## table of contents

t.select(1grass) | GRASS GIS User's Manual | t.select(1grass) |

# 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:¶

## 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 !: Bmeans: 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} BWe can now define arbitrary topological relations using the OR operator "|" to connect them:

C = A {:,equals|during|overlaps} BSelect 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} BSelect 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} BSame 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

# SOURCE CODE¶

Available at: t.select source code (history)

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