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The SEDRIS Data Representation Model
APPENDIX A - Classes Regular Axis |
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An instance of this DRM class is an <Axis> that uses a constant spacing between hash marks and numerical values.
Digital Terrain Elevation Data (DTED) is sampled in a regular grid of latitude and longitude points. Accordingly, the latitude and longitude <Axis> resulting <Data Table> are <Regular Axis> instances.
A table of wind-chill values has axes of air temperature and wind speed. Air temperature typically starts at 0 degree Celsius and is decremented in intervals of 1 degree. Wind speed starts at calm (0 mph) and is incremented at intervals of 5 mph. Consequently, the air temperature and wind-speed axes of this table are <Regular Axis> instances.
The axis_alignment field provides flexibility in specifying how cells are centred with respect to middle, edge, corner, and so on.
Consider the following 4 (of 9 possible) cases for 2-D grids.
The * indicates the location where the tick mark coordinates intersect in the cell relative to the cell edges. The grid of points determined by axis tick marks represent the location of, to name a few examples, the centre, lower left corner, upper right corner, or edge, respectively, of the corresponding cell. The axis_alignment field therefore indicates the axis alignment being used.
FACE CNTR LR LFT CORNER UP RT CORNER EDGE
Axis 1 ------- ------- ------* -------
^ | | | | | | | |
| | * | | | | | * |
| | | | | | | | |
--> Axis 0 ------- *------ ------- -------
Axis 0: SE_AXIS_ALNMNT_MIDDLE SE_AXIS_ALNMNT_LOWER
SE_AXIS_ALNMNT_UPPER SE_AXIS_ALNMNT_LOWER| EDCS_Attribute_Code | axis_type; | (notes) |
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| SE_Short_Integer_Positive | axis_value_count; | (notes) |
The axis_type field specifies the property being described by the given <Axis> instance.
The axis_value_count field specifies the number of "hash marks" along the given <Axis> instance.
The value_unit field specifies the unit of measurement of the given <Regular Axis> instance, which shall be compatible with the requirements imposed by axis_type. If axis_type does not require a unit of measurement (for example, if axis_type requires a value type of STRING), then value_unit shall be set to EUC_UNITLESS.
The value_scale field specifies the scale applicable to value_unit. If axis_type does not require a unit of measurement (for example, if axis_type requires a value type of STRING), then value_scale shall be set to ESC_UNI.
The interpolation_type field allows the data provider to indicate how best to interpolate the data to points that are between grid points on the axis. When a <Data Table> has more than one axis, the order of the interpolations is in the order of axis definitions.
The first_value field specifies the first numeric value on the axis.
The spacing field specifies the distance between tick marks. For SE_SPACING_TYP_ARITHMETIC, spacing is the arithmetic difference between tick marks, such that Tick(N) = first_value + (N * spacing) For SE_SPACING_TYP_GEOMETRIC, spacing is the difference between tick marks such that Tick(N) = first_value * (spacing^N)
The spacing_type field specifies how the spacing value is used to compute tick marks.
The axis_alignment field specifies the position of the axis with respect to the axis interval. Note that "lower" and "upper" refer to the axis INDEX; for example, "lower" means "aligned to the side with the lower index".
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