Whats the “Latitude of True Scale” that Mercator charts ask for?
The latitude of true scale in a Mercator projection is the one latitude for which distance measurements are correct. In general, a Mercator chart will have a latitude of true scale in the center of the chart. However, if the latitude of true scale given in the chart were applied, it would not be possible to put multiple Mercator charts in a single view. Therefore the developers have provided a means for allowing the user to set the latitude of true scale that will be used for all Mercator charts in a given view. When setting the latitude of true scale for a view, two options are suggested: 0.0 degrees and the latitude of true scale that is read from the chart being added. The drawback to using 0.0 degrees is the resulting inaccurate measurements. Few, if any, NOAA nautical charts actually have the equator in them; thus, the scale is wrong everywhere in the chart and the error increases with latitude. To allow the user to get reasonable distance measurements, a latitude of true scale ma
