[GRASS-dev] proposed improvements to v.select (was: How to find all vectors that overlay a given region)

[Michael Barton]

On 9/19/07 7:46 AM, "Patton, Eric" <epatton at nrcan.gc.ca> wrote:

>> Michael Barton wrote:
> 
>>> beware the case where the bounding boxes overlap but the vector features
>>> within
>>> do not. 
> 
>> Good point. I don't know how to account for that since vectors are not a
>> continuous field.
> 
>> Michael
> 
> Using v.select on the list of candidate maps whose regions superficially
> overlap and then v.info -t on the output will tell you if you have any vector
> geometry in the region.
> 
> ~ Eric.
> 

Eric's question and the very convoluted way of getting at an answer points
out a major weakness in the GRASS vector processing tool set. GRASS 6 is a
LOT better at dealing with vectors than any prior version. But there is
still a lack of sufficient vector query tools. So I want to suggest a kind
of roadmap to upgrading vector querying so that perhaps someone with C
programming skills, time, and an interest in vectors could perhaps take this
up (if I haven't eliminated everyone already). Maybe a summer of code
project?

v.select (query using a vector map):
1. 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.
2. 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.
3. 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.
4. 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):
1. add flags -p and -g for current behavior (-pg could be the default if we
wanted to do this before GRASS 7)
2. add "output=" option to allow v.what to create a new map from the results
of its query, like v.select does
3. allow multiple maps for input, as with the suggestion for v.select
4. allow coordinates to be read optionally as a line or area boundary (-l or
-a?) instead of only as individual points.
5. 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):
1. drop the ainput and binput. Replace with just input.
2. allow multiple maps to be entered into input, not just 2
3. 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).

I think that this should give us a full range of vector data spatial
management tools.

Opinions?

Michael

__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton