where result is the name of a raster map layer to contain the result of the calculation and expression is any legal arithmetic expression involving existing raster map layers, integer or floating point constants, and functions known to the calculator. Parentheses are allowed in the expression and may be nested to any depth. result will be created in the user's current mapset.
The formula entered to r.mapcalc by the user is recorded both in the result map TITLE (which appears in the category file for result) and in the history file for result.
Some characters have special meaning to the command shell. If the user is entering input to r.mapcalc on the command line, expressions should be enclosed within single quotes. See NOTES, below.
Operator Meaning Type Precedence ----------------------------------------------------------------------- % modulus (remainder upon division) Arithmetic 4 / division Arithmetic 4 * multiplication Arithmetic 4 + addition Arithmetic 3 - subtraction Arithmetic 3 == equal Logical 2 != not equal Logical 2 > greater than Logical 2 >= greater than or equal Logical 2 < less than Logical 2 <= less than or equal Logical 2 && and Logical 1 || or Logical 1 # color separator operator Arithmetic
The operators are applied from left to right, with those of higher precedence applied before those with lower precedence. Division by 0 and modulus by 0 are acceptable and give a 0 result. The logical operators give a 1 result if the comparison is true, 0 otherwise.
Most GRASS raster map layers meet this naming convention. However, if a raster map layer has a name which conflicts with the above rule, it should be quoted. For example, the expression
would be interpreted as: x equals a minus b, whereas
would be interpreted as: x equals the raster map layer named a-b
would create x filled with the number 3107, while
would copy the raster map layer 3107 to the raster map layer x.
Quotes are not required unless the raster map layer names look like numbers or contain operators, OR unless the program is run non-interactively. Examples given here assume the program is run interactively. See NOTES, below.
r.mapcalc will look for the raster map layers according to the user's current mapset search path. It is possible to override the search path and specify the mapset from which to select the raster map layer. This is done by specifying the raster map layer name in the form:
For example, the following is a legal expression:
The mapset specified does not have to be in the mapset search path. (This method of overriding the mapset search path is common to all GRASS commands, not just r.mapcalc.)
For example, suppose that the raster map layer soil.ph (representing soil pH values) has a category file with labels as follows:
cat label ------------------ 0 no data 1 1.4 2 2.4 3 3.5 4 5.8 5 7.2 6 8.8 7 9.4
Then the expression:
would produce a result with category values 0, 14, 24, 35, 58, 72, 88 and 94.
Note that this operator may only be applied to raster map layers and produces a floating point value in the expression. Also the category label must start with a valid number. Missing labels, or labels that do not start with a number will (silently) produce a 0 value for that category.
The # operator can be used to either convert map category values to their grey scale equivalents or to extract the red, green, or blue components of a raster map layer into separate raster map layers.
converts each category value in map to a value in the range 0-255 which represents the grey scale level implied by the color for the category. If the map has a grey scale color table, then the grey level is what #map evaluates to. Otherwise, it is computed as:
The # operator has three other forms: r#map, g#map, b#map. These extract the red, green, or blue components in the named raster map, respectively. The GRASS shell script blend.sh extracts each of these components from two raster map layers, and combines them by a user-specified percentage. These forms allow color separates to be made. For example, to extract the red component from map and store it in the new 0-255 map layer red, the user could type:
To assign this map grey colors type:
To assign this map red colors type:
function description type --------------------------------------------------------------------------- abs(x) return absolute value of x * atan(x) inverse tangent of x (result is in degrees) F cos(x) cosine of x (x is in degrees) F eval([x,y,...,]z) evaluate values of listed expr, pass results to z exp(x) exponential function of x F exp(x,y) x to the power y F float(x) convert x to floating point F if decision options: * if(x) 1 if x not zero, 0 otherwise if(x,a) a if x not zero, 0 otherwise if(x,a,b) a if x not zero, b otherwise if(x,a,b,c) a if x > 0, b if x is zero, c if x < 0 int(x) convert x to integer [ truncates ] I isnull(x) check if x = NULL log(x) natural log of x F log(x,b) log of x base b F max(x,y[,z...]) largest value of those listed * median(x,y[,z...]) median value of those listed * min(x,y[,z...]) smallest value of those listed * mode(x,y[,z...]) mode value of those listed * round(x) round x to nearest integer I sin(x) sine of x (x is in degrees) F sqrt(x) square root of x F tan(x) tangent of x (x is in degrees) F
Internal variables: row() current row of moving window col() current col of moving window x() current x-coordinate of moving window y() current y-coordinate of moving window ewres() current east-west resolution nsres() current north-south resolution null() NULL value
2.3 12. .81Floating point values in the expression are handled in a special way. With arithmetic and logical operators, if either operand is float, the other is converted to float and the result of the operation is float. This means, in particular that division of integers results in a (truncated) integer, while division of floats results in an accurate floating point value. With functions of type * (see table above), the result is float if any argument is float, integer otherwise.
Note: If you calculate with integer numbers, the resulting map will be integer. If you want to get a float result, add the decimal point to integer number(s).
ave = (a + b)/2To form a weighted average:
ave = (5*a + 3*b)/8.0To produce a binary representation of the raster map layer a so that category 0 remains 0 and all other categories become 1:
mask = a/aThis could also be accomplished by:
mask = if(a)To mask raster map layer b by raster map layer a:
result = if(a,b)To change all map values below 5 to NULL:
newmap = if(map<5, null(), 5)
new_elevation = elevation + 10Since a category value of 0 is used in GRASS for locations which do not exist in the raster map layer, the new raster map layer will contain the category value 10 in the locations that did not exist in the original elevation. Therefore, in this example, it is essential that the boundaries of the geographic region be set to agree with the cell header.
However, if there is a current mask, then the resultant raster map layer is masked when it is written; i.e., 0 category values in the mask force zero values in the output.
* ( ) > & |
It is advisable to put single quotes around the expression; e.g.:
result = 'elevation * 2'Without the quotes, the *, which has special meaning to the UNIX shell, would be altered and r.mapcalc would see something other than the *.
If the input comes directly from the keyboard and the result raster map layer exists, the user will be asked if it can be overwritten. Otherwise, the result raster map layer will automatically be overwritten if it exists.
Quoting result is not allowed. However, it is never necessary to quote result since it is always taken to be a raster map layer name.
For formulas that the user enters from standard input (rather than from the command line), a line continuation feature now exists. If the user adds \e to the end of an input line, r.mapcalc assumes that the formula being entered by the user continues on to the next input line. There is no limit to the possible number of input lines or to the length of a formula.
If the r.mapcalc formula entered by the user is very long, the map TITLE will contain only some of it, but most (if not all) of the formula will be placed into the history file for the result map.
When the user enters input to r.mapcalc non-interactively on the command line, the program will not warn the user not to overwrite existing map layers. Users should therefore take care to assign program outputs raster file names that do not yet exist in their current mapsets.
Error messages produced by r.mapcalc are almost useless. In future, r.mapcalc should make some attempt to point the user to the offending section of the equation, e.g.:
x = a * b ++ c ERROR: somewhere in line 1: ... b ++ c ...
Currently, there is no comment mechanism in r.mapcalc. Perhaps adding a capability that would cause the entire line to be ignored when the user inserted a # at the start of a line as if it were not present, would do the trick.
The function should require the user to type "end" or "exit" instead of simply a blank line. This would make separation of multiple scripts separable by white space.
Grey scale conversion is based on the C.I.E. x,y,z system where y represents luminance. See "Fundamentals of Digital Image Processing," by Anil K. Jain (Prentice Hall, NJ, 1989; p 67).