There are two restrictions on altitude which are important to IFR flight planning: Minimum reception altitude (MRA), an altitude which must be maintained across a flight segment (i.e. between two navigation radio transmitters) in order to assure reception of the required radio signals at all portions of that segment. Minimum obstacle clearance altitude (MOCA), an altitude which provides a predetermined vertical clearance from known obstacles within a predetermined corridor along the specified flight segment. For a published procedure, the greater of these two altitudes is the altitude which must be adhered to during that segment, and is called the Minimum Enroute Altitude (MEA). However, on an IFR en route chart, an asterisk appears next to the MOCA if there is also a MRA on that part of the route that uses a significantly higher altitude. One may choose to fly the lower MOCA altitude, but must keep in mind navaid reception is unlikely. One may do this if utilizing GPS and does not require navaids to navigate and would like the lower altitude. These altitudes are listed as "MSL" on IFR planning charts, and are thus the altitudes which would be indicated on the aircraft's barometric altimeter. Area minimum altitudes On Canadian IFR High- and Low-Level charts, area minimum altitudes (AMA) are published for quadrangle areas, which provide a buffer beyond the VFR maximum elevation figure. The AMA is the lowest off-airway altitude to be used under instrument meteorological conditions (IMC) that will provide a minimum vertical clearance of 1,000 feet (AGL), or in designated mountainous terrain 2,000 feet above all obstacles located in the area specified, rounded up to the nearest 100 foot increment. VFR flights Since VFR flights are not necessarily conducted on straight lines between ground-based radio navigation transmitters, the altitude restrictions for IFR flights are not applicable. Instead, a VFR flight can be conducted using pilotage, watching landmarks to determine position and desired direction. In this situation, the minimum reception altitude becomes moot, and the over-riding concern is for obstacle clearance. Pilotage in the United States is usually accomplished with the use of sectional charts, which show the ground with considerable accuracy, both for terrain levels and for man-made objects. The charts are marked with World Geographic Reference System or GEOREF grids, and at the center of each grid square a number shows the elevation (MSL) of the highest obstacle within that grid (the maximum elevation