---
name: t.select
description: Select maps from space time datasets by topological relationships to other space time datasets using temporal algebra.
keywords: [ temporal, metadata, time ]
---

# t.select

Select maps from space time datasets by topological relationships to other space time datasets using temporal algebra.

=== "Command line"

    **t.select**
    [**-sd**]
    [**type**=*name*]
    **expression**=*expression*
    [**--verbose**]
    [**--quiet**]
    [**--qq**]
    [**--ui**]

    Example:

    ```sh
    t.select expression=expression
    ```

=== "Python (grass.tools)"

    *grass.tools.Tools.t_select*(**type**=*"strds"*,
        **expression**,
        **flags**=*None*,
        **verbose**=*None*,
        **quiet**=*None*,
        **superquiet**=*None*)

    Example:

    ```python
    tools = Tools()
    tools.t_select(expression="expression")
    ```

    This grass.tools API is experimental in version 8.5 and expected to be stable in version 8.6.

=== "Python (grass.script)"

    *grass.script.run_command*("***t.select***",
        **type**=*"strds"*,
        **expression**,
        **flags**=*None*,
        **verbose**=*None*,
        **quiet**=*None*,
        **superquiet**=*None*)

    Example:

    ```python
    gs.run_command("t.select", expression="expression")
    ```

## Parameters

=== "Command line"

    **type**=*name*  
    &nbsp;&nbsp;&nbsp;&nbsp;Type of the input space time dataset  
    &nbsp;&nbsp;&nbsp;&nbsp;Allowed values: *strds, stvds, str3ds*  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *strds*  
    **expression**=*expression* **[required]**  
    &nbsp;&nbsp;&nbsp;&nbsp;The temporal mapcalc expression  
    **-s**  
    &nbsp;&nbsp;&nbsp;&nbsp;Check the spatial topology of temporally related maps and select only spatially related maps  
    **-d**  
    &nbsp;&nbsp;&nbsp;&nbsp;Perform a dry run, compute all dependencies and module calls but don't run them  
    **--help**  
    &nbsp;&nbsp;&nbsp;&nbsp;Print usage summary  
    **--verbose**  
    &nbsp;&nbsp;&nbsp;&nbsp;Verbose module output  
    **--quiet**  
    &nbsp;&nbsp;&nbsp;&nbsp;Quiet module output  
    **--qq**  
    &nbsp;&nbsp;&nbsp;&nbsp;Very quiet module output  
    **--ui**  
    &nbsp;&nbsp;&nbsp;&nbsp;Force launching GUI dialog

=== "Python (grass.tools)"

    **type** : str, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Type of the input space time dataset  
    &nbsp;&nbsp;&nbsp;&nbsp;Used as: *name*  
    &nbsp;&nbsp;&nbsp;&nbsp;Allowed values: *strds, stvds, str3ds*  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *strds*  
    **expression** : str, *required*  
    &nbsp;&nbsp;&nbsp;&nbsp;The temporal mapcalc expression  
    &nbsp;&nbsp;&nbsp;&nbsp;Used as: *expression*  
    **flags** : str, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Allowed values: *s*, *d*  
    &nbsp;&nbsp;&nbsp;&nbsp;**s**  
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Check the spatial topology of temporally related maps and select only spatially related maps  
    &nbsp;&nbsp;&nbsp;&nbsp;**d**  
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Perform a dry run, compute all dependencies and module calls but don't run them  
    **verbose** : bool, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Verbose module output  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *None*  
    **quiet** : bool, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Quiet module output  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *None*  
    **superquiet** : bool, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Very quiet module output  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *None*  

    Returns:

    **result** : grass.tools.support.ToolResult | None  
    If the tool produces text as standard output, a *ToolResult* object will be returned. Otherwise, `None` will be returned.

    Raises:

    *grass.tools.ToolError*: When the tool ended with an error.

=== "Python (grass.script)"

    **type** : str, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Type of the input space time dataset  
    &nbsp;&nbsp;&nbsp;&nbsp;Used as: *name*  
    &nbsp;&nbsp;&nbsp;&nbsp;Allowed values: *strds, stvds, str3ds*  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *strds*  
    **expression** : str, *required*  
    &nbsp;&nbsp;&nbsp;&nbsp;The temporal mapcalc expression  
    &nbsp;&nbsp;&nbsp;&nbsp;Used as: *expression*  
    **flags** : str, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Allowed values: *s*, *d*  
    &nbsp;&nbsp;&nbsp;&nbsp;**s**  
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Check the spatial topology of temporally related maps and select only spatially related maps  
    &nbsp;&nbsp;&nbsp;&nbsp;**d**  
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Perform a dry run, compute all dependencies and module calls but don't run them  
    **verbose** : bool, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Verbose module output  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *None*  
    **quiet** : bool, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Quiet module output  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *None*  
    **superquiet** : bool, *optional*  
    &nbsp;&nbsp;&nbsp;&nbsp;Very quiet module output  
    &nbsp;&nbsp;&nbsp;&nbsp;Default: *None*

## 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](r.mapcalc.md). 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:  

```sh
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:  

```sh
equals            A ------
                  B ------

during            A  ----
                  B ------

contains          A ------
                  B  ----

starts            A ----
                  B ------

started           A ------
                  B ----

finishes          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.

```sh
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

```sh
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

```sh
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:

```sh
{"temporal selection operator", "topological relations", "temporal operator"}
```

Examples:

```sh
C = A {:, equals} B
C = A {!:, equals} B
```

We can now define arbitrary topological relations using the OR operator
"\|" to connect them:

```sh
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.

```sh
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:

```sh
C = A : B
C = A {:} B
C = A {:,equal} B
C = A {:,equal,l} B
```

Same for the complementary selection:

```sh
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.  

```sh
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

```sh
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.

```sh
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:

```sh
|| -> | and && -> &
```

Examples:

```sh
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.  

```sh
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.

```sh
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.

```sh
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.

```sh
tmap()
```

For example:

```sh
 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.

```sh
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.

```sh
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.

```sh
D = if(contains, td(buff_t(A, "1 days")) == 3, B, C)
```

## REFERENCES

[PLY(Python-Lex-Yacc)](https://www.dabeaz.com/ply/)

Gebbert, S., Leppelt, T., Pebesma, E., 2019. A topology based
spatio-temporal map algebra for big data analysis. Data 4, 86.
<https://doi.org/10.3390/data4020086>

## SEE ALSO

*[r.mapcalc](r.mapcalc.md)*

## AUTHORS

Thomas Leppelt, Sören Gebbert, Thünen Institute of Climate-Smart
Agriculture

## SOURCE CODE

Available at: [t.select source code](https://github.com/OSGeo/grass/tree/main/temporal/t.select)
([history](https://github.com/OSGeo/grass/commits/main/temporal/t.select))  
Latest change: Sunday Jul 20 18:06:47 2025 in commit [ad7f155](https://github.com/OSGeo/grass/commit/ad7f155d7ad744c74bde965cc51b9575862e08f4)
