[GRASS5] New Script Submission on Grass WIKI: d.hyperlink

Patton, Eric epatton at nrcan.gc.ca
Fri Feb 10 10:38:13 EST 2006

-----Original Message-----
From: Michael Barton
To: Patton, Eric
Sent: 2/9/2006 10:49 PM
Subject: Re: [GRASS5] New Script Submission on Grass WIKI: d.hyperlink

Have you looked at the new v.what module that Trevor Wiens just
finished? It
queries a vector without needing an xterm.



I had a look at v.what, and I'm kind of confused about how it supposed to
work. I don't think that having the user enter in x and y coodinates as a
vector query is very efficient, for a few reasons:

1) There's no guarantee the user has any idea what the coordinates are for
the vectors in the first place. I certainly don't know beforehand. I might
have the idea that I want to see the hyperlink (or attributes) for Station
#20, but I'm stopped immediately from querying without xy's written down

2) The case might be made for using xy as the query of choice by saying that
that coords can just be found out from the attribute table. But there's no
guarantee that they exist there either - for instance, most data I receive
is collected by DGPS, so it's always Lat-Long. I import the data into an XY
Location, and reproject into a UTM Location with v.proj. But the coords in
the table are still Lat-Long. And uploading grid coords into the vector
table still requires an extra step, which IMO is less efficient.

3) Suppose the user does happen to know they are interested in the hyperlink
(or attributes) at east 5304500 north 4500750. Will whole integer coords
match the internal vector coords? What I mean is, are the vector coordinates
stored as floats or ints (will a 5304500 easting in the user query match
5304500.02)? Forgive my ignorance here, I don't know very much about how
Grass vectors' coordinates are related to the geometry, so this question
might not even make much sense. 

4) Lastly, I think being able to query a vector based on an ID is probably
more intuitive than using xy's, which is why I've built in the ability to
query based on SQL 'where' statement. 

Don't get me wrong, I think an xy query still could be useful provided my
concern in #3 above is invalid. And if my understanding of v.what is way off
base, then I apologize and please excuse my rant ;)

~ Eric.

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