# t.select.1grass man page

**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** [**-s**] [**type**=*name*] **expression**=*expression* [**--help**] [**--verbose**] [**--quiet**] [**--ui**]

### Flags

**-s**- Activate spatial topology
**--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 differnt 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 ealier 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: 2015-01-25 18:56:33 +0100 (Sun, 25 Jan 2015) $*

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