McIDAS Programmer's Manual

Appendix A - Guidelines for Writing Helps

This appendix provides the guidelines to follow when creating or modifying a help to meet the McIDAS-X core standards. You will learn:

the general guidelines to use for all command helps
the specific guidelines to use for parameters, keywords and remarks

A sample help for the McIDAS-X command ZA is also included. See the McIDAS User's Guide and online command helps for additional examples.

This appendix consists of these sections:

General guidelines for all helps

Confirm that all spelling and punctuation are correct.

Verify that your help contains the sections listed below. Only include examples if the help is very short.

command name with functional description
command formats
parameters
keywords
remarks

Keep the length of each line to 76 columns or less, and the total length of the help to 79 lines or less. This will accommodate the width of the default McIDAS-X Text and Command Window, which is formatted to 80 columns.

Verify that the first line of the help contains the command name, one or two hyphens and an accurate functional description. For example:

SFCMG - Displays a surface meteorogram 
    -or-
AVGI -- Reduces image resolution by averaging

Use a series of 10 hyphens to indicate the end of the help. For example:

C ? ----------

Don't include the owner's initials in the help.

Don't put tabs in the help. The programs that make the help files will skip lines with tabs, resulting in help files with missing information. To check for tabs in vi , use :set list .

See the following commands as examples if you're writing a complex help: MDX, SCFPLOT, BATCH, ZA, SFCMG, LVF, IMGCOPY, IMGOPER and GRDDISP.

Specific guidelines

Parameters and keywords

Check existing helps when naming parameters and keywords, since many McIDAS-X commands share parameter and keyword names and definitions. If you use a parameter or keyword with the same meaning as one in another core program, use the same name. For example, use the parameter day and keyword DAY instead of date or DATE.

Choose parameter and keyword names that are unambiguous. For example, sarea and darea , for source area and destination area, are better choices for parameter names than area1 and area2 , or source and dest .

Don't write parameter and keyword descriptions as complete sentences. If the description has more than one comment, link them with semicolons.

Place default values for the parameters and keywords in parentheses at the end of the line using this format: (default=value1 value2 ... ).

Use a vertical bar character ( | ) between the end of a parameter name or keyword name and its description. If possible, align these vertical bars. The vertical bar is required by the help program to list the parameters and keywords when a user requests an abbreviated help by pressing Alt ? .

Parameters only

List parameters individually, in the order they appear in the command formats, not alphabetically.

Specify a parameter name in uppercase only if it is the exact value that the user will enter; for example, the MAKE, LIST, DIR, and DEL options of the MDU command. Otherwise, parameter names are lowercase.

Keywords only

Alphabetize keywords. If a command has many keywords and you group them by function. Alphabetize them within each group. See the GRDDISP and IMGDISP commands.

Specify keywords in uppercase with an equals sign, followed by a vertical bar character and the description. For example:

COLOR= | graphics color level for contours; use positive
         numbers for solid, negative numbers for dashed (def=2)

If a keyword has several values, don't repeat the keyword name on every line. Simply list subsequent values below the first one. For example:

FORM=ALL | lists an expanded directory
    =STN | lists the standard directory (def)

If a keyword accepts many values or a range of values, use the format shown below.

LEV=l1..ln  | copies grids with these levels; for example,
              SFC, 1000, 850
BAND=b1..bn | applies the function to the specified image
              bands (def=8 for GOES IR, 4 for POES)

Remarks

Place remarks in the order of importance to the user. Since space is limited in online helps, include only those remarks that help the user understand how the program works or provide a useful suggestion.

Write the remarks section in complete sentences with proper punctuation. A remark can be several sentences long. Do not indent the beginning of the remark; instead, separate the remarks with a single blank line.

Sample help

Help for the ZA command is provided below as an example. Also look at the helps for the DSINFO, DATALOC, GRDINFO, GRDCOPY, MFPLOT, NAVDISP and TXT2MD commands. They contain a variety of standard parameters and keywords and are good examples of how to write your helps.

ZA command

C ? ZA -- Place annotation on graphics frame at cursor position
C ?   ZA color height justify <keywords> "text
C ? Parameters:
C ?  color   | graphics color level (def=1)
C ?  height  | pixel height to draw text/symbols (def=10 for
C ?            text; cursor height for TCYC= symbol)
C ?  justify | L=left, R=right, C=center, V=vertical, M=mouse
C ?            N=numerous (def=L)
C ?  "text   | text to write on the graphics frame
C ?  "$text  | displays weather symbols or arrows on the 
C ?            graphics frame
C ? Keywords:
C ?  ANG=   | angle to write text when justify=L
C ?  ENCL=C | draws a circle around the text in the N mode
C ?      =E | draws an ellipse around the text in the N mode
C ?  GRA=   | graphics frame number (def=current)
C ?  TCYC=color hemisphere |
C ?           color - plot a tropical cyclone/hurricane symbol
C ?                   with the specified graphics color level
C ?           hemisphere - determines the direction of spirals;
C ?                   N=northern hemisphere
C ?                   S=southern hemisphere (def)
C ?   WID=  | graphics line width (def=current value; maximum=64)
C ? Remarks:
C ?   When justify = M(ouse), the text positioning will be
C ?   under the control of the mouse position and the clicking
C ?   of the buttons. The MIDDLE button will cause the start
C ?   of the text to be repositioned to the cursor location,
C ?   while the RIGHT button will reposition the end of the
C ?   text. The height will be scaled as needed.
C ?
C ?   When justify = N(umerous), text positioning will be
C ?   centered at the cursor location when the MIDDLE mouse
C ?   button is clicked. The last drawn symbol will be erased
C ?   when the right button is clicked.
C ?
C ?   When ENCL= is used, the practical maximum number of
C ?   characters is 3.
C ? ----------

Appendix B

Image Information

This appendix contains links to the current SSEC-assigned sensor source numbers used in McIDAS-X. It also contains links to the current band information for satellite data ingested in real time by the SSEC McIDAS-X servers.

Sensor source numbers

For a current list of Sensor Source numbers, see Appendix C: Sensor Source Numbers, in the McIDAS User's Guide.

Bands for satellite imagery

For a current description of the bands for satellite data ingested in real time by the McIDAS-X servers, see Appendix D: Bands for Satellite Imagery, in the McIDAS User's Guide.

The tables containing the band information use terms that you may be unfamiliar with. They are defined below.

The band is the number given to the wavelength channel of the satellite data.
The central wavelength is the wavelength falling in the middle of the range over which the satellite collects data.
The absorbing gases column lists the primary gases that absorb and emit radiation at the central wavelength. The term window used in this column refers to a channel that is transparent to atmospheric radiation.
The sounding level is the average level where most of the radiant energy sensed by the satellite originates, assuming a cloud-free atmosphere.
The column labeled meteorological objective or primary use provides examples of how a band is used in physical applications.
The term resolution refers to the maximum resolution of the instrument, which is not always the same as the resolution of the ingested data in the image file.

	For more information about the format of the satellite image data, see the AREAnnnn data file described in Chapter 6, Format of the Data Files.
	See the image-specific characteristics section in Chapter 6 for aspects of McIDAS-X area files that are specific to their image type.

Appendix C

Common Data Parameter Names

The table provided in this appendix lists common data parameter names used in McIDAS-X, along with a description of each.

Parameter	Description
ACFT	aircraft type (ASCII)
ASIZ	aircraft size
CA	cloud amount; octants of the sky covered
CAI1	cloud amount of the first layer
CAI2	cloud amount of the second layer
CAI3	cloud amount of the third layer
CAI4	cloud amount of the fourth layer
CC	cloud cover
CC1	cloud cover; first cloud layer non-ceiling
CC2	cloud cover; second cloud layer non-ceiling
CCH	high cloud cover (0=CLR, 1=SCT, 2=BKN, 3=OVC)
CCL	low cloud cover (0=CLR, 1=SCT, 2=BKN, 3=OVC)
CCM	middle cloud cover (0=CLR, 1=SCT, 2=BKN, 3=OVC)
CG1	genus of the first cloud (ASCII)
CG2	genus of the second cloud (ASCII)
CG3	genus of the third cloud (ASCII)
CG4	genus of the fourth cloud (ASCII)
CH	type of high level clouds (ASCII)
CIG	categorical forecast of the ceiling height
CIGC	cloud cover of the ceiling
CIGH	ceiling height
CL	type of low level clouds (ASCII)
CM	type of middle level clouds (ASCII)
CMAX	number of columns containing data
CO	country ID (ASCII); see the McIDAS-X CCODE command for more information
DAY	year and Julian day
DIR	wind direction
FLV1	first flight level
FLV2	second flight level
GUS	wind gusts
HMS	actual time of the observation
ICE1	first icing code
ICE2	second icing code
ICTP	icing type (ASCII)
ID	station identifier (ASCII)
IDA	part one of a two-part ID (ASCII)
IDB	part two of a two-part ID (ASCII)
IDN	station ID number
IFQ1	first icing frequency (ASCII)
IFQ2	second icing frequency (ASCII)
IL11	first icing base
IL12	first icing top
IL21	second icing base
IL22	second icing top
LAT	latitude, +North ( -90 to +90)
LEV	level: SFC, TRO, or same as P (ASCII)
LLWS	low level wind shear
LON	longitude, +West ( -180 to +180 )
MOD	modification flag (1=modified)
NREC	number of records in a row
OBSV	categorical forecast of obstructions to vision
P	atmospheric pressure
P1	temperature for significant temperature level
P1	wind direction for significant wind level
P2	dew point temperature for significant temperature level
P2	wind speed for significant wind level
P3	pressure for significant temperature level
P3	height for significant wind level
PC	characteristic of pressure tendency
PCP	3-hour precipitation total
PP06	6-hour probability of precipitation
PP12	12-hour probability of precipitation
PRE	pressure
PSD	direction the predominant swell comes from
PSH	height of the predominant swell
PSL	sea level pressure
PSNO	probability of snow
PSP	period of predominant swell
PST	station pressure
PT	3-hour pressure change
PTIM	time period for precipitation
PTYP	precipitation type (ASCII)
PZR	probability of freezing rain
QP06	6-hour quantitative precipitation forecast
QP12	12-hour quantitative precipitation forecast
SN06	6-hour snow probability
SN12	12-hour snow probability
SNO	cumulative snow depth
SPD	wind speed
ST	state ID (ASCII)
STI	insitu sea surface temperature
SV06	6-hour severe weather probability
SV12	12-hour severe weather probability
SWH	height of sea or wind waves
SWP	period of sea or wind wave
T	temperature
TBTP	turbulence type (ASCII)
TD	dew point temperature
TFQ1	first turbulence frequency (ASCII)
TFQ2	second turbulence frequency (ASCII)
TH06	6-hour thunderstorm probability
TH12	12-hour thunderstorm probability
TIME	time
TL11	first turbulence base
TL12	first turbulence top
TL21	second turbulence base
TL22	second turbulence top
TMAX	maximum temperature
TMIN	minimum temperature
TRB1	first turbulence intensity
TRB2	second turbulence intensity
TYPE	type of data
VDAY	Julian day of forecast verification
VIS	visibility
VTIM	time of forecast verification
WX1	weather - first four characters (ASCII)
WX2	weather - second four characters (ASCII)
WXP	present weather type (ASCII)
Z	height above sea level
ZC1	base of the first cloud layer
ZC11	first cloud base
ZC12	first cloud top
ZC2	base of the second cloud layer
ZC21	second cloud base
ZC22	second cloud top
ZC3	base of the third cloud layer
ZC4	base of the fourth cloud layer
ZCB	height of the cloud base
ZCL1	height of the first non-ceiling
ZCL2	height of the second non-ceiling
ZS	surface elevation

Appendix D

POES AVHRR Calibration Information

This appendix contains the following additional POES AVHRR calibration information resulting from the launch of NOAA-15 satellite in May 1998.

AVHRR Calibration Background

Since NOAA-12 and -14 AVHRR use the older TIRO calibration while the NOAA-15 AVHRR uses the newer AVHR calibration, changes have been made in McIDAS-X area structure between the NOAA-12 and -14 areas and the NOAA-15 areas. All bands of the NOAA-15 AVHRR/3 has calibration that differs from NOAA-14 AVHRR, but most of the changes are internal to the area structure and calibration module and are transparent to McIDAS-X users.

All POES images are now produced and delivered only through SDI (SSEC Desktop Ingestor). Areas created from NOAA-15 data will have the AVHR calibration type; areas created from NOAA-12 and -14 data will have the TIRO calibration type. The AVHR calibration exactly duplicates the output of the TIRO calibration in all respects. For NOAA-12 and NOAA-14, the TIRO and AVHR calibration appears interchangeable.

The advantage of the new calibration type is that only relatively minor changes need to be made once to SDI software (assuming the raw data stream format does not change). Any further changes required by NOAA-15 orbit performance is made only in the AVHR calibration module, rather than the SDI ingestor. The TIRO and QTIR calibration modules will gradually become obsolete; they will be used only for archived data.

SDI can now deliver NOAA-12 and -14 ADDE areas in either the TIRO or AVHR format; a logical switch has been installed to produce either calibration type. Only the AVHR format can be delivered by SDI for NOAA-15. For those installations that do not use SDI, an updated SATBAND file, an updated kbprep.f , and kbxavhr.dlm still will not be able to manipulate NOAA-15 imagery,because kbxtiro.dlm will not be upgraded.

TIRO and AVHR Differences

The changes from TIRO formats for AVHR are summarized below:

The TIRCAL line-by-line calibration code is now moved to the SDI. This allows retension of the use of the platinum resistance thermometers (PRT) and samples from target and space looks, but it identifies channel three as either a near IR or thermal IR sensor (using bit 10 of word 7 in the HRPT minor frame). It renames channel three to channel six if it is identified as near IR by bit 10 of word 7, which can switch while going from one line to another, but normally switches only twice per orbit.
Following a switch of bit 10 in word 7, there may be one or two bad lines in the raw data, and up to 10 bad lines of poorly calibrated data (worst case), due to the five-point subcommutation of the PRT temperatures between reference values. These bad lines appear totally black or totally white. After a reference line is encountered, it takes four more lines to acquire all of the PRT temperatures and to calculate their weighted average. If this is done on-the-fly , there is no way to recover any bad or poorly calibrated lines already sent. However, if you use post-processing of a completed area, it is possible to backfill the erroneous line prefix data.

Each image still contains five bands or less, but they are numbered 1,2,3,4,5 or 1,2,6,4,5 with corresponding changes in the line prefix LEV section. All calibration constants that are generated by SDI and passed in the line prefix CAL section are converted to scaled integers with scaling of 100,000 for slopes and offsets (except for the band 3 slope, which is scaled by 10,000,000) and a scaling of 100 for temperatures.

The DOC section content is unchanged, as compared to XSD; it is copied just as received, but left-shifted five bits. The length of the DOC section does not change with NOAA-15. For each line, the DOC section captures the raw data in an HRPT minor frame that does not constitute imagery that is converted to interleaved pixels.
Prior to NOAA-15, every AVHRR channel had a linear calibration that required a single gain and a single offset constant in the line prefix for each channel. With NOAA-15, this changes to a bi-linear calibration for the visible (1,2) and near IR (6) channels, which requiries two slopes and two offsets (four constants and two conversion equations) for each visible and near IR channel identified in the line prefix.
The three thermal IR channels (channels 3, 4 and 5) from the AVHRR/3 instrument have a new non-linear correction method applied, which requires a constant from the line prefix CAL section that contains the average space count. TIRCAL used this constant to calculate the slope-offset constants for previous AVHRR data in bands 3,4, and 5. Since the NOAA-15 raw data stream continues to look exactly like NOAA-12 and NOAA-14, the slope-offset calculations currently done in TIRCAL are preserved in SDI for all AVHRR data, including AVHRR/3. This permits the AVHR calibration module to be used for NOAA-12 and NOAA-14 data from SDI as well as for NOAA-15 data. At SSEC, all POES data has been switched from TIRO to the new AVHR calibration type within SDI after NOAA-15 began operation; TIRO is calibration used only as a fall-back position.
An inverse Planck function developed by SSEC is used for converting to emission temperature in both SDI and kbxavhr.dlm.
The line prefixes for AVHR calibration data have been reformatted to allow eight constants per line for each of the channels (160 bytes total) instead of two constants (40 bytes) for TIRO. For the visible channels, the first four constants parameterize the bilinear calibration, while the last four constants are unused. For the IR channels, we use only the first two constants for linear calibration. The fifth and sixth constants are five scan averages of the space look and internal target. The internal target temperature is now redundantly written into the CAL section of the line prefix for bands 3,4, and 5 (at constant position 7) instead of overwriting the channel 3 patch temperature and spare word. The eighth constant is an unused spare in all channels. Note that the scaling factors for the constants are also different from TIRO. These line prefix changes will make the AVHR calibration type images incompatible with TIRO calibration.
The CAL=QUICK option is no longer available for AVHR.

Glossary

Miscellaneous, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

Miscellaneous

0-based
A counting sequence that begins with zero.

1-based
A counting sequence that begins with one.

A

ADDE
Abstract Data Distribution Environment software in McIDAS-X that lets a workstation act as a client, efficiently accessing data from multiple McIDAS-X servers.

alias
A short, user-defined name representing an ADDE dataset name; for example, the alias GV1 could represent the dataset name
SSEC-RT/GOES8-1KVIS.

ancillary data
Additional information needed to identify, quantify and manipulate data; for example, directory, navigation and calibration blocks.

API
Application Program Interface.

applications program
A program that runs from the McIDAS-X command line.

area
The McIDAS-X image file format.

ASCII file
American Standard Code for Information Interchange file containing only text; for example, schema definition files and scripts.

asynchronous
McIDAS-X commands that run asynchronously will return control to the original calling program before they have run to completion. Also see synchronous.

B

band
The spectral channels measured by a scanning instrument; for example, band 4 for the GOES-9 imager is 10.7 microns (infrared). Also see spectral band.

band map
Region of the line prefix that contains an ordered list of the spectral bands comprising the data portion of an image line.

big-endian
Used interchangeably with network-byte-order to mean the most significant byte in a word comes first; opposite of little-endian where the least significant byte comes first.

binary file
A file containing binary information; for example, areas and executable programs.

blank-padded
Describes the practice of replacing unused characters at the end of a string with spaces.

block
A collection of data records.

blow down
To decrease image resolution by sampling or averaging data. For example, a blowdown of two drops out every other data point along the line and every other line in an image.

blow up
To increase image resolution by replicating data point values, much like enlarging a 3 x 5 photograph to an 8 x 10.

buffer
Any memory storage.

byte
An 8-bit memory segment; a 16-bit memory segment is called a half-word; a 32-bit memory segment is called a word.

C

calibration
The conversion of data values received from an instrument to useful, physical quantities such as temperature, radiance or albedo.

calibration block
The image object block that holds the information for transforming image data from its internal quantities to more common physical quantities, such as radiance or albedo.

calibration module
A group of subroutines that are specific to a type of image data; this module is used to perform a calibration.

celestial coordinates
Identical to earth coordinates except the x-axis passes through the longitude of vernal equinox rather than the prime meridian so that the celestial system is fixed relative to the stars. The transformation from celestial to terrestrial involves a single axis rotation about the z-axis, equivalent to a scalar shift in longitude. Satellite orbital predictions are typically made in a celestial system.

client
The workstation in a distributed system that initiates a request, then receives and displays the requested data.

client routing table
The table that holds the list of group names configured by the user with the McIDAS-X DATALOC command.

clipping region
See viewport.

command line
The command or series of commands entered in the McIDAS-X Text and Command Window. It may consist of positional parameters, keywords and quoted text. In McIDAS-X, the number of characters permitted in a command line is workstation dependent, although there is no practical limit.

comment block
An image object block containing a variety of textual information, such as a list of commands run on the image object to-date.

compatibility library
A file where obsolete McIDAS-X library functions are placed for one year when they are no longer referenced by any core programs. Also see local library and McIDAS-X library.

conformal projection
A map projection in which angles are preserved; for example, parallels of latitude and meridians of longitude intersect at right angles. McIDAS-X supports Mercator, Lambert conformal, polar stereographic and tangent- cone conformal projections.

connection
The initialization that occurs in a distributed system when a client determines the location of the dataset server and issues a request for a data exchange. The server examines the request and determines its validity; if the request is valid, the connection is opened and the client is authorized to begin its transaction.

contrast stretching
The process of changing an image's gray scale to emphasize a feature for analysis; for example, thunderstorm cloud tops. Unlike data stretching, contrast stretching does not change the image data values.

coordinate systems
The four systems used to define the location of data points within an image; they include image, file, earth and frame coordinates. A fifth coordinate system, called world coordinates, is used with graphics.

cursor
The mouse-driven, highlighted mark that appears on the McIDAS-X display. Users manipulate the cursor to interact with McIDAS-X commands and the McIDAS-X Image Window. Several cursor sizes, types and colors are available.

D

data block
The block containing the actual data values.

data point
A collection of one or more bytes.

data point size
The number of bytes needed to accurately represent a data point; usually 1, 2 or 4 bytes.

data stretching
The process of changing an image's gray scale by stretching image data values to brightness values. To stretch image data values, a table defining the values to stretch must be created with the McIDAS-X SU command.

dataset
A collection of one or more files with a common format; for example, one dataset may contain image data, while another dataset may contain point data.

dataset name
The name used by the ADDE server to identify the type of data the user wants to access and the range or names of files to search. It consists of a group name and a descriptor name separated with a slash, such as SSEC-RT/GOES8-1KVIS.

decoder
The software that parses data from one format into a common format for use by another process such as a plotter or lister, or software that further manipulates data.

default
The parameter value accepted by the program if the user doesn't specify a value. To use the default for a positional parameter, the user types the letter X in the command line.

descriptor
The name used to reference a dataset in ADDE; for example, a dataset of images containing GOES-7 visible data at 4-km resolution might have the descriptor G7-VIS-4K.

directory block
An image object block containing the list of ancillary information about the image, such as the number of lines and data points, the satellite ID and the number of spectral bands.

disk file
McIDAS-X file for storing information that applications can randomly access by byte address using standard system library calls. Formerly called LW (Large Word) array files.

display
The device used to output image and graphical data in McIDAS-X; usually a workstation monitor or an X Terminal.

distributed data system
A computing system in which data is received, processed and stored, and then distributed among multiple workstations. Data can be received and processed on the same machines that store and serve it.

DLL
Dynamic Link Library; the library used in dynamic linking. OS/2 has true dynamic linking, while Unix modules are statically linked only giving the appearance of dynamic linking.

double precision
A two-word storage representation for floating-point numbers.

dynamic linking
Subprograms loaded at run time.

E

earth coordinates
A coordinate system having its origin at the Earth's center, its x-axis through the intersection of the equator and prime meridian, its z-axis through the north pole, and its y-axis completing a right-handed system. Locations in this system may be Cartesian (distances x, y and z from the origin along each axis) or spherical (a distance from the origin or a reference radius, and two angles from the x-axis). The most common spherical form is longitude, geodetic latitude and height above the reference geoid.

element
The image coordinate that makes up each division of the image along a scan line. Elements run vertically up and down the frame; they are numbered left to right with the leftmost element numbered one.

environment
The place where applications programs reside, along with McIDAS-X resident programs and shared memory.

equal-area projection
A projection in which areas are preserved; two equal areas on the Earth are also equal on the projection, even though their shapes are different. McIDAS-X supports the sinusoidal equal-area projection.

extended format
McIDAS-X commands run with an extended format can contain a semicolon, indicating the start of a sequence of commands, or one or more pound signs, indicating a required string substitution.

F

file coordinates
The coordinates of a data point in an area file referenced sequentially by lines and elements. The top line and leftmost element have the file coordinates (0,0).

file redirection
The process that lets users identify the location of individual files on a workstation.

format
File format. McIDAS-X file formats include image, grid, point and text; non-McIDAS-X formats include HDF and NetCDF.

frame
Contains a representation of an image sector displayed on the McIDAS-X Image Window. Users can define the number and size of frames; the default is four frames that are 480 lines by 640 elements.

frame coordinates
The native coordinates of a frame referenced sequentially by lines and elements. The frame's upper-left corner has coordinates (1,1). The number of lines and elements on the frame is determined by the frame size.

frame object
A memory-based collection of information that completely describes the contents and appearance of a frame to the mcimage process, which realizes it into a visible picture. Frame objects are stored in McIDAS-X shared memory.

FTP
File Transfer Protocol. A method of transferring files between workstations on a network.

full image
The entire image transmitted by a sensor source.

full resolution
One image data point represents one satellite sensor data point. Also see image resolution and satellite resolution.

function
The term used in this manual to describe C procedures and functions, and Fortran functions and subroutines.

G

geocentric latitude
The angle between the equatorial plane and a ray through the point from the Earth's center.

geodetic latitude
The angle between a line perpendicular to the surface of the geoid through a point and the Earth's equatorial plane. Due to the Earth's oblateness, geodetic latitudes (the most common form of earth location) are slightly greater than geocentric latitudes except at the equator and poles where they are identical.

geoid
The spheroid (surface formed by rotating an ellipse about the polar or Z-axis of the terrestrial coordinate system) that most closely approximates the Earth's surface.

geostationary satellite
A satellite that remains above a fixed location on the Earth's surface, usually about 36,000 km above the equator. It is limited in view, approximately 60° either side of the equator. GOES-8 and -9 view North America; Meteosat views Europe and Africa; GMS views the western Pacific.

global keyword
A keyword that can be used with any McIDAS-X command.

global string
A string name whose first character is a question mark; useful for defining strings that you don't want accidentally deleted. Global strings remain in the string table even if the current string table is replaced with another.

GMS
Geostationary Meteorological Satellite.

GOES
Geosynchronous Operational Environmental Satellite.

graphics
Text, symbols and line segments drawn in color on the McIDAS-X Image Window.

gray scale
The range of black-to-white gray shades available for displaying image data on the McIDAS-X Image Window. The range is 0 (black) to 255 (white).

gray shading
The most common method of displaying image data.

grid
A lattice of regularly spaced data points superimposed on a projection of the Earth. Grids are generated from numerical models or observational data.

grid header
The part of the grid object that contains the ancillary information about the grid, such as the parameters and physical quantities of the data in the grid, the level in the atmosphere or ocean the data represents, the grid navigation information and the time.

grid object
The actual gridded data along with the ancillary information contained in the grid header.

group name
In ADDE, the name used by the client to identify the server machine to get the data from. The server uses it to identify the data that the client is requesting.

H

half-word
A 16-bit memory segment; a 32-bit memory segment is called a word; an 8-bit memory segment is called a byte.

handle
A computer term used to describe a variable in a program that points to a specific structure. Handles are often used with input and output events.

help
A block of comments describing an applications purpose, its positional parameters and keywords, and other notable remarks.

I

image
Information that is usually represented as shades of gray in a two-dimensional matrix, such as satellite images, radar images or images derived from grids.

image coordinates
The native coordinates of remotely sensed data expressed as lines and elements. Each image is a series of lines and elements arranged from top to bottom, forming a grid for displaying data points on a McIDAS-X frame. Lines run horizontally across the frame; elements run vertically up and down the frame. The top line and leftmost element have the image coordinates (1,1). This coordinate system is independent of McIDAS-X and forms the basis for other McIDAS-X coordinate systems.

image object
The actual image along with its ancillary data.

image object block
A collection of image objects; each block contains image data or ancillary information.

image resolution
The number of satellite scan lines represented in each data point of an image line. Resolution can be increased or decreased; see blow up and blow down. Also see full resolution and satellite resolution.

image sector
A rectangular subset of a full image with the same coordinate system.

Image Window
The window used for displaying frames containing McIDAS-X-generated images and graphics.

include files
Files that hold definitions of constants specific to the McIDAS-X software; for example, mcidas.h and fileparm.inc .

ingestor
A process that listens to data received by a communications port and reformats the information for further processing.

interface documentation block
The template to use when writing a new McIDAS-X library function in Fortran or C.

I/O
Input/Output.

J

Julian day
Calendar date based on a 365-day year, usually in the form ccyyddd; for example, 1996056 is February 25, 1996.

K

keywords
Alphanumeric values that provide input to a McIDAS-X command; useful for clarifying commands with many complicated options. Keywords are always followed by an equals sign or a comma and the assigned value. They are optional for most commands and can be entered in any order as long as they follow command positional parameters and precede quoted text in the command line.

L

line
Image line; each image line contains an optional line prefix and the actual data values. Lines run horizontally across a McIDAS-X frame; they are numbered from top to bottom with the top line numbered one.

line prefix
Optional information that precedes the data on an image line; contains ancillary data about the line, such as navigation or calibration parameters.

line prefix block
The image object block containing information about an image that may vary on a line-by-line basis, such as documentation or calibration information.

little-endian
The least significant byte in a word comes first; opposite of big-endian where the most significant byte comes first.

local library
A file for keeping your local functions with their application's source code. A local library is useful for referencing functions that SSEC moves to the compatibility library. Also see compatibility library and McIDAS-X library.

lock
A unique, alphanumeric name (usually a legal file name) used to ensure that two programs can't access the same resource simultaneously.

looping
Continuous, automatic stepping through a sequence of image and/or graphics frames, much like a movie loop.

M

macro
A McIDAS-X command that runs a series of McIDAS-X commands (embedded in Fortran code) in a predefined sequence.

makefile
A description file that defines the relationships or dependencies between applications and functions; it simplifies the development process by automatically performing tasks necessary to rebuild an application when you modify code.

map files
Outlines of political or geographic boundaries that can be superimposed on the McIDAS-X Image Window using the MAP command.

McIDAS
Man computer Interactive Data Access System; a collection of tools for acquiring, analyzing and displaying meteorological data, created by the Space Science and Engineering Center of the University of Wisconsin-Madison. McIDAS-X runs on Unix workstations.

McIDAS-X library
A file called libmcidas.a that contains all the object code for the functions and subroutines that make up the McIDAS-X Application Program Interface (API). Also see local library and compatibility library.

McPATH
The environment variable in McIDAS-X that defines directories for commands to search when looking for data and help files.

memory overflow
Writing beyond the memory allocated for a variable.

Meteosat
European geosynchronous meteorological satellite.

mouse
See pointing device.

N

navigation
The process of transforming image coordinates (lines and elements) to earth coordinates (latitude and longitude) and vice versa.

navigation block
A McIDAS-X data structure containing the projection type and set of parameters for computing transformations between earth and image coordinates. Sometimes called a navigation codicil.

navigation module
A group of subroutines that are specific to a type of image data; this module is used to perform navigation.

navigation transform
A set of equations for converting a dataset's image or grid coordinates to and from earth coordinates.

network-byte-order
See big-endian.

null-terminated
Describes the practice of placing a zero (ASCII NULL character) at the end of a character string; this is the standard representation in the C language.

P

parsing
Breaking down an observation into its most elementary parts.

pipe
Shared space that accepts the output of one program for input into another.

pixel
A point on a McIDAS-X frame assigned a unique pair of line and element coordinates.

POES
Polar Orbiting Environmental Satellite.

point object
The actual point data values along with their ancillary information.

point data
Atmospheric or oceanographic data occurring at irregularly spaced locations on the Earth or vertically within the atmosphere or ocean. Most data gathered by direct measurements, such as weather balloons and synoptic reports, is stored as point data.

pointing device
A three-button mouse; the leftmost button is used by the window manager and the middle and right buttons are used by the McIDAS-X mouse interface.

polar orbiting satellite
A satellite that provides complete coverage of the Earth's surface twice per day. It normally orbits 800 to 900 km above the Earth and has a field of view that is about 2400 km, centered on the orbit path.

position
In ADDE, the absolute or time-relative position of a file in a dataset; position numbers greater than zero represent an absolute position in the dataset; numbers less than or equal to zero represent a relative position, 0 is most recent and -1 is next most recent; for example, if a dataset has four images with times 13, 14, 15 and 12, they have the positions -2, -1, 0 and -3.

positional parameters
Alphanumeric values that provide input to a McIDAS-X command; they must be entered in the exact order specified. Useful for minimizing the number of keystrokes a user types.

physical quantity
Radiance, temperature, albedo, etc.; sometimes, in error, called unit.

process chain
A series of processes run synchronously.

projection
A set of equations relating earth locations (three variables) to a location in Cartesian coordinates on the projection plane. Also see conformal projection, equal-area projection and earth coordinates.

projection parameters
One or more constants contained in projection equations; specifying values for these constants defines an instance of the projection.

pseudo-Mercator projection
A projection in which latitude and longitude vary uniformly with line (or row) and element (or column). This projection is distinct from a true Mercator and is neither conformal nor equal-area.

Q

quoted text
The last part of the command that the user enters; each application can contain only one quote string. Quoted text is preceded by double quote marks (") and is most often used when strings entered by a user require whitespace.

R

radar data
Information related to the strength of the reflected radar signal; usually correlated with rainfall intensities. Radars use active sensors that emit short-wave radiation and sample the signals reflected back to the radar antenna. Modern radars also sense the radial component of droplet velocity.

real-time data
Data that is available to users as soon as it is received by the system.

redirect tables
Part of the shared memory block that resides in the McIDAS-X environment; file redirection information from the McIDAS-X REDIRECT command is stored in them. Also see file redirection.

resident programs
Programs that are basic to McIDAS-X and its environment. For example, in McIDAS-X, the resident program mctext controls command line input from the keyboard and displays text output on the Text and Command Window.

resolution
See full, image, or satellite resolution.

S

satellite resolution
The size of the smallest feature that the satellite's sensors can detect; this is determined by the geographic width of each scanned slice of the Earth's surface observed by the satellite. Also see full resolution and image resolution.

scheduler
The part of the McIDAS-X system that initiates and ends user-defined command sequences.

selection clause
A text string used by an application to restrict information sent from the server to the client.

sensor source
A satellite device that collects a specific wavelength of radiation; for example, visible, infrared, microwave, solar protons or X-ray.

sensor source number
The number assigned to an image data source; it is stored in word 3 of the area directory; for example, 70 is the GOES-8 imager.

sensor type
Sensor name consisting of up to four characters, stored in word 52 of the area directory; for example, GVAR is the sensor on GOES-8.

server
The machine in a distributed system that stores data and supplies it to the client upon request. Each McIDAS-X workstation session acts as both a client and a local server. It can also be configured as a remote server, supplying data to all clients.

server mapping table
The table containing the list of dataset names on the server. Users assign these names with the McIDAS-X DSSERVE command.

shared memory
A component of the McIDAS-X environment that consists of User Common, redirect tables and frame objects. Resident programs use it to communicate with applications.

shell script
A program containing a set of executable commands; useful for running a series of McIDAS-X commands outside of McIDAS-X.

sleep
An application tells the operating system it doesn't want to be considered ready to be dispatched for a period of time.

slot number
The number 1, 2 or 3; allows loading of up to three navigation and calibration modules.

spectral band
The wavelength in which a scanning instrument measures data; for example, band 4 for the GOES-8 imager is 10.7 microns (infrared).

static data
Database information that changes little over time. Examples include map files, station tables, or font files.

static linking
Subprograms included at compile/link time.

station tables
A cross reference list of reporting stations.

stdin
Standard input.

stdout
Standard output.

stretching
See contrast stretching and data stretching.

string
A named character string defined by a user with the McIDAS-X TE command. A character string can be accessed by programs using parameter retrieving functions. It has two uses: it provides a shorthand method of entering commands and it allows programs to access keyword values predefined by the user that are not actually entered when the program begins. String names may contain no more than 12 alphanumeric characters; strings may not exceed 160 characters.

string table
A table of named character strings; useful for passing information between commands run at different times. An individual string table may contain no more than 256 strings.

synchronous
McIDAS-X commands that run synchronously will run to completion before control is returned to the original calling program. Also see asynchronous.

T

TCP/IP
Transmission Control Protocol/Internet Protocol. A set of communications protocols used to network dissimilar systems. The TCP protocol controls the transfer of data. The IP protocol provides the routing mechanism.

text output
Usually refers to the three types of messages that applications use to communicate status information to the user: text, error and debug messages.

Text and Command Window
The window used for entering McIDAS-X commands, displaying command output and showing workstation status information. When a session is started, 10 different text frames can be displayed in this window.

toggle
Turning on and off a McIDAS-X function, such as graphics or image frames; similar to turning a light switch on and off.

token
The smallest entity to which an observation may be parsed.

transaction
Any ADDE exchange; it implies a transfer between an ADDE client and server.

transaction logging
The record keeping done by ADDE servers for each transaction.

type
Data type: image, grid, point or text.

U

unit
See physical quantity.

Unix
Multitasking operating system originally developed by AT&T; McIDAS-X requires this system.

User Common (UC)
A component of the McIDAS-X shared memory that is used in applications to alter the display and make the applications interact with each other in predictable ways.

UTC
Coordinated Universal Time; same as GMT (Greenwich Mean Time).

V

validity code
Region of the line prefix that determines if data exists for an image line.

viewport
The region of a frame to be displayed; graphics outside this region will not appear even if drawn. Viewports are used in McIDAS-X programming to generate graphical output in panels. Also called a clipping region.

W

weather text
Data transmitted in alphanumeric form; it can be user-generated or computer-generated and contains forecasts, observations, weather advisories or other public information.

whitespace
A subset of the ASCII character set, including space, end-of-line, vertical tab, horizontal tab and form-feed characters.

word
A 32-bit memory segment; a 16-bit memory segment is called a half-word; an 8-bit memory segment is called a byte.

world coordinates
The coordinate system as viewed by a graphics program; world coordinates may be defined to be convenient for the application. Their purpose is to generate attractive, properly positioned output regardless of the size of the frame.