Why do distances calculated using projected coordinates in systems like Mercator not always match the actual ground distance as reported by the Measure Tool?
Your are being confused by the 2 different types of “meters” being used. The Measure Tool always calculates accurate distances along the surface of the ellipsoid and is not affected by any distortion caused by a projection system, like Mercator. When you use a projection system, like UTM or Mercator, it projects coordinates from the 3D surface of the earth to a 2D plane in some linear units, like meters. However, because any projection from an ellipsoidal surface to a 2D plane must have some sort of inaccuracies, the linear values reported in that system cannot be 100% accurate. For a projection like UTM the difference from ground truth is quite small so long as you are within or near the UTM zone. The Mercator projection on the other hand is quite accurate in the Y-direction Mercator meters relative to ground truth regardless of location, but the reported Mercator meters in the X direction will be significantly stretched as you get far from the equator, so that the reported distance i
