The arc command is used to define an arc of a circle within a line segment.
The arc command uses the position of the previous point in the segment to define the start position of the arc. Therefore it cannot appear as the first item in the segment - it must follow an explicit point or another arc command. The arc command does not terminate the current segment.
Note that the .EWM file format has no concept of arcs. MapComp converts the specification of the arc command into a sequence of short straight lines in the .EWM file. In doing this MapComp uses a simple "Mercator" calculation for determining a point in terms of a distance and bearing from the centre of the arc, so arcs of large circles will not appear accurately.
The syntax of the arc command is:
| arc <centre> <radius> <end-point> [ <point-count> ] |
| <centre> | The latitude and longitude (in the standard format) of the centre of the arc. |
| <radius> | The radius of the arc in nautical miles. Minimum 1nm. Positive values indicate arcs clockwise from the start point, negative values indicate anti-clockwise. |
| <end-point> | The end point of the arc as a latitude and longitude in the standard format. |
| <point-count> | Optional. The number of points generated in the .EWM file for the arc. This defaults to one point for each 3° of arc which generally gives a smooth looking curve at any display scale. |
For examples of the arc command see the Brize Norton example.
There is redundancy in the information provided by the parameters of the arc command in that the distances from the start (previous) point to the centre or from the end point to the centre could be used to define radius in place of the radius parameter. (This redundancy follows from the way in which arcs are defined for airspace in the UK AIP (Air Information Publication) and presumably in the AIPs of other states).
MapComp uses the bearings from the centre of the arc to the end points combined with the radius parameter to define the actual end points used. It the generates an additional line segment to connect the last point of the arc to the specified end-point. In normal use this additional line segment should be very short and not visible.
As an additional check, MapComp will report an error if either the distance from the start point to the centre or the distance from the end point to the centre are less than 95% or more than 105% of the value of the radius parameter.