The following system variables contain predefined constants or values for use by IDL routines. System variables can be used just like other variables. For example, the command:

PRINT, ACOS(A) * !RADEG

converts a result expressed in radians to one expressed in degrees.

!COLOR

A read-only structure variable that allows you to retrieve color values [red, green, blue] by providing one of the color name strings listed in the table below.

PRINT, !COLOR.burlywood

The following table lists !COLOR names and values (these are essentially the same as those in W3C’s list of web colors):

Color Name

Hex Value

Decimal Value

alice_blue

#F0F8FF

240,248,255

antique_white

#FAEBD7

250,235,215

aqua

#00FFFF

0,255,255

aquamarine

#7FFFD4

127,255,212

azure

#F0FFFF

240,255,255

beige

#F5F5DC

245,245,220

bisque

#FFE4C4

255,228,196

black

#000000

0,0,0

blanched_almond

#FFEBCD

255,235,205

blue

#0000FF

0,0,255

blue_violet

#8A2BE2

138,43,226

brown

#A52A2A

165,42,42

burlywood

#DEB887

222,184,135

cadet_blue

#5F9EA0

95,158,160

chartreuse

#7FFF00

127,255,0

chocolate

#D2691E

210,105,30

coral

#FF7F50

255,127,80

cornflower

#6495ED

100,149,237

cornsilk

#FFF8DC

255,248,220

crimson

#DC143C

220,20,60

cyan

#00FFFF

0,255,255

dark_blue

#00008B

0,0,139

dark_cyan

#008B8B

0,139,139

dark_goldenrod

#B8860B

184,134,11

dark_gray

#A9A9A9

169,169,169

dark_grey

#A9A9A9

169,169,169

dark_green

#006400

0,100,0

dark_khaki

#BDB76B

189,183,107

dark_magenta

#8B008B

139,0,139

dark_olive_green

#556B2F

85,107,47

dark_orange

#FF8C00

255,140,0

dark_orchid

#9932CC

153,50,204

dark_red

#8B0000

139,0,0

dark_salmon

#E9967A

233,150,122

dark_sea_green

#8FBC8F

143,188,143

dark_slate_blue

#483D8B

72,61,139

dark_slate_gray

#2F4F4F

47,79,79

dark_slate_grey

#2F4F4F

47,79,79

dark_turquoise

#00CED1

0,206,209

dark_violet

#9400D3

148,0,211

deep_pink

#FF1493

255,20,147

deep_sky_blue

#00BFFF

0,191,255

dim_gray

#696969

105,105,105

dim_grey

#696969

105,105,105

dodger_blue

#1E90FF

30,144,255

firebrick

#B22222

178,34,34

floral_white

#FFFAF0

255,250,240

forest_green

#228B22

34,139,34

fuchsia

#FF00FF

255,0,255

gainsboro

#DCDCDC

220,220,220

ghost_white

#F8F8FF

248,248,255

gold

#FFD700

255,215,0

goldenrod

#DAA520

218,165,32

gray

#7F7F7F

127,127,127

grey

#7F7F7F

127,127,127

green

#007F00

0,127,0

green_yellow

#ADFF2F

173,255,47

gray

#808080

128,128,128

honeydew

#F0FFF0

240,255,240

hot_pink

#FF69B4

255,105,180

indian_red

#CD5C5C

205,92,92

indigo

#4B0082

75,0,130

ivory

#FFFFF0

255,255,240

khaki

#F0E68C

240,230,140

lavender

#E6E6FA

230,230,250

lavender_blush

#FFF0F5

255,240,245

lawn_green

#7CFC00

124,252,0

lemon_chiffon

#FFFACD

255,250,205

light_blue

#ADD8E6

173,216,230

light_coral

#F08080

240,128,128

light_cyan

#E0FFFF

224,255,255

light_goldenrod

#FAFAD2

250,250,210

light_gray

#D3D3D3

211,211,211

light_green

#90EE90

144,238,144

light_gray

#D3D3D3

211,211,211

light_grey

#D3D3D3

211,211,211

light_pink

#FFB6C1

255,182,193

light_salmon

#FFA07A

255,160,122

light_sea_green

#20B2AA

32,178,170

light_sky_blue

#87CEFA

135,206,250

light_slate_gray

#778899

119,136,153

light_slate_grey

#778899

119,136,153

light_steel_blue

#B0C4DE

176,196,222

light_yellow

#FFFFE0

255,255,224

lime

#00FF00

0,255,0

lime_green

#32CD32

50,205,50

linen

#FAF0E6

250,240,230

magenta

#FF00FF

255,0,255

maroon

#7F0000

127,0,0

medium_aquamarine

#66CDAA

102,205,170

medium_blue

#0000CD

0,0,205

medium_orchid

#BA55D3

186,85,211

medium_purple

#9370DB

147,112,219

medium_sea_green

#3CB371

60,179,113

medium_slate_blue

#7B68EE

123,104,238

medium_spring_green

#00FA9A

0,250,154

medium_turquoise

#48D1CC

72,209,204

medium_violet_red

#C71585

199,21,133

midnight_blue

#191970

25,25,112

mint_cream

#F5FFFA

245,255,250

misty_rose

#FFE4E1

255,228,225

moccasin

#FFE4B5

255,228,181

navajo_white

#FFDEAD

255,222,173

navy

#000080

0,0,128

old_lace

#FDF5E6

253,245,230

olive

#808000

128,128,0

olive_drab

#6B8E23

107,142,35

orange

#FFA500

255,165,0

orange_red

#FF4500

255,69,0

orchid

#DA70D6

218,112,214

pale_goldenrod

#EEE8AA

238,232,170

pale_green

#98FB98

152,251,152

pale_turquoise

#AFEEEE

175,238,238

pale_violet_red

#DB7093

219,112,147

papaya_whip

#FFEFD5

255,239,213

peach_puff

#FFDAB9

255,218,185

peru

#CD853F

205,133,63

pink

#FFC0CB

255,192,203

plum

#DDA0DD

221,160,221

powder_blue

#B0E0E6

176,224,230

purple

#7F007F

127,0,127

red

#FF0000

255,0,0

rosy_brown

#BC8F8F

188,143,143

royal_blue

#4169E1

65,105,225

saddle_brown

#8B4513

139,69,19

salmon

#FA8072

250,128,114

sandy_brown

#F4A460

244,164,96

sea_green

#2E8B57

46,139,87

seashell

#FFF5EE

255,245,238

sienna

#A0522D

160,82,45

silver

#C0C0C0

192,192,192

sky_blue

#87CEEB

135,206,235

slate_blue

#6A5ACD

106,90,205

slate_gray

#708090

112,128,144

slate_grey

#708090

112,128,144

snow

#FFFAFA

255,250,250

spring_green

#00FF7F

0,255,127

steel_blue

#4682B4

70,130,180

tan

#D2B48C

210,180,140

teal

#008080

0,128,128

thistle

#D8BFD8

216,191,216

tomato

#FF6347

255,99,71

turquoise

#40E0D0

64,224,208

violet

#EE82EE

238,130,238

wheat

#F5DEB3

245,222,179

white

#FFFFFF

255,255,255

white_smoke

#F5F5F5

245,245,245

yellow

#FFFF00

255,255,0

yellow_green

#9ACD32

154,205,50

The following image displays the colors described in the table above.

!CONST

A read-only structure variable that contains physical constants. For example:

HELP, !CONST.C, /FULL

IDL prints:

<Expression>    DOUBLE    =        299792458.00000000

The following table lists !CONST names and values. All values are in double precision and are given in Meter-Kilogram-Second (MKS) units. The values for fundamental physical constants are taken from the Committee on Data for Science and Technology, "2010 CODATA Recommended Values," available from the Physical Measurement Laboratory at the National Institute of Standards and Technology (NIST). The values for au, G, M_earth, M_sun, parsec, and R_earth are derived from the IAU 2009 System of Astronomical Constants, based upon the Barycentric Coordinate Time (TCB) values. The light-year distance was computed using the speed of light during 1 Julian year of 365.25 days*86400 seconds. The value for ∏ (pi) was computed using Buffon's needle method with approximately 253 tosses of 4 cm toothpicks onto parallel lines.

Name

Description

Value

alpha

Fine structure constant

7.2973525698 x 10-3

au

astronomical unit

1.49597870700 x 1011m

c

Speed of light in a vacuum

299792458 m/s

DtoR

Degrees to radians

0.0174532925199433

e

elementary charge

1.602176565 x 10-19 C

eps0

electric vacuum permittivity

8.854187817 x 10-12 F/m

euler

Euler's number

2.7182818284590452

F

Faraday constant NAe

96485.3365 C/mol

G

Gravitation constant

6.67384 x 10-11 m3/kg/s2

gn

Earth standard gravity

9.80665 m/s2

h

Planck constant

6.62606957 x 10-34 J s

hbar

h/(2∏)

1.054571726 x 10-34 J s

i

Imaginary number

dcomplex(0,1)

k

Boltzmann constant R/NA

1.3806488 x 10-23 J/K

ly

Light-year distance

9.460730472580800 x 1015 m

M_earth

Earth mass

5.972186390 x 1024 kg

M_sun

Sun mass

1.98841586057 x 1030 kg

me

electron mass

9.10938291 x 10-31 kg

mn

neutron mass

1.674927351 x 10-27 kg

mp

proton mass

1.672621777 x 10-27 kg

mu0

magnetic vacuum permeability

12.566370614 x 10-7 N/A2

n0

Loschmidt's number NAp0/(RT0)

2.6867805 x 1025 m-3

Na

Avogadro constant NA

6.02214129e23 mol-1

p0

Standard atmosphere

101325 Pa

parsec

Parsec distance

3.0856775814671912 x 1016 m

phi

golden ratio

1.6180339887498948

pi

∏

3.1415926535897932

R

molar gas constant

8.3144621 J/mol/K

R_earth

Earth radius (spherical)

6378136.6 m

re

classical electron radius

2.8179403267 x 10-15 m

RtoD

Radians to degrees

57.295779513082323

rydberg

Rydberg constant R∞

10973731.568539 m-1

sigma

Stefan-Boltzmann constant

5.670373 x 10-8 W/m2/K4

T0

Standard temperature

273.15 K

u

unified atomic mass unit

1.660538921 x 10-27 kg

Vm

Molar volume, ideal gas at STP

22.413968 x 10-3 m3/mol

!DPI

A read-only variable containing the double-precision value of ∏ (pi).

!DTOR

A read-only variable containing the floating-point value used to convert degrees to radians (∏/180 @ 0.01745).

!FALSE

A read-only variable containing the boolean value of false (0).

!MAP

A variable containing the information needed to effect coordinate conversions between points of latitude and longitude and map coordinates. The values in this structure are established by the MAP_SET procedure; the user should not change them directly.

The !MAP structure contains the following fields:

A

A double-precision floating point integer specifying the equatorial radius or semimajor axis in meters. (The default is 6370997 m.)

COSO

A double-precision floating point integer specifying the cosine of the center latitude in degrees.

COSR

A double-precision floating point integer specifying the cosine of the rotation angle.

E2

A double-precision floating point integer specifying the eccentricity squared of the ellipsoid.

FILL_METHOD

A long-integer variable that indicates the fill order. True (1) fills counterclockwise, False (0) fills starting with the closest polygons.

LL_BOX

A four-element, double-precision floating point array that specifies the map range in degrees (latmin, lonmin, latmax, lonmax).

P

A 16-element, double-precision floating point array indicating additional projection parameters.

P0LAT

A double-precision floating point integer specifying the center latitude in degrees.

P0LON

A double-precision floating point integer specifying the center longitude in degrees.

PIPELINE

A four by four, double-precision floating point array specifying the graphics clipping and splitting pipeline.

POLE

A seven-element, double-precision floating point array that indicates Pole location parameters. Longitude and latitude are given in radians, sine and cosine of polar latitude, and X,Y,Z coordinates.

PROJECTION

A long-integer variable indicating the projection number (for GCTP only).

ROTATION

A double-precision floating point integer specifying the rotation angle in degrees, clockwise of pole.

SEGMENT_LENGTH

A double-precision floating point integer specifying the maximum segment length.

SIMPLE

A long-integer variable indicating the projection number. For IDL projections, True (1) indicates the projection is centered on the Equator or no rotation. For GCTP, this number is the GCTP projection number.

SINO

A double-precision floating point integer specifying the sine of the center latitude.

SINR

A double-precision floating point integer specifying the sine of the rotation angle.

U0

A double-precision floating point integer specifying the center longitude in radians.

UP_FLAGS

A double-precision floating point integer that sets the user projection flags (currently unused).

UP_NAME

A string that specifies the projection name (used only for GCTP).

UV

A two-element, double-precision floating point array specifying the U,V coordinates of the last point.

UV_BOX

A four-element, double-precision floating point array that contains the map range in meters (xmin, ymin, xmax, ymax).

Note: The UV_BOX values are not just the LL_BOX values converted into the map projection. For most projections, converting the UV_BOX values back into lat/lon coordinates will not return the same values as LL_BOX. Instead, the UV_BOX gives the approximate maximum extent of the map projection.

V0

A double-precision floating point integer specifying the center latitude in radians.

!NULL

A variable of type Undefined. The value of !NULL can be:

  • Assigned to another variable: var = !NULL
  • Used to test whether a variable exists: IF var NE !NULL THEN...

In addition, variables with the value !NULL are ignored when added to an array. That is: 

array = [1, !NULL, 2, !NULL, 3]

is the same as 

array = [1, 2, 3]

See The Null Variable for additional information.

!PI

A read-only variable containing the single-precision value of ∏ (pi).

!RADEG

A read-only variable containing the floating-point value used to convert radians to degrees (180/∏ @ 57.2958).

!TRUE

A read-only variable containing the boolean value of true (1).

!VALUES

A read-only variable containing the IEEE single- and double-precision floating-point values Infinity and NaN (Not A Number). !VALUES is a structure variable with the following fields:

** Structure !VALUES, 4 tags, length=24:
   F_INFINITY      FLOAT          Infinity
   F_NAN           FLOAT               NaN
   D_INFINITY      DOUBLE         Infinity
   D_NAN           DOUBLE              NaN

where the meaning of the fields as follows:

F_INFINITY

The single-precision floating point value Infinity.

F_NAN

The single-precision floating point value NaN (Not a Number).

D_INFINITY

The double-precision floating point value Infinity.

D_NAN

The double-precision floating point value NaN (Not a Number).