. Im pretty sure its a variation on the SOH CAH TOA.
Again, it seems like a theoretical idea when applied to a large scale chart where the accuracy is more than could ever be applied to a paper chart.
But I’d like to find out if there’s more…
I think I understand what your asking, you want do know the position on a rhumb line at a particular lat or long. I don’t know of any particular formula off hand- it would be a “sailing” problem, plane sailing - mercator sailing or Mid-Latitude. I would think plane sailing would do the trick in your case. It is accurate for several hundred miles. Mercator sailing is more accurate but more work.
Rhumb lines are mathematical more complicated then great circle.
The easiest way to solve this problem that I know of is to use Waypoint for Windows. In the “nav tools” you can plug in start and end points and it will spit out the lat (or long) of various points along the track. That’s how I laid down track-lines when I was second mate.
The other way is graphically on the chart, basically using trail and error, which is how it was done before WfW
K.C.
If accuracy was desired!
Knowing my math skills (or lack thereof), and that this is a theoretical problem, and that If I were looking for accuracy: I would stick to the tried and true method, and get out the charts and plot the DR line, and move from one chart to another.
What are you trying to do? Nav problems? What level exam? Trying to impress the guys down at the bar?
[QUOTE=cappy208;69390]If accuracy was desired!
Knowing my math skills (or lack thereof), and that this is a theoretical problem, and that If I were looking for accuracy: I would stick to the tried and true method, and get out the charts and plot the DR line, and move from one chart to another.
What are you trying to do? Nav problems? What level exam? Trying to impress the guys down at the bar?[/QUOTE]
The problem is that when you lay down track-lines by using the course and then picking off the lat and long to transfer from one chart to the next the track-line will not be as accurate as it will be if you can compute points along the rhumb line. For example if you are running easterly, if you know the latitude near the edges of the chart you can connect the points on the chart which is more accurate then if you have one point and a course. This is why lazy second mates prefer courses using the four cardinal directions or if they can’t get away with that there is the trick of putting waypoints on each chart. This is why you see waypoints at the chart edges when there is no practical reason to have a course change there.
K.C.
[QUOTE=Kennebec Captain;69392]The problem is that when you lay down track-lines by using the course and then picking off the lat and long to transfer from one chart to the next the track-line will not be as accurate as it will be if you can compute points along the rhumb line.
K.C.[/QUOTE]
At what accuracy are you thinking about on a chart where the pencil width is about a quarter mile? And assuming you are using these charts well offshore is it worth quibbling over? I understand for an exam. But in reality, if the ‘Argo Merchant’ wasn’t worried about 6 days between a star fix, what is this about?
the original question was specifically asked in reference to large scale charts where, presumably, a pencil width is most definitely NOT a quarter mile.
But I do agreed this is a theoretical problem, taking away from more pleasurable 2nd mate duties such as staring out the bridge windows, chart corrections, fiddling with radars, studying for CMM, recognizing cloud types, etc.
Sorry, Incomplete thought. Why, if one is coasting on Large scale chart to large scale chart, why wouldn’t one use a small scale chart? If you were using large scale charts, it would seem to be more prudent to use the dividers and pick off the corresponding coordinates to accomplish this. On the other hand if one was close to shore and hopping from chart to chart, would the accuracy of said DR track be of any real importance? Since you ‘should’ be checking via the window, gps, or range and bearings anyway to make such quibbling over where ‘it’ is irrelevant towards the accomplishment of the voyage and the DR.
I think he wanted to lay out super super accurate track lines as part of his voyage planning.
[QUOTE=cappy208;69397]At what accuracy are you thinking about on a chart where the pencil width is about a quarter mile? And assuming you are using these charts well offshore is it worth quibbling over? I understand for an exam. But in reality, if the ‘Argo Merchant’ wasn’t worried about 6 days between a star fix, what is this about?[/QUOTE]
With regards to ocean navigation, say a trip across the Atlantic - It used to be before GPS we used to solve navigation problems graphically, that is we would plot positions on a plotting sheet and then determine speed and course made good and the course to steer to regain the track.
With GPS (we don’t have ECDIS) it is no longer done that way, mates determine the course to steer and get speed etc directly off the GPS. Paper charts are used only to compare the position to the DR as a check. We no longer use plotting sheets at all as they are no longer needed to accurately determine speed or course made good as this is now done electronically. Typically we don’t lay down track-lines on the larger scale charts in advance as the route might change due to weather or schedule. Instead each watch lays down the track for that watch, the actual navigation is done on the computer on Waypoint for Windows and then a list of waypoints is printed out for the mate to plot and the points are entered into the GPS. If the route is changed the waypoints are changed on the computer and a new list is printed out. Each mate is responsible to lay out the track for their watch.
In other words navigation is all done on the computer or the GPS. The paper chart is only used to compare the position to the DR
[QUOTE=cappy208;69397]Sorry, Incomplete thought. Why, if one is coasting on Large scale chart to large scale chart, why wouldn’t one use a small scale chart? If you were using large scale charts, it would seem to be more prudent to use the dividers and pick off the corresponding coordinates to accomplish this. On the other hand if one was close to shore and hopping from chart to chart, would the accuracy of said DR track be of any real importance? Since you ‘should’ be checking via the window, gps, or range and bearings anyway to make such quibbling over where ‘it’ is irrelevant towards the accomplishment of the voyage and the DR. [/QUOTE]
Running coast wise is a whole other program. We usually have two charts out, a small scale (large area) one that shows the entire coastwise for planning and then an appropriate large scale (small area) chart for the nav plot. Coast wise I want the planned track on the chart to match the voyage plan waypoint list and to match what is in the GPS exactly. The reason is that there are two things about coast wise, there is less margin for error and the workload on the mate is higher (traffic, navigation, communication).
When navigating coast wise you want the mate forward at the windows and radar. At certain intervals (usually 1/2hr or at each turn) the mate has to leave the connng station and put a fix down on the chart to verify the ship is safe. Each time this happens he/she has to lose situation awareness, plot the fix, then return to the conning station and regain the picture. The reason why I want the track to be balls-on accurate is because if there is any mismatch with regards to the three methods (GPS, waypoint list and chart plot) the mate has to reconcile the mismatch. I don’t want the watch mate to have to do that. For example say there is a fishing boat on the next leg of the track. The mate goes past the track then turns. The fishing boat passes down the side 1/2 mile. The GPS should also show 1/2 mile right and when the mate throws down a fix it should also show 1/2 mile right. If it doesn’t the mate has to re-plot the fix, or resolve the problem. I don’t want that to happen. I don’t want the mate trying to figure why things don’t match. I want him/her to be able to throw a fix down on the chart, see that everything matches and then go back forward.
K.C.
Here is a link from NGA with some handy nautical calculators.
Use the drop down box to find the formula you seek.
Obviously it is possible to find points along a rhumb line mathematically but I’ve never done it. As I said upthread it can be done on WayPoint for Windows which is available as shareware and can be downloaded here.
Waypoint has not been upgraded from Windows XP to Windows 7 but the site has a work around.
To find the points along the track go to “tools” - “nav book” - “Mercator points”
It won’t give you the point at the edge of the chart but it will give you the Lat (Long) at each degree of Long (Lat).
This is the only way I know of to do the problem. Plus Waypoint while dated, it’s a is a very good program.
K.C.
Missed some of the conversation as we had to go out for a day. I’m heading down from CA to Panama and will be gone soon. But I think I may be able to explain it better now… I know that there are many ways to go about drawing up something on a chart, I was just looking for it because I’m nit picky sometimes & try challenging myself when bored on a boat.
Mercator sailing: Say I have position 1 Lat/Long, the course, & position 2 Lat/Long. I then have a larger scale Coastal chart that is between these positions. Say I’m heading on a SSEly direction, the course is going to start at the top (north) part of this chart so I know the Lat of both the north and south segments of three chart I’m working on. How would I find the Long. that the course would intercept the Lat.?
Did I make it clearer or more confusing. I remember there being some equation on this when I was back in school, so it’s notanything I’m in urgent need of, just enrichment. Thanks guys.
I see some other replies with posts. I’ll check those. Thanks again guys.
We have that formula in an excell sheet, it gives you the intercepts, Iv never used it though, way point for windows does the job just fine you can get the lat or long per degree or 2,5,10. Just use that and go to the.next charts. Send me your email and I could send the excel sheet to you