AvMap ekp iii c ユーザーズマニュアル
EKP III USER MANUAL
109
Appendix C Map Datum
A map datum is a mathematical description of the earth or a part of the earth and is based on
the ellipsoid or the arc of an ellipsoid that most closely represents the area being described. In
addition, the datum is centered at a specific location (the datum origin). A datum may describe
a small part of the earth, such as California or Iceland or may describe the entire earth, such as
WGS84, depending on which ellipsoid or ellipsoidal arc is selected.
Since datums use different ellipsoids and origins, the LAT/LON coordinates of the same posi-
tion differs from one datum to another. The difference may be slight or great, depending on
the datums involved, but will affect the apparent accuracy of the positioning information pro-
vided by a GPS receiver.
the ellipsoid or the arc of an ellipsoid that most closely represents the area being described. In
addition, the datum is centered at a specific location (the datum origin). A datum may describe
a small part of the earth, such as California or Iceland or may describe the entire earth, such as
WGS84, depending on which ellipsoid or ellipsoidal arc is selected.
Since datums use different ellipsoids and origins, the LAT/LON coordinates of the same posi-
tion differs from one datum to another. The difference may be slight or great, depending on
the datums involved, but will affect the apparent accuracy of the positioning information pro-
vided by a GPS receiver.
GPS (and all of the EKP-III cartridges) use the WGS84 datum, which is the model of
the earth that is the closest possible average of the planet as a whole.
Your chart datum is usually found in the legend. If the datum is not WGS84 this means
that position coordinates determined with the EKP-III may not appear to agree with coordi-
nates determined from a printed chart.
nates determined from a printed chart.