.. index:: ! polespotter .. include:: ../module_supplements_purpose.rst_ *********** polespotter *********** |polespotter_purpose| Synopsis -------- .. include:: ../../common_SYN_OPTs.rst_ **gmt polespotter** [ |-A|\ *abyssalhills* ] [ |-D|\ *spacing* ] [ |-E|\ **a**\|\ **f**\ *sigma* ] [ |-F|\ *fracturezones* ] [ |-G|\ *outgrid* ] [ |SYN_OPT-I| ] [ |-N| ] [ |SYN_OPT-R| ] [ |-S|\ **l**\|\ **p**\|\ **s**\ [*modifiers*] ] [ |SYN_OPT-V| ] [ |SYN_OPT-bi| ] [ |SYN_OPT-di| ] [ |SYN_OPT-e| ] [ |SYN_OPT-h| ] [ |SYN_OPT-i| ] [ |SYN_OPT-o| ] [ |SYN_OPT-r| ] [ |SYN_OPT-:| ] [ |SYN_OPT--| ] |No-spaces| Description ----------- **polespotter** reads line segments (abyssal hill fabric lineaments and/or fracture zone lineaments) and performs one of three types of scans. In spot mode it computes great circles for each individual segment along these lines. The bisector great circles to fracture zones and the great circle extensions of abyssal hills are expected to intersect at potential rotation poles. The assumption is that abyssal hill lines are meridians and fracture zones are parallels with respect to the rotation pole. Line density may be computed and returned via a grid, the great circle lines may be returned via standard output, and the intersections of the great circles may be saved to file. In line mode it will determine which line segments are compatible with a given trial pole, while in pole mode it will compute chi-squared misfits for all the poles defined by the grid. Optional Arguments ------------------ .. _-A: **-A**\ *abyssalhills* File with multiple segments of abyssal hill lineaments. These are assumed to reflect the great circle direction towards the rotation pole in effect when the seafloor was formed. .. _-D: **-D**\ *step* Sets the line increment for all great circles produced, in km [5]. Actual spacing will be adjusted to give an integer number of steps along the full circle. .. _-E: **-E** Provide different 1-sigma angular uncertainty (in degrees) in the orientation of **a**\ byssal hills or **f**\ racture zones. Give **-Ea**\ *sigma* to set the former [1] and **-Ef**\ *sigma* for the latter [1]. These *sigma* values are then used to form weights = 1/*sigma*. .. _-F: **-F**\ *fracturezones* File with multiple segments of fracture zone lineaments. These are assumed to reflect small circles about the rotation pole in effect when the seafloor was formed. .. _-G: .. |Add_outgrid| replace:: Specify name for output grid. For spot mode we will accumulate great circle line density for the grid. Each bin that is crossed by a great circle is incremented by 1, multiplied by cos(latitude), the length of the fracture zone or abyssal line segment used to define the great circle, and any overall weight set via |-E|. In pole mode we return the chi-squared misfit surface. Not used in line mode. .. include:: /explain_grd_inout.rst_ :start-after: outgrid-syntax-begins :end-before: outgrid-syntax-ends .. _-I: .. include:: ../../explain_-I.rst_ .. _-N: **-N** Normalize the grid so max value equals 1 [no normalization]. .. _-R: .. |Add_-Rgeo| unicode:: 0x20 .. just an invisible code .. include:: ../../explain_-Rgeo.rst_ .. _-S: **-Sl**\|\ **p**\|\ **s**\ [*modifiers*] Set the scan mode for this run. Choose from **l**\ ines, **p**\ oles, or **s**\ pots. Depending on the mode there may be required and optional arguments as indicated below: **-Sl**\ *plon*\ /*plat*\ [**+m**] Line mode means we accept a *plon*\ /*plat* trial pole location and determine how compatible each data segment is with the predictions of small circles (for fracture zones) and meridians (for abyssal hills). By default we report summary statistics (*chi2*, *table*, *segment*, *type*) for each line segment. Append **+m** to instead report the misfit information (*mlon*, *mlat*, *del_angle*, *chi2*, *table*, *segment*, *type*) for each mid-point along all multi-point line segments. The information is written to standard output. **-Sp** Pole mode means we search for all poles on the given grid and determine the weighted chi-square misfit to all given line constraints. This mode requires |-G|, |-R|, |-I| (and optionally **-r**). **-Ss**\ [**+c**\ *xfile*][**+l**] Spot mode means we compute bisectors to fracture zones and meridians along abyssal hills and determine intersections of all these great circles. You can append any of two modifiers: **+l** will dump all great circles produced to standard output [no output], and **+c**\ *xfile* will compute the intersections of all great circles and write the locations to *xfile*. This output has 5 columns: *lon*, *lat*, *weight*, *cos*, *type*, where *weight* is the combined length weight from the two generating line segments, *cos* is the cosine of the angle between the intersecting lines, and *type* is either 0 (AH intersect AH), 1 (AH intersect FZ), or 2 (FZ intersect FZ), where AH means an abyssal hill great circle and FZ means a bisector great circle to a fracture zone. .. |Add_-V| replace:: |Add_-V_links| .. include:: /explain_-V.rst_ :start-after: **Syntax** :end-before: **Description** .. |Add_-bi| replace:: [Default is 5 input columns]. .. include:: ../../explain_-bi.rst_ .. |Add_-di| unicode:: 0x20 .. just an invisible code .. include:: ../../explain_-di.rst_ .. |Add_-e| unicode:: 0x20 .. just an invisible code .. include:: ../../explain_-e.rst_ .. |Add_-h| unicode:: 0x20 .. just an invisible code .. include:: ../../explain_-h.rst_ .. include:: ../../explain_-icols.rst_ .. include:: ../../explain_-ocols.rst_ .. |Add_nodereg| unicode:: 0x20 .. just an invisible code .. include:: ../../explain_nodereg.rst_ .. include:: ../../explain_colon.rst_ .. include:: ../../explain_help.rst_ .. include:: explain_geodetic.rst_ Notes ----- #. Each input line is expected to contain two or more points, and each consecutive pairs of points define a great circle line segment. For fracture zones, these points should be digitized often enough so that the great circle between then can approximate the small circle. #. All line segments are given equal angular uncertainty [1, unless changed by |-E|]. However, individual line segments can override this weight by adding a **-D**\ *sigma* argument in the segment headers (in degrees). Examples -------- To create a polespotting image from the abyssal hill and fracture zone fabric (*lon, lat*) data in the files hills.txt and fractures.txt, on a 1x1 degree grid for the northern hemisphere, sampling the great circles every 10 km, and also dump the great circles to standard output, try :: gmt polespotter -Ahills.txt -Ffractures.txt -D10 -Gpoles.nc -R0/360/0/90 -I1 -V -Ss+l > lines.txt This file can then be plotted with :doc:`grdimage `. See Also -------- :doc:`gmt `, :doc:`grdimage `, :doc:`grdrotater`, :doc:`grdspotter`, :doc:`project `, :doc:`mapproject `, :doc:`backtracker`, :doc:`gmtpmodeler`, :doc:`gmtvector `, :doc:`grdpmodeler`, :doc:`grdrotater`, :doc:`originater`