Why must a coastal navigator check the horizontal geodetic datum note on older Chilean paper charts (such as SAD-69 / Sudamericano 1969) before plotting coordinates obtained from a standard WGS-84 GPS receiver?

1. Different geodetic datums define Earth's reference ellipsoid differently, which can cause positional discrepancies of hundreds of meters if coordinates are plotted without applying the chart's datum shift correction
2. Older charts are printed in miles per hour instead of knots
3. GPS signals are completely blocked by charts drawn on the SAD-69 datum
4. SAD-69 charts use magnetic bearings instead of true bearings for all coordinate grids