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Top Forums UNIX for Dummies Questions & Answers How to get data only inside polygon created by points which is part of whole data from file? Post 302411426 by Corona688 on Thursday 8th of April 2010 12:45:38 PM
Old 04-08-2010
I don't have a simple program for you. It's not even a simple problem. Furthermore just considering this a polygon may not be strictly correct since we're concerned with points on a sphere not a plane; if you just consider latitude as "Y" and longitude as "X" your polygon would need curved lines. You'd have to map it into mercator projection or something to get a correct representation. But if a naive solution will do, you could do it in a similar way to how they draw polygons. Please excuse my awful ASCII art:

Code:
        /\      /--\
......./**\..../****\....
      /    \  /      \
      |     \/       /
      |             /
      \            /
       \          /
        \---------

So you loop through your list of edges left to right, stopping at the segment following it, starting again at the next segment, stopping at the next, etc, etc until you run out of edges, checking if your data point exists between any of the spans you'd be "drawing" in. Note that the order of segments left to right can change depending on the latitude of the data point in question, you have to keep re-sorting the list!

To speed this up you could first check the data points against a bounding box of [min, min]-[max,max] which your polygon fits inside. If it's outside that box you can skip it without the bother of re-sorting your list of edges and trawling them one by one.

Last edited by Corona688; 04-08-2010 at 01:52 PM..
 

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PLPOLY3(3plplot)						    PLplot API							  PLPOLY3(3plplot)

NAME
plpoly3 - Draw a polygon in 3 space SYNOPSIS
plpoly3(n, x, y, z, draw, ifcc) DESCRIPTION
Draws a polygon in 3 space defined by n points in x, y, and z. Setup like plline3(3plplot), but differs from that function in that plpoly3(3plplot) attempts to determine if the polygon is viewable depending on the order of the points within the arrays and the value of ifcc. If the back of polygon is facing the viewer, then it isn't drawn. If this isn't what you want, then use plline3(3plplot) instead. The points are assumed to be in a plane, and the directionality of the plane is determined from the first three points. Additional points do not have to lie on the plane defined by the first three, but if they do not, then the determination of visibility obviously can't be 100% accurate... So if you're 3 space polygons are too far from planar, consider breaking them into smaller polygons. 3 points define a plane :-). Bugs: If one of the first two segments is of zero length, or if they are co-linear, the calculation of visibility has a 50/50 chance of being correct. Avoid such situations :-). See x18c.c for an example of this problem. (Search for 20.1). Redacted form: plpoly3(x, y, z, code) This function is used in example 18. ARGUMENTS
n (PLINT, input) Number of points defining line. x (PLFLT *, input) Pointer to array with x coordinates of points. y (PLFLT *, input) Pointer to array with y coordinates of points. z (PLFLT *, input) Pointer to array with z coordinates of points. draw (PLBOOL *, input) Pointer to array which controls drawing the segments of the polygon. If draw[i] is true, then the polygon segment from index [i] to [i+1] is drawn, otherwise, not. ifcc (PLBOOL, input) If ifcc is true the directionality of the polygon is determined by assuming the points are laid out in a counter-clockwise order. Otherwise, the directionality of the polygon is determined by assuming the points are laid out in a clockwise order. AUTHORS
Geoffrey Furnish and Maurice LeBrun wrote and maintain PLplot. This man page was automatically generated from the DocBook source of the PLplot documentation, maintained by Alan W. Irwin and Rafael Laboissiere. SEE ALSO
PLplot documentation at http://plplot.sourceforge.net/resources. August, 2012 PLPOLY3(3plplot)
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