Enhancements to vector querying
|Reported by:||cmbarton||Owned by:|
About 9 months ago, I suggested an overhaul of the GRASS vector query framework (original post <http://lists.osgeo.org/pipermail/grass-dev/2007-September/033173.html>)
Now that work has started on GRASS 7 in ernest, I'd like to reiterate this. Besides giving GRASS a simpler and richer query ability, it would permit much easier GUI interface to vector querying--such as interactively drawing a box or polygon around points to select them for query in the new database table manager.
So here is the idea. It involves some upgrades to modules we already have, but no new modules. The nice thing is we are almost there. I'll also drop this in the bug tracker as an enhancement suggestion.
v.select (query using a vector map):
- add flags -p and -g for text output instead of creating a new map. It would report which map(s) and feature(s)/cat(s) meet the query criteria.
- allow multiple maps to be selected. This would directly address Eric's question. If the output is a map, it would be the equivalent of v.patch on all queried vector elements.
- add operators "contains" and "adjacent". Contains=all vector features whose nodes are completely inside a polygon (or inside or touching the boundary). Adjacent=all vector features who share a node/point or line/boundary with the selecting feature. Because GRASS is topologically correct, adjacency information is readily available.
- maybe change option names from ainput and binput to selector and selected or queried. This would have to wait until GRASS 7, of course. I find ainput and binput not very clear where used in other vector operations either (maybe I'm just dense).
v.what (query using coordinates):
- add flags -p and -g for current behavior (-pg could be the default if we wanted to do this before GRASS 7)
- add "output=" option to allow v.what to create a new map from the results of its query, like v.select does
- allow multiple maps for input, as with the suggestion for v.select
- allow coordinates to be read optionally as a line or area/boundary (-l or -a?) instead of only as individual points.
- add operators overlap, contains, adjacent. (This also would make possible interactive vector selection with a mouse drawn box or polygon from the GUI) In other words, have v.select and v.what work the same except that v.select uses a vector map for querying and v.what uses a set of coordinate points.
v.overlay (boolean combination of maps):
- drop the ainput and binput. Replace with just input.
- allow multiple maps to be entered into input, not just 2
- deprecate v.patch because v.overlay with the OR operator replaces it. (If we wanted to do this before GRASS 7, we'd have to create a new module, maybe named v.combine or something like that because this changes the default behavior of v.overlay)