Mapping toolbox: plotm strange behavior

Hi,

I've just just experimented a strange behavior with the plotm function of the Mapping Toolbox (Matlab R2008b)

I'm on a simple world map,

 figure;worldmap('w');

I try to plot some lines parallel to the equator using plotm (but the problem is the same with linem). Depending on their extend in longitude, the lines are plotted or not:

 plotm([30 30],[-10 10],'-k') % plotted
 plotm([30 30],[-80 80],'-k') % plotted
 plotm([30 30],[-90 90],'-k') % not plotted !
 plotm([30 30],[-180 -180],'-k') % not plotted !
 plotm([30 30],[-10 0],'-k') % plotted !
 plotm([30 30],[-90 0],'-k') % plotted !
 plotm([30 30],[-180 0],'-k') % not plotted !

The deduction I made from these examples were: plotm try to plot the shorter path, and if the shorter path is not inside the projection, it doesn't work. Well. But I experiment this:

 plotm([30 30],[-180 0],'-k') % not plotted, ok I understand
 plotm([30 30],[-170 0],'-k') % plotted, ok, I see
 plotm([30 30],[-171 0],'-k') % not plotted ! gne... oO ?

Do you know how to explain this ?

Answer 1

[Rob Comer]

Try using the newer geoshow function instead of plotm. All the lines are displayed if I replace

plotm(lat,lon,'-k')

with

geoshow(lat,lon,'Color','k')

Even for your case number 4, which has identical endpoints,

geoshow([30 30],[-180 -180],'Color','k')

results in a visible object --- but you'll have to turn off the frame

framem off

and look carefully at the edge of the map to see it, because it's just a single point.