Planar measurements are measurements made on a flat surface. When using geographic coordinates, planar measurements must cut through the curved surface of the earth, causing the measurement to be shorter than the true distance. This error becomes greater as the length of the line increases. If using a projected coordinate system, the properties of the underlying projection greatly influence the accuracy of measurements for a given area of the map. This type of measurement is in contrast to geodetic measurement, which uses a curved (nonplanar) surface to model the earth.
- geodetic measurement