Mercator charts in ship navigation


Mercator charts:

Mercator charts are charts used in navigation onboard ships. This chart is used for Rhumb line sailing onboard ships. Rhumb lines are straight lines which cut every line at the same angle. Mercator projection is also called as cylindrical orthomorphic projection. This projection was invented by Gerard Kremar and was named after his latin name meaning, 'mercator'. It is a cylindrical projection in which, the meridians are not curved and are represented as straight lines which is perpendicular to the equator .They divide the equator into 360 parts. The parallels of latitude are horizontal and are parallel to the equator. 
Actually on the earth surface, the meridians converge at the poles, and the distance between two meridians is maximum at the equator and minimum at the poles. But on a Mercator chart, since the meridians are represented as straight lines, the distance between meridians are same at the equator and also at the poles. It is also seen that the distance between parallels of latitude is not constant throughout the chart. The distances between parallels of latitudes increase when moving towards the poles. Therefore in a Mercator chart the latitude scale increases along with latitude. 

Advantages of Mercator chart :

  • On Mercator charts, Rhumb line courses are easily laid.                           
  • Scale of distance is equal to the scale of latitude enabling to measure the distances very easily.                         
  • For a point on the chart, the shapes of landmasses nearby are correctly shown.                                                       
  • Between earth and the chart,  angles of Rhumb lines are unaltered.               
  • Distortions occur, even though the directions remain correct.                      
Easy transferring of directions and position lines from one part of the chart to another part as parallel lines. This facility is not available on most of other projections. This feature helps the navigator to use the chart in position fixing by running fix.    
                                                                                                                                                                        Disadvantages of Mercator chart:   
       
  • Great circle courses are not easily laid off. They appear curved on a Mercator projection.                               
  • The distance between parallel of latitudes increases towards the poles . The polar regions have large extreme distortions. This regions are not correctly represented on a Mercator chart.                                       
  • Scale of distance increases with the scale of latitude. It is not constant and changes as going towards the poles.                                                         
  • Mercator chart have varying distortion and different areas of charts cannot be compared.