this.sdata = sdata;
this.recno = recno;
StructureMembers members = sdata.getStructureMembers();
obsTime = getTime(members.findMember(obsTimeVName), sdata);
nomTime = (nomTimeVName == null) ? obsTime : getTime(members.findMember(nomTimeVName), sdata);
// this assumes the lat/lon/alt is stored in the obs record
double lat = sdata.convertScalarDouble(members.findMember(latVName));
double lon = sdata.convertScalarDouble(members.findMember(lonVName));
double alt = (zcoordVName == null) ? 0.0 : altScaleFactor * sdata.convertScalarDouble(members.findMember(zcoordVName));