Quick Introduction to the Geometry Dictionary

From The SBN Wiki
Revision as of 17:26, 28 June 2016 by Raugh (talk | contribs) (Creation Safety Save)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search

If you're not already well-versed in geometry as it relates to observational meta-data, the Geometry Discipline Dictionary can be fairly intimidating. This page is here to help orient you to terminology and structure so you can navigate the Geometry Dictionary more easily. It is written for the non-specialist.

Terminology and Concepts

To start with, here are some key terms and concepts you'll find throughout the Geometry Dictionary.

Central Body
Sometimes the field of view contains one thing that is orbiting around another thing. When the smaller thing is the target of interest, Central Body is the term used to refer to the larger thing that the target is orbiting.
Clock Angle
Clock angles are a way of specifying the direction of something, like North or the Sun, from the center of an image. They are measured clockwise from "up" - a vertical running from the middle of the image to the center of the top edge. In order to correctly interpret a clock angle, you must be displaying the image correctly. "Correctly", in the PDS case, means according to the explicit display directions included in the <geom:Image_Display_Geometry> class.
Coordinate System vs Reference Frame
In the geometry dictionary, these two terms are distinct, and they are used here the same way they are used in the documentation for the NAIF SPICE toolkit (the software most missions and PDS nodes are using to calculate geometric values for PDS4 labels). A Reference Frame is defined by three orthogonal axes and an orientation in space; a Coordinate System is the result of fixing a Reference Frame to a specific origin. So (loosely speaking), "celestial coordinates" is a reference frame, "celestial coordinates centered on the Sun" is a coordinate system.
North Pole
For solar system objects, the IAU defines the "north" pole of a planet as the pole that is on the same side of the invariant plane of the solar system as the Earth's north pole. For larger planets and satellites, the poles are commonly referred to as "north" and "south" and you will see classes with "north pole" in their name or description. Only use these classes for things that have a well-defined "north". For small bodies, which tend to tumble and have complicated rotational states, "north" is not an applicable concept. For these cases, use classes with the "positive pole" notation, instead.
Object
In the Geometry Dictionary, the word "object" is used strictly to refer to the digital data object in the data file(s) referenced by the label. It will not be used to refer to physical objects like planets, comets, rings, dust, spacecraft, instruments, or any other thing that might be found in the field of view or involved in actually recording the observation. So if you are writing a label for an image of Titan, "object" will always mean the image, not Titan.
Positive Pole
This term is used to indicate the direction of positive angular momentum for a body that is rotating. The positive pole may or may not be considered the "north" pole, depending on a number of things including the overall rotational state of the body. Typically, asteroids and comets will be described in terms of their "positive" poles, while larger planets and their satellites will be described in terms of their "north" poles.
Specific vs Generic
Certain geometric quantities recur in data from widely varying sources, and are of particular interest to users, researchers, and analysts. In order to make those data quickly recognizable to both humans and program, many classes and attributes are define with "specific" names - names that indicate the observer and/or target. For example, <spacecraft_target_center_distance> is the attribute that contains the scalar distance between the spacecraft and the center of the target. "Generic" classes, on the other hand, allow a data prepare to define distances between arbitrary points, but require an explicit specification of start and end point. You should use the defined specific classes and attributes wherever they are applicable, because they support correlative studies across data sources, and use the generic classes only when there is no specific alternative.
SPICE
This is an acronym that has come to be the primary identifier for a toolkit that is widely used by missions and end-users to calculate observational geometry for spacecraft observations - though it has routines for ground-based observers as well. When you see this acronym used in a class or attribute name it indicates that the associated concepts are mapped directly from the SPICE toolkit and documentation. See the NAIF website for details: http://naif.jpl.nasa.gov/naif.
Target
The term target is used in this dictionary to refer to the thing of interest in the enclosing geometry class. It may or may not be the same thing named in the <Target_Identification> area elsewhere in your label. In fact, if you have multiple things of interest in your field of view, you may well have distinct <Geometry> classes in your label, each one of which provides geometry for one specific thing of interest ("target"). So target will be defined locally in your Geometry classes and subclasses. The <Geometry_Target_Identification> class is used throughout the Geometry dictionary structures to identify a thing of interest as the local target of reference.