Astrogeology Science Center

Glossary of Terms

A Priori: The term, "A Priori", refers to the initial or original value of a pixel coordinate or geographic location within a control network. Applications such as pointreg or jigsaw refer to the a priori values for an input pixel coordinate (pointreg) or geographic location (jigsaw) in an input control network.

Adjusted: The term, Adjusted, refers to the output pixel coordinate or output geographic location within a control network. Applications such as pointreg or jigsaw report and update the "Adjusted" output pixel coordinate (pointreg) or geographic location (jigsaw) in the output control network.

Albedo: Reflectivity of a surface or particle. Commonly in I/F units for radiometrically calibrated data.

Apollo sites: The landing sites on the Moon for Apollo Missions 11, 12, 14, 15, 16, and 17.

Azimuth: The angular direction of the rays as observed from directly over a location, expressed as the angle measured clockwise relative to a line drawn from the location towards the Moon's north pole. Sun angle conventionally refers to the elevation angle of the Sun's center. In addition to the elevation angle a second parameter is required to completely describe the direction of the incident sunlight

Basemap: A map or image mosaic generally (hopefully controlled) used as the background upon which geographic data is overlayed and analyzed.

Basin: Basins are craters that are 190 miles (300 kilometers) or more across.

Bit: Short for binary digit, which in a computer is the smallest unit of storage. Bits are either "0" or "1".

Bit Type: Refers to how many bits there are per single meaningful value in an image file. Here 8 bit represents values from 0 to 255, 16bit represents values from -32768 to 32767, and 32bit represents floating point values from -3.40282E+38 to 3.04282E+38. For more see.

Body Fixed Coordinate: A planetary coordinate system where the coordinates are not time varying. For a body-fixed reference frame, ISIS refers to the NAIF planet kernel (PCK) for high-accuracy orientation (rotation) data for any solar system target body. This kernel includes location of the pole, prime meridian, axis directions of a body-fixed, body-centered reference frame and spin rate.

Byte: Short for binary term. A byte is a collection of computer bits. On most modern computers, a byte is a group of eight bits. Typically, computers handle and store information in binary (base-2), and a group of eight bits represents an eight digit binary number. Within the ISIS environment, "Byte" refers to how many bits there are per single meaningful value in an image cube file. The Byte information for an image cube file is stored on the image cube labels in Group=Pixels, where the keywords Type and ByteOrder describe the byte format.

Clementine: The Clementine mission of 1994 mapped most of the lunar surface at a number of resolutions and wavelengths from UV to IR.

Color Hillshade: A shaded relief map of the terrain where elevation values are represented by color.

Control Island: Ideally a control network will represent a single island of a list of overlapping images all tied together through a network of control points and image measures (Refer to Control Measures). Multiple islands can be identified within a single network if groups of images are isolated from one another due to no overlap or the absence of possible image measures.

Control Measure: The sample and line pixel coordinate (measurement) within a single image cube that has been measured and associated with a Control Point and other overlapping image cubes.

Control Network: A network of control points. Within the ISIS environment, the control network file is binary file. The network contains multiple fields of information for control points and measures collected between any number of image cubes.

Control Point: One or more measurements (image coordinates) that identify the same feature or location in different overlapping images.

CUB: Extension name for cube image file format used by Astrogeology's Integrated Software for Imagers and Spectrometers software. A cube is a 3-dimensional image with axis; samples, lines, and bands. The physical dimensions of a cube are called the number of samples (NS), number of lines (NL), and number of bands (NB). For application and conversion support see.

Cube: A cube is a image file format used by Astrogeology's Integrated Software for Imagers and Spectrometers software. The image can be a 3-dimensional image with axis; samples, lines, and bands. The physical dimensions of a cube are called the number of samples (NS), number of lines (NL), and number of bands (NB). For application and conversion support see.

CxP sites: The NASA Constellation Program identified potential landing sites or other areas of interest for scientific study.

Declination: Declination (Dec) is one of two angles of the North Pole of a target body as a function of time.

Direct Spatial Reference Method: the format type used to represent space in the data set generally either raster (image) or vector (point, line, polygon).

Elevation: The height above or below a fixed point on the surface of a body.

Emission Angle: The emission angle (EMA) is the angle between the spacecraft and a vector drawn perpendicular to the planet's surface (surface normal). Using SPICE, ISIS applications compute this angle from the ellipsoid. Emission Angle is in degrees. The valid range is between 0 and 90.

Emissivity: A measure describing a substance's ability to absorb and radiate electromagnetic energy.

Ephemeris: An ephemeris (plural ephemerides) is a table of values that gives the positions of astronomical objects in the sky at a given time or times.

Ephemeris Time: Ephemeris Time (ET) as defined by NAIF (Toolkit used by ISIS), is the uniform scale represented by the independent variable in the differential equations that describe the motions of the planets, sun and moon. Ephemeris time is described as a count of ephemeris seconds past the ephemeris reference epoch (J2000). The basic spatial reference system for SPICE is the J2000 system. This is an inertial reference frame in which the equations of motion for the solar system may be integrated. This reference frame is specified by the orientation of the earth's mean equator and equinox at a particular ephoch...the J2000 epoch. This epoch is Greenwich noon on January 1 2000 Barycentric Dynamical Time. Ephemeris Time is in seconds.

Equatorial Radius: The distance from the geometric center of an ellipsoid to its equator.

Equirectangular: The projection maps meridians to vertical straight lines of constant spacing for meridional intervals of constant spacing, and circles of latitude to horizontal straight lines of constant spacing for constant intervals of parallels.

Equirectangular Projection: The equirectangular projection is a simple map projection that maps meridians to equally spaced vertical straight lines, and circles of latitude to evenly spread horizontal straight lines.

Euler Angles: All the NAIF orientation models use three Euler angles to describe body orientation. The Euler angles describe the orientation of the coordinate axes of the 'body equator and prime meridian' system with respect to the J2000 system. For more information, refer to Right Ascension (RA), Declination (Dec) and Prime Meridian (W)

FGDC: The Federal Geographic Data Committee (FGDC) is an interagency committee that promotes the coordinated development, use, sharing, and dissemination of geospatial data on a national basis. FGDC activities are administered through the FGDC Secretariat, hosted by the U.S. Geological Survey. The FGDC is responsible for the development of the Content Standard for Digital Geospatial Metadata, the standard USGS scientists use to document datasets.

GIS: A geographical information system (GIS) integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information. A GIS allows one to view, understand, question, interpret, and visualize data in many ways that reveal relationships, patterns, and trends in the form of maps, globes, reports, and charts.

Gridded Data Record: The LOLA GDR data products contain the binned, interpolated altimetric measurements, as well as albedo, roughness, and surface slope, in cylindrical and polar projections.

HillShade: An image portraying topography on a map by shading different locations with different levels of gray according to illumination by an assumed light source from the northwest, thus creating a pictorial representation of relief.

HIS: HIS [High Instrument Saturation] is an instrument special pixel value that is tracked and reported to the user. HIS is set during the ingestion of the image data into ISIS. The input image sensor acquired a value too high to be measured accurately. HIS represents "Oversaturated". HIS is set in 16-and 32-bit type image data. For 8-bit image data, HIS = NULL.

HR: High resolution camera utilized in the Lunar Orbiter Mission.

HRS: HRS [High Representation Saturation] is a representation special pixel value that is tracked and reported to the user. HRS is set when an ISIS application causes the output DN pixel to fall 'above' the possible range of valid values. HRS represents "Oversaturated". HRS is set in all 8, 16, 32-bit type image data.

IAU: The International Astronomical Union

ILIADS: Integrated Lunar Information Architecture for Decision Support (ILIADS) is a lunar GIS that executes on a user-provided laptop or desk side computer. ILIADS supports 3D visualizations of the lunar surface, a suite of quantitative analysis tools, and analysis functions that can be invoked by user-developed scripts.

Incidence Angle: The incidence angle (INC) is the angle between the sun and the surface normal. ISIS applications use SPICE to compute this angle from the ellipsoid or the elevation model (e.g., local slopes) at every pixel. Incidence Angle is in degrees. The valid range is between 0 - 90. Data can be illuminated beyond the 90 degrees in the deep terminator region. While this is valid, most photometric functions will not normalize beyond 90 degrees. Incidence Angle is in degrees.

JP2: Jpeg2000 is an image format which uses a compression techniques based on wavelet technology. Images served from here are generally lossless but can be lossy if the space savings is needed. For larger Jp2 files, generally only scientific applications can be used to view them. The GeoJpeg2000 contains a GeoTiff-type spatial header within the Jpeg2000 file for mapping applications.

JPG2000: Jpeg2000 is an image format which uses a compression techniques based on wavelet technology. Images served from here are generally lossless but can be lossy if the space savings is needed. For larger Jp2 files, generally only scientific applications can be used to view them. The GeoJpeg2000 contains a GeoTiff-type spatial header within the Jpeg2000 file for mapping applications.

Kaguya: The Japan Aerospace Exploration Agency (JAXA) launched "KAGUYA (SELENE)" by the H-IIA Launch Vehicle on September 14, 2007 from Tanegashima Space Center (TNSC). The major objectives of the "KAGUYA" mission are to obtain scientific data of the lunar origin and evolution and to develop the technology for the future lunar exploration.

Latitude Type: Latitudes can be represented either in Planetographic or Planetocentric form. Both latitudes are equivalent on a sphere (i.e., the equatorial radius is equal to the polar radius); however they differ on an ellipsoid body (e.g., Mars, Earth).

Layer: The visual representation of a specific type of data (e.g., topographic data, titanium concentration, surface temperature, and hydrogen concentration) in a visualization system. Layers can be combined (overlaid) to identify data correlations between data or to produce more complex/comprehensive integrated map products.

LIS: LIS [Low Instrument Saturation] is an instrument special pixel value that is tracked and reported to the user. LIS is set during the ingestion of the image data into ISIS. The input image sensor acquired a value too low to be measured accurately. LIS represents "Undersaturated". LIS is set in 16-and 32-bit type image data. For 8-bit, LIS = NULL.

LO: Five Lunar Orbiter (LO) missions were launched in 1966 and 1967 to study the Moon. The first three missions were devoted to mapping potential lunar landing sites. The missions of fourth and fifth were intended for broader scientific goals.

Local Radius: The distance from a point on the surface of a body to its geometric center. ISIS expects and reports the local radius value in meters.

Local Solar Time: How high the sun is in the sky as seen from a particular site on the surface of a planet. The local solar time at a site on a body is the angular difference between the planetocentric longitude of the site and the planetocentric longitude of the Sun as seen from the center of the body.

LOLA: The Lunar Orbiter Laser Altimeter (LOLA) is an instrument on the LRO. LOLA will provide a precise global lunar topographic model and geodetic grid.

Longitude Direction: Longitude direction of a target body indicates whether longitude increases (positive) to the east or the west. The default for Longitude Direction in ISIS is East. Longitude Direction will affect the reported values for longitude range (Minimum and Maximum Longitude) and Center longitude.

Longitude Domain: Longitude Domain specifies how longitudes should be interpreted and reported for a target body. Longitude Domain = 360 will report longitude values (including Minimum, Maximum and Center longitude) in a degree range between 0 to 360. Longitude Domain = 180 will report longitude values (including Minimum, Maximum and Center longitude) in a degree range between -180 to 180. For more details, refer to the interactive demonstration in "Learning About Map Projections" on the ISIS Workshop website. For potential challenges refer to the workshop discussion regarding image data that crosses the Longitude Seams. The default Longitude Domain in ISIS is 360.

LRO: Lunar Reconnaissance Orbiter launched on June 18, 2009 aboard an United Launch Alliance Atlas V rocket. The LRO instruments return global data about the Moon, such as day-night temperature maps, a global geodetic grid, high resolution color imaging and the Moon's UV albedo. However there is particular emphasis on the polar regions of the moon where continuous access to solar illumination may be possible and the prospect of water in the permanently shadowed regions at the poles may exist. Although the objectives of LRO are explorative in nature, the payload includes instruments with considerable heritage from previous planetary science missions, enabling transition, after one year, to a science phase under NASA's Science Mission Directorate.

LROC: The Lunar Reconnaissance Orbiter Camera (LROC) is one of seven instruments aboard NASA's Lunar Reconnaissance Orbiter. LROC was designed to assess meter scale features to facilitate selection of future landing sites on the Moon and acquire images of the poles every orbit to characterize the polar illumination environment over a full lunar year. In addition to these two main objectives, the LROC team plans to conduct meter-scale mapping of polar regions, 3-dimensional observations to enable derivation of meter-scale surface features, global multi-spectral imaging, and produce a global landform map. LROC images will also be used to map and determine current impact hazards by re-photographing Apollo images.

LRS: LRS [Low Representation Saturation] is a representation special pixel value that is tracked and reported to the user. LRS is set when an ISIS application causes the output DN pixel to fall 'below' the possible range of valid values. LRS represents "Undersaturated". LRS is set in 16-and 32-bit type image data. For output 8-bit, LRS = NULL.

Luna sites: The Luna Program was one of two lunar exploration programs conducted by the Soviet Union. This was a very long-running program, with the first mission flying in 1959 and the last flying in 1976. The series included flyby, lunar-orbiting, and soft-landing missions. Luna landing sites are part of the LMMP "Unofficial Nomenclature".

Lunar Mapper: Lunar Mapper is a web-based digital overlay tool designed to allow the novice GIS user to interact with the LMMP products and services.

Lunar Prospector: Lunar Prospector (LP) launched aboard a Lockheed Martin rocket called Athena II on January 6, 1998. The primary mission was dedicated to globally mapping lunar resources, gravity fields, and magnetic fields.

Map Projection: A map projection is an algorithm or equation for mapping a three dimensional coordinate (latitude, longitude, radius) into a two dimensional coordinate plane (x,y).

Metadata: Metadata is a description of the content, quality, lineage, contact, condition, and other characteristics of data. The description of the data is organized in a standardized format using a common set of terms. Metadata is literally "data about data". Metadata records are similar in concept to library catalog records: details about a book such as title, author, and publisher are recorded in a standard way to ease the search for information. Metadata ultimately makes information about data sets more easily accessible to scientists and researchers.

Mini-RF: Mini-RF stands for Miniature Radio Frequency. Mini-RF is an instrument on the Lunar Reconnaissance Orbiter focusing on the lunar poles. The Mini-RF will map the lunar poles, search for water ice, and demonstrate future NASA communication technologies.

Model: A theoretical and mathematical representation, algorithm and/or application that is used to visualize, describe and/or predict the behavior of a particular system using a given set of variables and logical and quantitative relationships (e.g., lighting model, thermal model, and gravity model).

MR: High resolution camera utilized in the Lunar Orbiter Mission.

NAIF: Navigation and Ancillary Information Facility.

Neogeography: Neogeography literally means "new geography", and is applied to the usage of geographical techniques and tools used for personal and community activities or for utilization by a non-expert group of users. Examples include Google Earth, Google Mars and NASA's World Wind.

North Azimuth: A clockwise angle from a point of origin to true North. The North Azimuth is measured in a positive clockwise direction from a reference line. The reference line is formed by a line drawn horizontally from the origin point (pixel location) to the right side of the image (3 o'clock axis). Refer to Azimuth.

NULL: NULL is a Special Pixel value that is tracked and reported to the user. NULL represents "No Data". NULL can be set in all 8, 16, 32-bit type image data. Common NoData mappings include 8bit as 0, 16bit as -32678 and 32bit as -3.40282E+38 (or the lowest value in the bit type).

Off Nadir Angle: From the spacecraft, the Off Nadir is the angle between the nadir vector (subspacecraft vector) and the look vector. Every pixel in the image will result in a different Off Nadir Angle. Off Nadir Angle is in degrees.

PDS: The Planetary Data System (PDS) archives and distributes scientific data from NASA planetary missions, astronomical observations, and laboratory measurements. The PDS is sponsored by NASA's Science Mission Directorate. Its purpose is to ensure the long-term usability of NASA data and to stimulate advanced research.

Phase Angle: The phase angle is the angle between the sun and the spacecraft at a point on the surface. ISIS applications will use SPICE to compute this angle from the ellipsoid or the elevation model (e.g., local slopes) at every pixel. Phase Angle is in degrees.

Pixel Resolution: The ground distance in meters from the left edge to the right edge of a pixel.

Planetocentric: Planetocentric Latitude is the angle between the equatorial plane and a line from the center of the body.

Planetographic: Planetographic Latitude is the angle between the equatorial plane and a line that is normal (perpendicular) to the surface body.

Polar Radius: The distance from the geometric center of an ellipsoid to either of its poles.

Polygon Thickness: The thickness of a polygon is defined as follows- Thickness = Area / max(Xextent,Yextent)**2. A polygon's thickness will be between 0 and 1.

Positive East: The reported longitude values for a target body increase positive to the East.

Positive West: The reported longitude values for a target body increase positive to the West.

PPD: Pixels per degree

Prime Meridian: Prime Meridian (W) location is the third Euler angle which is expressed as a rotation about the North Pole as a function of time. The reference frame is J2000. For more information, refer to Euler Angles

Projection X: Projection X is the x-coordinate (Easting) for a point on a geographic Cartesian coordinate system

Projection Y: Projection Y is the y-coordinate (Northing) for a point on a geographic Cartesian coordinate system

PVL: Parameter Value Language (PVL) is used extensively by ISIS as a standard keyword value type language for naming and expressing data values. PVL format in ISIS is compatible with syntax used by the Planetary Data System. For example, the keyword labels of a typical ISIS image cube is in PVL format (refer to catlab in order to view image cube labels). A parameter name (or keyword name) is used to reference the value. The value portion of an assignment statement may contain a simple value (i.e., numeric, string, date/time value), a set or a sequence of values.

Radiance: A measurement describing the amount of electromagnetic energy emitted from an area of a planet.

Raster value: A raster is a data structure that is based on the use of grid cells. The raster value is the data value of the cell.

Reference Measure: Reference Measure is a fundamental component of the control network. For every point within the network, there is always one and only one associated Reference Measure (Reference=True). The Reference Measure refers to a single image and it's pixel coordinate (measure) that best represents a control point location or feature. This specific element of the control network is exclusively used for co-registering all other overlapping and potential measures (images) to the Reference Measure. There are control network applications that can set the Reference=True explicitly (cnetref, qnet) or 'runtime' implicitly (If reference is not set, the implicit default is the first 'valid' measure listed within a control point, refer to the pointreg application for more information).

Reflectance: The ratio of reflected energy to incoming energy.

Right Ascension: Right Ascension (RA) is one of two angles of the North Pole of a target body as a function of time. For more information, refer to Euler Angles, Declination and Prime Meridian

Scale: The map resolution measured in pixels per degree

Serial Number: A unique identifier constructed and assigned to each individual image within a control network. The serial number (SN) is constructed by most of the control network applications "in real time" from specific instrument keywords on the image labels (Instrument keywords are populated by mission specific ingestion applications). The serial number associated with each control measure is included as a field in the binary control network. ISIS getsn application will return the SN value for a given image filename.

Slant Distance: The distance from the spacecraft to the point of interest on the surface of the planet. Slant Distance is reported in kilometers.

Slew Angle: From the spacecraft, the Slew is the angle between the boresight (i.e., center of framing camera) and the nadir vector (subspacecraft vector). The Slew angle will be a constant value across the image. Slew Angle is in degrees.

Solar Longitude: The Solar Longitude is the planetocentric longitude of the sun as seen from a point on a body. It is considered a seasonal angle. Also referred to as L_sub_S.

South Pole-Aitken Basin: The South Pole-Aitken basin is the largest and oldest recognized impact basin on the moon.

Spacecraft Position: The Spacecraft Position is the position of the spacecraft (x,y,z) in body-fixed rotating coordinate system. Spacecraft position in body-fixed frame kilometer units.

Special Pixels: Special Pixels are defined to distinguish valid pixels from non-valid pixels.

SPICE: Spacecraft & Planetary ephemerides, Instrument C-matrix and Event kernels. SPICE refers to all the information required by ISIS in order to compute and map each image pixel onto a surface with reference to spacecraft position, sun position, instrument, and mission activities. ISIS uses software (ToolKit) supplied by the Navigation and Ancillary Information Facility (NAIF) for SPICE access and kernel management (Refer to spiceinit). SPICE is available only for mission instruments supported by ISIS (camera models).

Stereographic Projection: Stereographic projections are used for mapping polar regions.

SubSolar Point: The subsolar point is the target surface intercept of the line from the Sun and the target body's center. A triaxial ellipsoid is used to model the surface of the target body.

Sun Angle: The angle of the Sun above (positive) or below (negative) the local horizontal as viewed from a location on the lunar surface.

Target Center Distance: The distance from the spacecraft to the target body center. Target Center Distance is in kilometers

TFW: TFW is a detached GIS world file which is used by geographic information systems (GIS) to georeference raster map images. Small-scale rectangular raster image maps can have an associated world file for GIS map software that describes the location, scale and rotation of the map. These world files are six-line files with decimal numbers on each line. For more see.

TIF: TIFF is a image format popular among scientific and graphical applications. GeoTIFF is a public domain metadata standard which allows georeferencing information to be embedded within a TIFF file. For files over 4GB, the TIFF variant used is BiGTIFF which allows for huge files.

Universal Coordinate: The ISIS default coordinate system

UTC: Coordinated Universal Time (UTC) is a system of time keeping that gives an understandable name to each instant of time. The names are formed from the calendar date and time of day. UTC format consists of year, month, day, hour, minutes and seconds. UTC is in string format.

UVVIS: UVVIS refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region.

WLD: WLD is a generic extension for a detached GIS world file which is used by geographic information systems (GIS) to georeference raster map images. Small-scale rectangular raster image maps can have an associated world file for GIS map software that describes the location, scale and rotation of the map. These world files are six-line files with decimal numbers on each line. For more For more see.