1STUMTA(3NCARG)                   NCAR GRAPHICS                  STUMTA(3NCARG)
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NAME

6       STUMTA - Given the coordinates of a point on the streamline in data,
7       user, and NDC space, and the interpolated, normalized components of the
8       vector at the point relative to data coordinate space, the user-
9       modifiable routine STUMTA finds the directional angle of the streamline
10       relative to NDC space at the point.
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SYNOPSIS

13       CALL STUMTA (XDA,YDA,XUS,YUS,XND,YND,DU,DV,TA,IST)
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DESCRIPTION

16       XDA         (REAL, input): The X coordinate of a point on the
17                   streamline in the data coordinate system.
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19       YDA         (REAL, input); The Y coordinate of a point on the
20                   streamline in the data coordinate system.
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22       XUS         (REAL, input): The X coordinate of the point in the user
23                   coordinate system.
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25       YUS         (REAL, input): The Y coordinate of the point in the user
26                   coordinate system.
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28       XND         (REAL, input): The X coordinate of the point in NDC space.
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30       YND         (REAL, input): The Y coordinate of the point in NDC space.
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32       DU          (REAL, input): The interpolated value of the normalized
33                   component of the vector at the point, with direction
34                   parallel to the X axis of the data coordinate system.
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36       DV          (REAL, input): The interpolated value of the normalized
37                   component of the vector at the point, with direction
38                   parallel to the Y axis of the data coordinate system.
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40       TA          (REAL, output): The directional angle of the streamline at
41                   the point relative to NDC space.
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43       IST         (REAL, output); Status of the mapping operation: 0
44                   indicates success, negative values indicate that the
45                   mapping failed; positive values are reserved and should not
46                   be used by the implementor of a mapping routine.
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USAGE

49       STUMTA is a user-modifiable routine provided to support custom mappings
50       of the data coordinate space. The user does not invoke it directly.
51       Instead, whenever the parameter MAP specifies a mapping not handled by
52       Streamlines internally (i.e., when MAP is set to a value other than 0,
53       1, or 2), Streamlines calls STUMTA once for each incremental step in
54       the creation of a streamline. The default version of STUMTA simply
55       returns the angle implied by the incremental vector components passed
56       to it: that is, it returns ATAN2(DV,DU). In order to implement a custom
57       mapping, you must pick a unique mapping code (a positive integer
58       greater than 2), and then modify each of the three routines, STUMXY,
59       STUIXY, and STUMTA to recognize and respond consistently to the chosen
60       code. In the standard distribution of NCAR Graphics, these three
61       routines reside in a single file, ´stumxy.f´.  STUMXY maps a point from
62       data to user coordinate space and STUIXY inversely maps a point from
63       user to data coordinate space. STUMTA, which is likely to be the most
64       difficult to implement, finds the tangent angle of the streamline at a
65       point in NDC space.
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67       STUMTA has access to a common block called STMAP that contains a number
68       of variables used to record the current transformation state.  In order
69       to accommodate a variety of mapping implementations, STMAP provides
70       more information than normally required. Consider the values stored in
71       STMAP as strictly read-only. One essential member of this common block
72       is IMAP, which contains the value currently assigned to the MAP
73       parameter.
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75       When implementing a non-linear mapping, an iterative differential
76       technique will most likely be required. Look at the routine, STMPTA, in
77       ´stmpxy.f´, which handles the pre-defined mappings, for examples of the
78       method. Both the default transformation (MAP set to 0), in order to
79       account for possible log scaling of the user coordinate axes, and also
80       the Ezmap projection (MAP set to 1) use such a technique.  Basically
81       the idea is that the vector components must be proportionally reduced
82       in size enough that an effectively "instantaneous" angle can be
83       calculated, although they must not become so small that the calculation
84       is adversely affected by the floating point precision available for the
85       machine. Additionally, checks must be put in place to prevent the
86       increment from stepping off the edge of the coordinate system space.
87       The pre-defined mappings step in the opposite direction to find the
88       angle whenever an increment in the original direction would fall off
89       the edge.
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ACCESS

92       To use STUMTA, load the NCAR Graphics libraries ncarg, ncarg_gks, and
93       ncarg_c, preferably in that order.
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SEE ALSO

96       Online: stgetc, stgeti, stgetr, stinit, stream, streamlines,
97       streamlines_params, strset, stsetc, stseti, stsetr, stuixy, stumsl,
98       stumxy, ncarg_cbind.
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100       Hardcopy: NCAR Graphics Fundamentals, UNIX Version
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103       Copyright (C) 1987-2009
104       University Corporation for Atmospheric Research
105       The use of this Software is governed by a License Agreement.
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109UNIX                              April 1993                    STUMTA(3NCARG)
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