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## [task #14639] Table arithmetic operator to find rows within given region

**From**: |
Mohammad Akhlaghi |

**Subject**: |
[task #14639] Table arithmetic operator to find rows within given region |

**Date**: |
Sun, 29 Mar 2020 14:51:27 -0400 (EDT) |

**User-agent**: |
Mozilla/5.0 (X11; Linux x86_64; rv:74.0) Gecko/20100101 Firefox/74.0 |

Update of task #14639 (project gnuastro):
Category: New program => Table
Summary: Program to identify points inside/outside a region
=> Table arithmetic operator to find rows within given region
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Follow-up Comment #3:
Good point! We now have all the low-level tools for this task!
Previously I had mentioned this task as a separate program, but given all the
improvements in Gnuastro since 2.5 years ago, I think it now fits nicely
within the Table program (as a Column Arithmetic
<https://www.gnu.org/software/gnuastro/manual/html_node/Column-arithmetic.html>
operator.
After all the input will always be a table, the user just has to specify the
columns containing the coordinates and the polygon in the same units as the
coordinates.
The only complication that I can think of now is that Right Ascension
<https://en.wikipedia.org/wiki/Right_ascension> and Declination
<https://en.wikipedia.org/wiki/Declination> are defined on the spherical
coordinate system of the sky, so if the region is sufficiently large, a
Euclidean-space line between the vertices may miss some points. To address
this, we have to add a spherical implementation of the `gal_polygon_is_inside'
function.
But until then, we can say that it assumes a euclidean space and implement the
low-level column arithmetic operator (very easy!). For spherical coordinates,
we should define a new operator.
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