Diagonal lines (Rhumb/Nav Lines)

[aeronox]

On maps, besides the long/lat grid, there is often diagonal lines that radiate from multiple points on the map... often with a fancy rose at the center.

What are they for?

[Robbie]

I believe you're referring to ley lines maybe? which are lines drawn connecting ancient sites...

Or nav lines...which I'll quote from the profantasy tome of ultimate mapping:

Navigation lines (navlines) were used on early maps to give sailors waypoints and bearings
when traveling relatively short distances. They only appear over bodies of water. They give
a map an authentic historical look. If you prefer purely functional maps, you’ll probably want
to do without them.

[Robbie]

I also see them referred to as Rhumb lines...but I dont' think thats correct, because a rhumb line is a line of constant bearing on a sphere (loxodrome)...kind of spirals around a map or something...but if the lines are drawn using rhumb line formulas (cosines and crap) then that makes sense.

I think its something worth looking into, because there may be a useful science we've all been overlooking here that could improve all our maps.

[Gamerprinter]

I was reading about Rhumb lines today - er, yesterday while researching pirate maps.

True Rhumb lines are 11.25 degrees of a compass, so 32 lines form a star (?). They are used for dead reckoning between known locations using a sextant. Rhumb line centers in the maps I found were located near major known ports of the Caribbean - Havana, Santo Domingue, etc.

I'm sure there's more science than that.

After the adoption of Parallels and Meridians, Rhumb Lines were nolonger used.

On my regional map for the December challenge - there's only 6 Rhumb lines, but I based that on an actual 1761 map of St. Christopher, with center of Rhumb lines near the port area. So that's what I chose to do.



The second map has true Rhumb Lines on it.

[pyrandon]

What a sweet thread! Interesting, scholarly, and useful! Thanks, aeronox, for posting it!

[Midgardsormr]

Wikipedia has a nice article on Rhumb lines: http://en.wikipedia.org/wiki/Rhumb_line

According to that article, even following the adoption of parallels and meridians, Rhumb lines continued to be useful, since they could be used calculate a straight-line course based on an angle of bearing from a parallel.

Additionally, and not to be contrary, there are examples of maps that place navigation lines on land. I have a book in front of me right now with a map of Africa from 1529 that has them emanating from Ethiopia. Those lines appear to be decorative, though.

I saved the following useful post from the CC2 discussion list a few weeks ago:

----
Look at some old Charts for placement.. NAV Lines were used to aid in
Plotting Course. Often used in conjunction with tables of values to help the
Navigator maintain their bearing or determine their location in reference to
a Navigation Line. The Rosette of the Nav Lines would normally appear at a
Rally Point or Navigation Beacon. This could be the entrance to the Channel
to a Harbor or a point off shore where the Navigator would set the long
range course. In the latter cases, the Nav Lines would only radiate in the
Safe Sailing directions (not back to Port) These sometimes would be used to
indicate distances or time between Ports, though those type rarely included
the Rosette or the Starburst of Lines along the Cardinal directions..
(rarely was a Port directly along one of the Cardinal Directions from
another Port!). The type of Nave line included with CC is more decorative. A
similar type was used to provide a Reference as to Compass Bearing on a
Chart at any point, so the Navigator would not need to refer to the Compass
Rose when plotting.

Placing them on a Map as a decorative feature should be used so as to not
distract from the Map. Placing one under the Compass Rose for the the Map,
if that is on the Sea can be effective... using two on opposite corners to
Frame the Map are also effective in looks. If recreating the Chart Look,
make sure you have the other features associated with Charts. NAV Lines
should never cross land, no matter how small... Technically they should
never break for Land (no Island breaking a Nav Line) With that said, I have
seen this on a few Charts of the South Asia Islands, but I am not sure if
they were true Charts or Decorative Charts to be hung up as art... (I have
seen some Decorative Charts that moved Land Masses or deleted them all
together so the Nav Lines added would be unbroken)

You can use the Trim To Command and select the Lines and Trim (either
shortening or lengthening) the lines to an entity, such as the Coast Line or
the Map Border.

John Csaky

[ravells]

Cool topic. Rhumb lines / nav lines are a perennial source of enquiry and it's good to have all the explanations in one place. One of the master mariners at my office showed me how compass roses and graticules are used with parallel rulers to mark courses on sea charts. I guess the they used rhumb / nav lines for the same thing.

[arakish]

Sorry for the necromancy on this topic, but I found it interesting to read what others wrote about something I already knew.

On the wikipedia link above, they have a diagram that show what I made in images some time ago (perhaps years) when a friend asked me why put them on a map I made for him. As mentioned in a previous post, rhumb lines cross latitudes and longitudes at a constant angle. But when you look at it on a globe, the line spirals to the pole. Attached are some images I made showing this.

rmfr

[Hai-Etlik]

Sorry for the necromancy on this topic, but I found it interesting to read what others wrote about something I already knew.

On the wikipedia link above, they have a diagram that show what I made in images some time ago (perhaps years) when a friend asked me why put them on a map I made for him. As mentioned in a previous post, rhumb lines cross latitudes and longitudes at a constant angle. But when you look at it on a globe, the line spirals to the pole. Attached are some images I made showing this.

rmfr

That's not actually a true loxodrome. Your map is in Equirectangular projection and a diagonal in it is not a line of constant bearing. A true loxodrome in this projection has decreasing slope as it gets closer to the poles. You need a Normal Mercator projection to get all loxodromes shown as straight lines.

[Blacky]

I have what's I'm sure is a pretty dumb question. But hey, we're here to learn.

What Photoshop technique do you use to draw your nav lines? I'm thinking about this for some time now, and for the life of me, I can't find a nice tool to say to PS: draw n infinite lines from this point each with the same angle...