Why are the elevation values I downloaded much less than zero (-20 to -40 meters)?
• The data you downloaded are most likely referenced to an ellipsoid surface. The values you are seeing are about the expected range for that vertical reference frame. To get values that are closer to “0=mean sea level” try either the NAVD 88 or NGVD 29 datum in the Vertical Datum selection box in the LDART applet. For help with the concept of a vertical datum see the National Geodetic Survey FAQ site or this University of Colorado site.
