Provided by: grass-doc_7.6.1-3_all bug

NAME

       t.vect.algebra   -  Apply  temporal  and  spatial operations on space time vector datasets
       using temporal vector algebra.

KEYWORDS

       temporal, algebra, vector, time

SYNOPSIS

       t.vect.algebra
       t.vect.algebra --help
       t.vect.algebra  [-s]  expression=expression   basename=basename    [--help]    [--verbose]
       [--quiet]  [--ui]

   Flags:
       -s
           Check  the  spatial  topology  of  temporally  related maps and process only spatially
           related maps

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       expression=expression [required]
           Spatio-temporal mapcalc expression

       basename=basename [required]
           Basename of the new generated output maps
           A numerical suffix separated by an underscore will be  attached  to  create  a  unique
           identifier

DESCRIPTION

       t.vect.algebra  performs  temporal  and spatial overlay and buffer functions on space time
       vector datasets (STVDS) by using the temporal vector algebra. New STVDS  can  be  created,
       which are expressions of existing STVDS.

   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 (STVDS) 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 vector algebra.
       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 VECTOR 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.

   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 tmap 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’

   Spatial vector operators
       The module supports the following boolean vector operations:
        Boolean Name   Operator Meaning         Precedence   Correspondent function
       ----------------------------------------------------------------------------------
        AND            &        Intersection          1      (v.overlay operator=and)
        OR             |        Union                 1      (v.overlay operator=or)
        DISJOINT OR    +        Disjoint union        1      (v.patch)
        XOR            ^        Symmetric difference  1      (v.overlay operator=xor)
        NOT            ~        Complement            1      (v.overlay operator=not)
       And vector functions:
        buff_p(A, size)           Buffer the points of vector map layer A with size
        buff_l(A, size)           Buffer the lines of vector map layer A with size
        buff_a(A, size)           Buffer the areas of vector map layer A with size

   Combinations of temporal, vector and select operators
       We  combine the temporal topology relations, the temporal operators and the spatial/select
       operators to create spatio-temporal vector operators:
       {"spatial or select operator" , "list of temporal relations", "temporal operator" }

       For multiple topological relations or several related maps the  spatio-temporal  operators
       feature  implicit  aggregation.   The  algebra evaluates the stated STDS by their temporal
       topologies and apply the given spatio temporal operators in a aggregated form.  If we have
       two  STDS A and B, B has three maps: b1, b2, b3 that are all during the temporal extent of
       the single map a1 of A, then the following overlay calculations would implicitly aggregate
       all maps of B into one result map for a1 of A:
       C = A {&, contains} B --> c1 = a1 & b1 & b2 & b3
       Keep attention that the aggregation behaviour is not symmetric:
       C = B {&, during} A --> c1 = b1 & a1
                               c2 = b2 & a1
                               c3 = b3 & a1

   Examples:
       Spatio-temporal intersect all maps from space time dataset A with all maps from space time
       dataset B which have equal time stamps and are temporary before Jan.  1.  2005  and  store
       them in space time dataset D.
       D = if(start_date(A) < "2005-01-01", A & B)
       Buffer all vector points from space time vector dataset A and B with a distance of one and
       intersect the results with overlapping, containing, during and equal temporal relations to
       store the result in space time vector dataset D with intersected time stamps.
       D = buff_p(A, 1) {&,overlaps|overlapped|equal|during|contains,i} buff_p(B, 1)
       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)

REFERENCES

       PLY(Python-Lex-Yacc)

SEE ALSO

        t.select

AUTHORS

       Thomas Leppelt, Soeren 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.vect.algebra source code (history)

       Main index | Temporal index | Topics index | Keywords index | Graphical index | Full index

       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual