Map Algebra
How does Map Algebra work?
* Map Algebra uses math-like expressions
containing operators and functions with raster
data. Map Algebra operators, which are
relational, Boolean, logical, combinatorial, and
bitwise, work with one or more inputs to develop
new values. Functions perform specialized
tasks, such as computing slope from elevation,
and they usually return numeric values.
* You don't have to be a programmer to know how
to use operators and functions effectively, you
just have to be taught how to use them.
Map Algebra
* Map Algebra is an analysis
language based loosely on the
map algebra concepts presented
by Dr. Dana Tomlin in his book
entitled Geographic Information
Systems and Cartographic
Modeling (Tomlin, 1990). Map
Algebra is a high-level
computational language used for
performing cartographic spatial
analysis using raster data.
* Simply put, Map Algebra is math
applied to rasters, a practice thaťs
possible because rasters are
geographically referenced arrays
of numbers. If you stack rasters on
top of each other as if they were a
mathematical sandwich, you can
perform simple arithmetic to the
most sophisticated algorithms with
them.
You probably know more about Map
Algebra operators than you think.
Map Algebra operators are generally
the same operators found on
scientific calculators. The operators
used most often (arithmetic,
relational, Boolean, and logical) are
also the simplest. Two less
commonly used operators are
combinatorial and bitwise.
Map Algebra Arithmetic operators
* Arithmetic operators allow
addition, subtraction,
multiplication, and
division. For example,
three rasters measuring
three different types of
fire risk could be added to
create an overall risk
analysis raster.
* Arithmetic operators can
also be used to convert
values from one
measurement to another
(e.g., feet x 0.3048 =
meters).
43
21
x 2
86
42
=
Map Algebra Relational operators
* Relational operators allow
you to build logical tests,
returning values of true (1)
and false (0). For example,
this type of operator can be
used to find vegetation
"equal to" Sierra-type mixed
coniferous forest.
* You can ether use the
Relational Operator (=,<,>)
of the Abbreviation (EQ,LT,
GT)
43
21
EQ 4
10
00
=
Map Algebra Boolean operators
* Boolean operators such
as "and", "or", and "not"
allow you to chain logical
tests. Like relational
operators, Boolean
operators return values of
true and false. For
example, you could find
all slopes that are
"greater-than" 45 degrees
"and" that have an
elevation that is "greaterthan"
5000 meters.
4655
2165
Slope
40007500
65005500
Elevation
AND
01
01
=
Map Algebra Logical operators
* The logical operators DIFF, IN, and
OVER, also allow you to build
logical tests on a cell-by-cell basis,
but are implemented with specific
rules.
­ A DIFF B: If a cell value in raster A
and raster B are different, the cell
value in raster A is returned. If the
cell values are the same, the value
zero is returned.
­ A IN {value list): If a cell value in
raster A is in the value list, the cell
value in raster A is returned.
Otherwise, NoData is returned.
­ A OVER B: If a cell value in raster
A is not equal to zero, the cell
value is raster A is returned.
Otherwise, the cell value in raster
B is returned.
43
21
53
25
A B
DIFF
40
01
=
Understanding logical values
Logical values simply keep
track of the values, true and
false. In Map Algebra, any
non-zero input value is
considered to be a logical
true, and zero is considered
a logical false. Some Map
Algebra operators and
functions evaluate input cell
values and return logical 1
values (true) and logical 0
values (false). The relational
and Boolean operators all
return logical values.
In this example, cells are evaluated as to whether their
value is greater than 15. The value 1, or logical true, is
assigned to those cells that are greater than 15. The
value 0, or logical false, is assigned to all other cells.
Map Algebra Combinatorial
operators
* Combinatorial operators
combine the attributes of
multiple input rasters.
These operators find all
unique combinations of
values and assign a
unique ID to each, which
is then returned to the
output grid. The output
VAT will have the Value
fields from all the input
grids.
CAND Operator
Map Algebra Functions
* The term "functions" might sound like programmers' speak,
but whether you realize it or not, you've probably already
used them. In the previous module, you derived hillshade,
slope, and aspect rasters using choices on the Surface
Analysis menu. These choices are simply dialogs that
implement Map Algebra functions--programs that perform
specific tasks, such as calculating slope.
* While you can do a lot through the Spatial Analyst user
interface, many more functions are available through Map
Algebra. Functions are the major element of the Map
Algebra language, and there are over a hundred of them.
* The ArcGIS online help system provides a listing of all of the
functions and a help topic for each. Here is where you can
find a description of what the function does, as well as its
syntax.
* Most of the functionality available from the Spatial Analyst
toolbar are also available as Map Algebra functions.
Map Algebra Functions Examples
* ASPECT: identifies the direction of maximum rate of change in z value from each cell.
* BOUNDARYCLEAN: smoothes the boundary between zones by expanding and shrinking the boundary.
* CON: performs one or more conditional if/else evaluations.
* EQUALTO: evaluates, on a cell-by-cell basis, the number of times in an argument list that the input grid
values are equal to the value specified by the first argument.
* GREATERTHAN: evaluates, on a cell-by-cell basis, the number of times in an argument list that the input
grid values are greater than the value specified by the first argument.
* HILLSHADE: creates a shaded relief grid from a grid by considering the sun illumination angle and
shadows.
* INT: converts input floating-point values to integer values through truncation.
* ISNULL: returns `1' if the input value is NODATA, and `0' if it is not.
* LESSTHAN: evaluates, on a cell-by-cell basis, the number of times in an argument list that the input grid
values are less than the value specified by the first argument.
* MEAN: uses multiple input grids to determine the mean value on a cell-by-cell basis.
* MERGE: merges multiple, possibly non-adjacent input grids into a single grid based upon order of input.
* MOSAIC: merges multiple adjacent continuous grids and performs interpolation in the overlapping areas.
* NIBBLE: replaces areas in a grid corresponding to a mask, with the values of the nearest neighbors.
* REGIONGROUP: records for each cell in the output the identity of the connected region to which it belongs.
A unique number is assigned to each region.
* SETNULL: returns NODATA if the evaluation of the input condition is `TRUE'; if it `FALSE', returns the value
specified by the second input argument.
* SLICE: `slices' (or changes) a range of values of the input cells by specified ranges, zones of equal area, or
zones with equal intervals.
* SLOPE: identifies the rate of maximum change in z value from each cell.
* ZONALAREA: calculates the area of each zone in the input grid.
Building Map Algebra Expressions
* You build expressions in the Raster Calculator by typing
directly into the expression box, clicking layers in the
Layers list, and clicking buttons on the keypad. When
you click an element it is added to the expression in the
expression box.
* If you specify an output grid name in your expression
(e.g., NewElev = ), a permanent grid will be created in
your working directory. If you do not specify an output
name, a grid will be written to your working directory, but
the grid will be temporary.
* Output grids are either integer or floating point,
depending on the expression
Raster Calculator
Building Expressions Help
Building Map Algebra Expressions
Building Map Algebra Expressions
Building Map Algebra Functions
Building Map Algebra Functions
Working with NoData
* Similar to logical values,
NoData values also
influence the evaluation of
expressions. The NoData
value is the only non-zero
value that is not interpreted
as a true condition.
* This special value indicates
that there is no information
associated with the cell. In
general, a Map Algebra
expression will return
NoData for a cell if any of
the corresponding input
cells have NoData.
In general, Map Algebra expressions assign the value
NoData to the output cell if any corresponding input cell
is NoData.
Summary
* While you can do a lot from the Spatial Analyst user interface, most
of the power of the Spatial Analyst is found within Map Algebra,
which you can access through the Raster Calculator dialog. You can
construct a Map Algebra expression using the dialog buttons and
controls, or by typing directly into the expression box. The
expression processes when you click Evaluate.
* Map Algebra is the analysis language for Spatial Analyst. It has a
simple syntax similar to any algebra. The inputs to an expression
may be as simple as a single raster layer or shapefile, or as complex
as a compound expression using many operators, functions, and
input data.
* With Map Algebra, not only can you access functions not available
in the user interface, you can also build complex expressions and
process them as a single command. For instance, you can calculate
the sine of an input raster dataset or raster layer and add that to two
other input raster datasets or raster layers. Like all languages, Map
Algebra is defined by rules. By understanding the basic rules, you
will be able to use Spatial Analyst in new ways.