Current Page: NeXtMidas User's Guide Result Parameters System Modeling Data Frame of Reference
| #### Frame of Reference A Type 5000 file is a set of records comprising at most 14 explicit components and one or two computable, linear components that are most often treated as independent variables for uniformly sampled data. A component can be any non-record element type, and component type can vary within a Type 5000 record; however, all records within a given file must have a common format. The Type 5000 format is designed to store geometry data, which is only interpreted and combined with other geometry data after a frame of reference is specified. For example, knowing that a vector has the value (x,y,z) = (1,2,3) means little unless we know something about the origin and orientation of each of the axes. This is what frame of reference describes. The current Type 5000 format recognizes four different frames of reference that are distinguished by the following keywords in Type 5000 adjunct header: `ECR` - Earth-centered rotational, which has x and y in the equatorial plane at 0 and 90 degrees east longitude, respectively, and z north along the earth's rotational axis `ECI` - Earth-centered inertial, which has x toward the vernal equinox, y 90 degrees east of x, and z north along the earth's rotational axis `TOPOCENT` - Topocentric, which has x due south in the local horizon plane, y east in the horizon plane, and z perpendicular to the horizon plane in the upward direction `TOP` - Modified topocentric, with x due east in the local horizon plane, y north in the horizon plane, and z perpendicular to the horizon plane in the upward direction All but the ECR frame of reference require additional parameters to complete their definitions and permit conversion to different frames of reference. ECI needs epoch time and the hour angle of the vernal equinox as viewed from Greenwich. Both topocentric systems need the latitude, longitude, and altitude of an origin. The TOP system accepts additional augmentation in the form of optional azimuth, elevation, and roll angles. The Type 5000 format also allows NULL to be the specified frame of reference when frame of reference is not relevant to a particular dataset. NeXtMidas assumes that a non-NULL frame of reference that doesn't match one of the keywords above is the name of a file containing transformation matrices that specify a custom reference frame. Knowing frame of reference is sufficient for manipulation of geometry data specified in Cartesian coordinates, but there are additional coordinate systems in common use. It is important to recognize frame of reference and coordinate system as different concepts.