A projection widely used for aeronautical charts is the Lambert conformal conic, with two standard parallels. As the name implies, a cone is placed over the Earth; the cone intersects the Earth’s surface at two parallels of latitude. Scale is exact everywhere along the standard parallels, but scale decreases between the parallels and scale increases beyond the parallels. Shape and area distortion is minimal at the standard parallels, but the distortion increases away from the standard parallels.

All meridians are straight lines that meet at a point beyond the map; parallels are concentric circles. Meridians and parallels intersect at right angles. The chart is considered conformal because scale is almost uniform around any point; scale error on any chart is so small that distances can be considered constant anywhere on the chart. A straight line from one point to another very closely approximates a great circle.

**Polar stereographic projection**

The standard Lambert is too inaccurate for navigation above a latitude of approximately 75° to 80°. The polar stereographic projection is sometimes used for polar regions. A plane tangent to the Earth at the pole provides the projection. Meridians are straight lines radiating from the pole, and parallels are concentric circles. A rhumb line is curved and a great-circle route is approximated by a straight line. Directions are true only from the center point of the projection. Scale increases away from the center point. The projection is conformal, with area and shape distortion increasing away from the pole.

**Horizontal datum**

Cartographers need a defined reference point upon which to base the position of locations on a chart. This is known as the horizontal datum, or horizontal constant datum or horizontal geodetic datum. The horizontal datum used as a reference for position is defined by the latitude and longitude of this initial point. Prior to 1992, the horizontal datum for the United States was located at Meades Ranch, Kansas, referred to as the North American Datum 1927 (NAD 27).

With the introduction of geodetic satellites for mapping the Earth’s surface and satellite navigation systems for innumerable applications, there were recommendations to revise NAD 27. Beginning October 15, 1992, the horizontal geodetic referencing system applied to all charts and chart products was changed from the North American Datum of 1927 to the North American Datum of 1983 (NAD 83). This resulted in differences of only approximately 1000 feet between NAD 27 and NAD 83 positions. The greatest coordinate shifts occurred in Alaska and Hawaii where latitude was moved by as much as 1200 feet and longitude by up to 950 feet. In the conterminous United States, the maximum change was approximately 165 feet in latitude and 345 feet in longitude.