Provided by: grass-doc_7.4.0-1_all 
NAME
t.select - Select maps from space time datasets by topological relationships to other
space time datasets using temporal algebra.
KEYWORDS
temporal, metadata, time
SYNOPSIS
t.select
t.select --help
t.select [-sd] [type=name] expression=expression [--help] [--verbose] [--quiet]
[--ui]
Flags:
-s
Check the spatial topology of temporally related maps and select only spatially
related maps
-d
Perform a dry run, compute all dependencies and module calls but don’t run them
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
type=name
Type of the input space time dataset
Options: strds, stvds, str3ds
Default: strds
expression=expression [required]
The temporal mapcalc expression
DESCRIPTION
t.select performs selection of maps that are registered in space time datasets using
temporal algebra.
PROGRAM USE
The module expects an expression as input parameter in the following form:
"result = expression"
The statement structure is similar to r.mapcalc, see r.mapcalc. Where result represents
the name of a space time dataset (STDS)that will contain the result of the calculation
that is given as expression on the right side of the equality sign. These expression can
be any valid or nested combination of temporal operations and functions that are provided
by the temporal algebra.
The temporal algebra works with space time datasets of any type (STRDS, STR3DS and STVDS).
The algebra provides methods for map selection from STDS based on their temporal
relations. It is also possible to temporally shift maps, to create temporal buffer and to
snap time instances to create a valid temporal topology. Furthermore expressions can be
nested and evaluated in conditional statements (if, else statements). Within if-statements
the algebra provides temporal variables like start time, end time, day of year, time
differences or number of maps per time interval to build up conditions. These operations
can be assigned to space time datasets or to the results of operations between space time
datasets.
The type of the input space time datasets must be defined with the input parameter type.
Possible options are STRDS, STVDS or STR3DS. The default is set to space time raster
datasets (STRDS).
As default, topological relationships between space time datasets will be evaluated only
temporal. Use the s flag to activate the additionally spatial topology evaluation.
The expression option must be passed as quoted expression, for example:
t.select expression="C = A : B"
Where C is the new space time raster dataset that will contain maps from A that are
selected by equal temporal relationships to the existing dataset B in this case.
TEMPORAL ALGEBRA
The temporal algebra provides a wide range of temporal operators and functions that will
be presented in the following section.
TEMPORAL RELATIONS
Several temporal topology relations between registered maps of space time datasets are
supported:
equals A ------
B ------
during A ----
B ------
contains A ------
B ----
starts A ----
B ------
started A ------
B ----
finishs A ----
B ------
finished A ------
B ----
precedes A ----
B ----
follows A ----
B ----
overlapped A ------
B ------
overlaps A ------
B ------
over booth overlaps and overlapped
The relations must be read as: A is related to B, like - A equals B - A is during B - A
contains B
Topological relations must be specified in {} parentheses.
TEMPORAL OPERATORS
The temporal algebra defines temporal operators that can be combined with other operators
to perform spatio-temporal operations. The temporal operators process the time instances
and intervals of two temporal related maps and calculate the result temporal extent by
five different possibilities.
LEFT REFERENCE l Use the time stamp of the left space time dataset
INTERSECTION i Intersection
DISJOINT UNION d Disjoint union
UNION u Union
RIGHT REFERENCE r Use the time stamp of the right space time dataset
TEMPORAL SELECTION
The temporal selection simply selects parts of a space time dataset without processing
raster or vector data. The algebra provides a selection operator : that selects parts of
a space time dataset that are temporally equal to parts of a second one by default. The
following expression
C = A : B
means: Select all parts of space time dataset A that are equal to B and store it in space
time dataset C. The parts are time stamped maps.
In addition the inverse selection operator !: is defined as the complement of the
selection operator, hence the following expression
C = A !: B
means: select all parts of space time time dataset A that are not equal to B and store it
in space time dataset (STDS) C.
To select parts of a STDS by different topological relations to other STDS, the temporal
topology selection operator can be used. The operator consists of the temporal selection
operator, the topological relations, that must be separated by the logical OR operator |
and the temporal extent operator. All three parts are separated by comma and surrounded
by curly braces:
{"temporal selection operator", "topological relations", "temporal operator"}
Examples:
C = A {:, equals} B
C = A {!:, equals} B
We can now define arbitrary topological relations using the OR operator "|" to connect
them:
C = A {:,equals|during|overlaps} B
Select all parts of A that are equal to B, during B or overlaps B.
In addition we can define the temporal extent of the result STDS by adding the temporal
operator.
C = A {:, during,r} B
Select all parts of A that are during B and use the temporal extents from B for C.
The selection operator is implicitly contained in the temporal topology selection
operator, so that the following statements are exactly the same:
C = A : B
C = A {:} B
C = A {:,equal} B
C = A {:,equal,l} B
Same for the complementary selection:
C = A !: B
C = A {!:} B
C = A {!:,equal} B
C = A {!:,equal,l} B
CONDITIONAL STATEMENTS
Selection operations can be evaluated within conditional statements.
Note A and B can either be space time datasets or expressions. The temporal
relationship between the conditions and the conclusions can be defined at the
beginning of the if statement. The relationship between then and else conclusion
must be always equal.
if statement decision option temporal relations
if(if, then, else)
if(conditions, A) A if conditions are True; temporal topological relation between if and then is equal.
if(conditions, A, B) A if conditions are True, B otherwise; temporal topological relation between if, then and else is equal.
if(topologies, conditions, A) A if conditions are True; temporal topological relation between if and then is explicit specified by topologies.
if(topologies, conditions, A, B) A if conditions are True, B otherwise; temporal topological relation between if, then and else is explicit specified by topologies.
The conditions are comparison expressions that are used to evaluate space time datasets.
Specific values of temporal variables are compared by logical operators and evaluated for
each map of the STDS.
Important: The conditions are evaluated from left to right.
Logical operators
Symbol description
== equal
!= not equal
> greater than
>= greater than or equal
< less than
<= less than or equal
&& and
|| or
Temporal functions
The following temporal function are evaluated only for the STDS that must be given in
parenthesis.
td(A) Returns a list of time intervals of STDS A
start_time(A) Start time as HH::MM:SS
start_date(A) Start date as yyyy-mm-DD
start_datetime(A) Start datetime as yyyy-mm-DD HH:MM:SS
end_time(A) End time as HH:MM:SS
end_date(A) End date as yyyy-mm-DD
end_datetime(A) End datetime as yyyy-mm-DD HH:MM
start_doy(A) Day of year (doy) from the start time [1 - 366]
start_dow(A) Day of week (dow) from the start time [1 - 7], the start of the week is Monday == 1
start_year(A) The year of the start time [0 - 9999]
start_month(A) The month of the start time [1 - 12]
start_week(A) Week of year of the start time [1 - 54]
start_day(A) Day of month from the start time [1 - 31]
start_hour(A) The hour of the start time [0 - 23]
start_minute(A) The minute of the start time [0 - 59]
start_second(A) The second of the start time [0 - 59]
end_doy(A) Day of year (doy) from the end time [1 - 366]
end_dow(A) Day of week (dow) from the end time [1 - 7], the start of the week is Monday == 1
end_year(A) The year of the end time [0 - 9999]
end_month(A) The month of the end time [1 - 12]
end_week(A) Week of year of the end time [1 - 54]
end_day(A) Day of month from the start time [1 - 31]
end_hour(A) The hour of the end time [0 - 23]
end_minute(A) The minute of the end time [0 - 59]
end_second(A) The second of the end time [0 - 59]
Comparison operator
The conditions are comparison expressions that are used to evaluate space time datasets.
Specific values of temporal variables are compared by logical operators and evaluated for
each map of the STDS and the related maps. For complex relations the comparison operator
can be used to combine conditions:
The structure is similar to the select operator with the extension of an aggregation
operator: {"comparison operator", "topological relations", aggregation operator, "temporal
operator"}
This aggregation operator (| or &) define the behaviour if a map is related the more than
one map, e.g for the topological relations ’contains’. Should all (&) conditions for the
related maps be true or is it sufficient to have any (|) condition that is true. The
resulting boolean value is then compared to the first condition by the comparison operator
(|| or &&). As default the aggregation operator is related to the comparison operator:
Comparison operator -> aggregation operator:
|| -> | and && -> &
Examples:
Condition 1 {||, equal, r} Condition 2
Condition 1 {&&, equal|during, l} Condition 2
Condition 1 {&&, equal|contains, |, l} Condition 2
Condition 1 {&&, equal|during, l} Condition 2 && Condition 3
Condition 1 {&&, equal|during, l} Condition 2 {&&,contains, |, r} Condition 3
Hash operator
Additionally the number of maps in intervals can be computed and used in conditional
statements with the hash (#) operator.
A{#, contains}B
This expression computes the number of maps from space time dataset B which are during the
time intervals of maps from space time dataset A.
A list of integers (scalars) corresponding to the maps of A that contain maps from B will
be returned.
C = if({equal}, A {#, contains} B > 2, A {:, contains} B)
This expression selects all maps from A that temporally contains at least 2 maps from B
and stores them in space time dataset C. The leading equal statement in the if condition
specifies the temporal relation between the if and then part of the if expression. This is
very important, so we do not need to specify a global time reference (a space time
dataset) for temporal processing.
Furthermore the temporal algebra allows temporal buffering, shifting and snapping with the
functions buff_t(), tshift() and tsnap() respectively.
buff_t(A, size) Buffer STDS A with granule ("1 month" or 5)
tshift(A, size) Shift STDS A with granule ("1 month" or 5)
tsnap(A) Snap time instances and intervals of STDS A
Single map with temporal extent
The temporal algebra can also handle single maps with time stamps in the map function.
tmap()
For example:
C = A {:,during} tmap(event)
This statement select all maps from space time data set A that are during the temporal
extent of single map ’event’
Examples
Select all maps from space time dataset A which have equal time stamps with space time
dataset B and C and are earlier that Jan. 1. 2005 and store them in space time dataset D.
D = if(start_date(A) < "2005-01-01", A : B : C)
Select all maps from space time dataset A which contains more than three maps of space
time dataset B, else select maps from C with time stamps that are not equal to A and store
them in space time dataset D.
D = if(A {#, contains} B > 3, A {:, contains} B, C)
Select all maps from space time dataset B which are during the temporal buffered space
time dataset A with a map interval of three days, else select maps from C and store them
in space time dataset D.
D = if(contains, td(buff_t(A, "1 days")) == 3, B, C)
SEE ALSO
r.mapcalc
REFERENCES
PLY(Python-Lex-Yacc)
AUTHORS
Thomas Leppelt, Sören Gebbert, Thünen Institute of Climate-Smart Agriculture
Last changed: $Date: 2016-11-14 00:05:32 +0100 (Mon, 14 Nov 2016) $
SOURCE CODE
Available at: t.select source code (history)
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