PROJECTIONS AND GRIDS 97 



PROJECTIONS AND GRIDS 



THE MERCATOR PROJECTION 



The parallels of latitude and the meridians of longitude constitute the framework for 

 placing accurately all details which enter into the construction of the finished chart. They 

 are essential to the navigator in plotting and scaling his position. 



With few exceptions, nautical charts are constructed on the Mercator projection. On this 

 projection, meridians of longitude and parallels of latitude are straight lines intersecting at right 

 angles. The distances between meridians are equal throughout the chart, but distances be- 

 tween parallels increase progressively from the equator toward the poles, proportionately as 

 the scales of the parallels increase, so that the scale at any point on the chart is the same in 

 all directions. 



This projection has a number of advantages, among which are conformality, simplicity of 

 construction, convenience in plotting positions from the border divisions, and the fact that 

 on it alone a course can be laid off from any meridian or compass rose within its borders. Its 

 principal advantage, however, and the one responsible for its world-wide use for nautical 

 charts is that any straight line drawn on it in any direction is a rhumb line (loxodromic curve). 

 Thus the rhumb line, or the track of a ship on a constant course, is a straight Hne on the pro- 

 jection and will pass all features along that line exactly as they are charted. This is a great 

 advantage in coastal navigation, smce the straight line representing a constant course to be 

 made good will indicate at once the distance at which dangers will be passed abeam. 



Disadvantages of the Mercator projection are that it makes comparison of areas very 

 misleading when large differences of latitude are involved; that great circle routes cannot be 

 plotted conveniently on it without the use of an auxiliary gnomonic chart. Other disadvan- 

 tages are that the scale is constantly changing \vith the latitude, with the result that a graphic 

 scale cannot be used on smaller scale charts, making it necessary to measure distances along 

 the border divisions for the latitudes in which the distance lies; and that for distances over 

 200 miles, bearings must be adjusted before plotting. 



On all nautical charts, a minute of latitude is considered to be approximately 1 nautical 

 mile. 



